H. E. WRIGHT, JR. Dept. Geology, University of Minnesota, Minneapolis, Minn.
Late Pleistocene Climate of Europe: A Review
Abstract: The Pleistocene periglacial environment tional zonation, the maps are considered useful as a of Europe is believed to have been marked by in- basis for discussion of Pleistocene atmospheric circu- tensive frost action and wind action and by the lation patterns. spread of arctic and subarctic plants and animals The sequence of climatic changes for the last south to the Alps. The frost action is recorded by cold period is inferred largely from the stratigraphy ice-wedge casts, involutions, solifluction deposits, of periglacial loesses and intercalated soils, and from block fields, and related deposits, some of which their correlation with the Alpine glacial sequence. may record permafrost. The occurrence of extensive The controversy concerning the classification of the sand dunes and loess deposits may be related to the Wiirm period as single (following Penck) or as distribution of glaciofluvial plains, strong winds, multiple (following Soergel) is reviewed; the and the absence of forest cover. The occurrence of validity of the Gottweig interstadial is questioned a tundra zone in the basal levels of Late-glacial on the basis of recent field studies and radiocarbon pollen sequences in central Europe indicates a frigid dates in Austria and adjacent areas. climate, and molluscan and mammalian faunas pro- The reduction of mean annual temperature dur- vide similar information. ing the late Pleistocene, as inferred from the snow- Poser and Biidel have separately attempted to line depression in the mountains, amounted to represent the Pleistocene climatic-vegetational 5°-8°C under the general assumption of a 0.5°- zones of Europe on maps. Poser located the south- 0.7°C/100 m vertical temperature gradient (lapse ern limit of permafrost according to the distribution rate) in the atmosphere. Calculations based on the of ice-wedge casts and involutions, and the northern occurrence of frost features and fossil plants in limit of forest from pollen-analytical evidence sum- lowlands, however, suggest a temperature depres- marized by Firbas. These two map lines were then sion of 10°-12° C. The discrepancy may result from used to help delineate several vegetational-climatic different lapse rates over mountains and over low- provinces. An additional study on the Late-glacial lands, according to Mortensen. Studies of modern sand dunes led to the presentation of a map of mean lapse rates in arctic, subarctic, and subtropical summer air pressure for Europe. regions are reviewed in order to evaluate this Biidel located the Pleistocene frost limit on the hypothesis. basis of its presumed relation to the 1000-m snow Maps of mean atmospheric pressure and circu- line, which is identifiable in the mountains of central lation patterns are commonly based on extrapola- Europe. His representation differs from Poser's tion from those modern patterns that are related chiefly tor Hungary and France, where there is a to snowy winters, cool summers, and other condi- dearth of pollen studies from which vegetational tions favorable to glaciation, on the assumption that relations can be inferred. Biidel distinguishes several the Pleistocene climate was marked primarily by a vegetational zones throughout Europe on the basis different frequency of certain air-mass movements of loess distribution and also from pollen diagrams and related storm tracks compared to today. where available. Biidel believes that the ice sheet Possible causes for circulation changes are still had only a minor effect on the climatic zonation of highly speculative, although much recent interest Europe during the Pleistocene. is centered on variations in solar radiation such as Although exception is taken to many details of are exhibited in Sunspots. the criteria used by Poser and Biidel for vegeta-
Zusammenfassung: Es wird allgemein angenom- Flugsand- und Lossablagerungen muss an der men, dass die periglaziale Zone Europas wahrend Verbreitung fluvioglazialer Flachen, starken Win- des Pleistozans starker Frost- und Windwirkung den und dem Fehlen einer Waldbedeckung liegen. ausgesetzt war, infolge welcher arktische und Die Pollenspektren in den untersten Teilen subarktische Pflanzen sich damals in siidlicher spatglazialer Ablagerungen in Mitteleuropa weisen Richtung auf die Alpen zu verbreiteten. Die auf ein kaltes Klima hin, ebenso wie die Mollusken Frostwirkung zeigt sich in Eiskeilnetzen, Taschen- und Saugetiere. boden, Fliesserden, Blockfeldern und ahnlichen Unabhangig vonemander haben Poser und Biidel Ablagerungen. Einige davon sind Anzeichen eines versucht, Karten der klimatisch bedingten Vegeta- Dauerfrostbodens. Das Vorkommen ausgebreiteter tionsgiirtel Europas wahrend des Pleistozans
Geological Society of America Bulletin, v. 72, p. 933-984, 9 figs., June 1961 933
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zusammenzustellen. Die Siidgrenze des Dauerfrost- ungen andererseits abgeleitet. Die Streitfrage iiber bodens 1st von Poser auf Grund der Verbreitung die Wiirmeiszeit als ein einmaliger (nach Penck) von Eiskeilnetzen und Taschenboden gezogen oder mehrmaliger (nach Soergel) Kaltevorstoss worden, wahrend die Nordgrenze des Waldes mit wird diskutiert, wahrend die Haltbarkeit des Hilfe der pollenanalytischen Daten nach Firbas Gottweiger Interstadials auf Grund von rezenten festgestellt wurde. Diese beiden Linien wurden Untersuchungen im Gelande und von Radio- benutzt, um die verschiedenen Vegetations- und karbon Datierungen in Osterreich und in angren- Klimaprovinzen abzugrenzen. Eine erganzende zenden Gebieten in Frage gestellt wird. Untersuchung spatglazialer Binnendiinen hat zu Unter der iiblichen Annahme eines Reduktions- der Zeichnung einer Karte der mittleren Luft- faktors von 0.5 bis 0.7° C. fiir 100 m lasst sich aus druckverteilung wahrend des Sommers in Europa der Senkung der Schneegrenze im Gebirge eine gefuhrt. Erniedrigung der mittleren Jahrestemperature von Biidel hat die pleistozane Frostgrenze auf Grund 5 bis 8° C. berechnen. Aus dem Vorkommen von der angenommenen Beziehung zur 1000 m-Schnee- Frosterscheinungen und fossilen Pflanzen im grenze festgestellt, die in den Mittelgebirgen Flachland lasst sich dagegen ein Temperaturriick- nachgewiesen werden kann. Seine Darstellung gang von 10 bis 12° C. berechnen. Nach Mortensen weicht von der Posers hauptsachlich fur Ungarn konnte diese Diskrepanz ungleichen Reduktions- und Frankreich ab, wo noch wenig pollenanaly- faktoren im Gebirge und im Flachland zugeschrieb- tische Untersuchungen zur Bestimmung der Vege- en werden. Um diese Hypothese auszuwerten, tation durchgefiihrt worden sind. Auf Grund der werden in diesem Aufsatz die Untersuchungen Lossverbreitung sowie auf Grund von Pollendia- uber den jetzigen Reduktionsfaktor in arktischen, grammen, wo solche zur Verfiigung standen, subarktischen und subtropischen Gebieten bes- unterscheidet Biidel verschiedene Vegetationszonen prochen. in Europa. Biidel meint, dass das Inlandeis nur Karten der mittleren Luftdruckverteilung und einen untergeordneten Einfluss auf die Klimazonen der Windrichtungen sind meistens auf einer Ex- Europas wahrend des Pleistozans ausgeiibt haben trapolation derjenigen rezenten Luftdrucklagen kann. basiert, die schneereiche Winter, kiihle Sommer Obwohl man in manchen Einzelfragen Bedenken und andere fiir Vergletscherung giinstige Bedingun- gegen die von Poser und Biidel fur die Vegeta- gen hervorrufen. Dabei wird angenommen, dass tionszonierung benutzten Normen haben kann, das pleistozane Klima im wesentlichen durch eine miissen diese Karten doch als sehr wertvoll fiir von der gegenwartig abweichenden Haufigkeit die Diskussion der Luftdruckverteilung und von Stromungen bestimmter Luftmassen und Windrichtung wahrend des Pleistozans beachtet damit in Verbindung stehender Sturmbahnen werden. charakterisiert war. Uber die moglichen Ursachen Die Aufeinanderfolge der Klimaschwankungen der A'nderungen des Stromungsbildes lasst sich wahrend der letzten Eiszeit wird hauptsachlich noch wenig sagen, obwohl sich das heutige Interesse aus der Stratigraphie der periglazialen Lossschichten besonders auf die Schwankungen der Sonnen- und der eingeschalteten Boden einerseits und aus strahlung, wie sie in Sonnenflecken zum Ausdruck ihrer Verkniipfung mit den alpinen Vergletscher- kommt richtet.
Resume: On considere en general que, pendant le la limite sud du sol perpetuellement gele d'apres Pleistocene, le milieu periglacial d'Europe etait la repartition des fentes en coin et des involutions, caracterise par une action du gel intense, par et la limite nord de la foret d'apres les indications Faction du vent, et par 1'extension vers le sud, de 1'analyse pollmique telles qu'elles sont resum6es jusqu'aux Alpes, de plantes et d'animaux arctiques par Firbas. Ces deux lignes sont ensuite utilisees et subarctiques. L'action du gel est indiquee par pour permettre de separer plusieurs provinces de des fentes en coin, par des involutions, par des climat et de vegetation. Une etude supplemental depots de solifluxion, par des champs de blocs et des dunes de sable du Tardiglaciaire permit de autres depots analogues, dont certains peuvent dresser une carte de la pression atmospherique indiquer un sol gele en permanence. La presence moyenne d'ete pour 1'Europe. de dunes sableuses importantes et de loess est Biidel determine la limite de gel du Pleistocene peut-etre liee a la repartition de plaines fluvio- d'apres ses rapports avec la limite des neiges glaciaires, a des vents violents, et a 1'absence de permanentes de 1000 m, qui se reconnait bien couvert forestier. La presence d'une zone de tundra dans les montagnes de 1'Europe centrale. Sa dans les niveaux inferieurs des diagrammes pol- representation differe de celle de Poser surtout liniques de 1'epoque Tardiglaciaire de 1'Europe en Hongrie et en France, ou les etudes polliniques Centrale indique un climat froid, et les faunes de sont insuffisantes pour qu'on puisse en deduire des mollusques et de mammiferes fournissent des renseignements sur la vegetation. Biidel distingue renseignements semblables. plusieurs zones de vegetation dans toute 1'Europe Poser et Biidel ont independamment essaye de d'apres la repartition du loess et aussi d'apres des representer sur des cartes les zones de climat et de diagrammes polliniques lorsqu'ils existent. Biidel vegetation de 1'Europe au Pleistocene. Poser place pense que la calotte glaciaire n'a eu qu'un effet
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mmime sur la Donation climatique de 1'Europe au d'altitude. Toutefois les calculs bases sur la presence Pleistocene. de formes de cryoturbation et de plantes fossiles Bien qu'on puisse critiquer bien des details des dans les plaines suggerent un abaissement de la criteres utilises par Poser et Biidel pour la zonation temperature de 10 a 12°C. La difference provient de la vegetation, leurs cartes se montrent utiles peut-etre, d'apres Mortensen d'une difference comme bases de discussion des systemes de circu- entre les taux de decroissement au-dessus des lation atmospherique du Pleistocene. montagnes et au-dessus des plaines. L'auteur L'auteur deduit la succession des changements resume les Etudes des taux actuels dans les regions de chmat pour la dermere periode de froid et arctiques, subarctiques et subtropicales pour evaluer general d'apres la stratigraphie des loess peri- cette hypothese. glaciaires et des sols mtercales, et de leur correlation En general, on dresse les cartes de pression avec la succession glaciaire des Alpes. II resume la atmospherique moyenne et des systemes de circu- controverse qui entoure la classification de la lation par extrapolation a partir de ceux des periode du Wiirm en une seule glaciation (suivant systemes contemporains qui se trouvent lies a des Penck) ou en glaciations multiples (d'apres Soergel); hivers neigeux, des etes frais et autres conditions il met en question la validite de 1'inter-stadiaire du favorables a la glaciation, ceci etant base sur la Gottweig en vue des etudes recentes sur le terrain supposition que le climat Pleistocene etait carac- et des dates au radiocarbone obtenues en Autnche terise surtout par une frequence de certains mouve- et dans les regions avoisinantes. ments des masses d'air et des trajectoires de tern- L'abaissement de la temperature moyenne petes differente de ce qui se passe de nos jours. annuelle pendant le Pleistocene recent, deduite de Les causes possibles du changement de circulation 1'abaissement de la limite des neiges dans les sont encore du domaine de la conjecture, quoiqu'on montagnes, est de 5 a 8°C, si Ton admet 1'existence s'interesse actuellement beaucoup aux variation d'un gradient vertical ("lapse rate") de tempera- du rayonnement solaire telles qu'elles s'observent ture de 1'atmosphcre de 0,5 a 0,7°C par 100 m dans les laches du Soleil.
Resumen: El medio ambiente periglacial del difiere de la de Poser principalmente para Hungria Pleistoceno de Europa se cree que ha sido marcado y Francia, donde escasean los estudios de polen de por los efectos intensos de heladas y de vientos y los que se podria inferir las relaciones vegetacionales. por la dispersion de plantas y animales articos y Biidel distingue varias zonas vegetacionales en toda subarticos hacia el sur, hasta los Alpes. El efecto Europa a base de la distribucion de loess y tambien de las heladas esta registrado por los moldes de de los diagramas de polen disponibles. Biidel cree curias de hielo, por involuciones, por depositos de que el casquete de hielo solo tuvo un efecto menor solifluccion, por campos de bloques y por depositos en la zonificacion climatica de Europa durante el relacionados, algunos de los cuales pueden registrar Pleistoceno. la helada permanente. La presencia de extensos Aunque se duda de muchos de los detalles de los medanos arenosos y depositos de loess puede estar criterios usados por Poser y Biidel para la zonifica- relacionada con la distribucion de llanuras glacio- cion de la vegetacion, los mapas se consideran fluviales. con fuertes vientos y con la falta de utiles como una base para la discusion de los cubierta forestal. La apariencia de Una zona de regimenes de circulacion atmosferica del Pleisto- tundra en los niveles basales de los diagramas de ceno. polen hacia el final de la glaciacion en Europa La sucesion de los cambios chmaticos en el central indica un clima frigido. Las faunas de ultimo periodo frio se infiere principalmente de la moluscos y de mamiferos proporcionan datos estratigrafia de los loess periglaciales y suelos parecidos. intercalados, asi como de su correlacion con la Poser y Biidel, por separado, han intentado sucesion glacial alpma. Se hace una resena de la representar en mapas las zonas climatico-vegeta- controversy acerca del periodo Wiirm, clasifican- cionales pleistocemcas de Europa. Poser localize dolo como uno solo (siguiendo Penck) o como el limite austral de la helada permanente de acuerdo multiple (siguiendo Soergel); se pone en duda la con la distribucion de moldes de curias de hielo y validez del interestadial de Gottvveig, basandose de involuciones, asi como el limite boreal de los en estudios recientes de campo y en fechas radio bosques de acuerdo con las evidencias analiticas metricas de carbon procedentes de Austria y de de polen resumidas por Firbas. Estos dos limites areas adyacentes. fueron aprovechados para delinear varias provincias La dismmucion de la temperatura media anual climatico-vegetacionales. Un estudio adicional de durante el Pleistoceno tardio, inferida de la de- los medanos arenosos del final de la glaciacion dio presion de la linea de las nieves en las montanas, como resultado la presentaci6n de un mapa de la alcanzo de 5° a 8° C., bajo la suposicion general de presion atmosferica media estival de Europa. un gradiente vertical de temperatura en la atmos- Biidel localize la linea de las heladas pleistocenicas fera de 0.5° a 0.7° C. por 100 m. Los cakulos a base de las relaciones supuestas con la linea de basados en la presencia de rasgos de heladas y nieve de 1,000 m., que es identificable en las de plantas fosiles en las tierra bajas, sin embargo, montafias de Europa central. Su representacion sugieren una disminucion de la temperatura de
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10° a 12° C. La discrepancia puede deberse a nevados, los veranos frescos y otras condiciones diferentes relaciones de tardanza que existian en favorables a la glaciaci6n, suponiendo que el clima las montanas y en las tierras bajas, de acuerdo con pleistocenico fue marcado sobre todo por una Mortensen. Con el objecto de evaluar esta hip6tesis, frecuencia diferente de la actual de ciertos movi- se hace una resena de los estudios de las relaciones mientos de masas de aire y de los cursos de las modernas de tardanza en las regiones articas, tormentas relacionadas. Las causas posibles de los subaxticas y subtropicales. cambios de circulacion son aun altamente especula- Mapas de presi6n atmosferica media y de tivas, aunque recientemente se ha enfocado mucho regimenes de circulaci6n atmosf6rica generalmente interes en las variaciones de radiaci6n solar, como se basan en la extrapolacion a partir de los regimenes las exhibidas por las manchas solares. modernos que estan relacionados con los inviernos
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CONTENTS
Introduction 938 Inverted lapse rates 968 Geologic relations .... 938 Effects of precipitation 969 The periglacial region . 938 Mediterranean region 970 Introduction .... 938 General atmospheric circulation 972 Periglacial processes . 939 Change in solar radiation as a cause for climatic Frost soils 940 change 974 Wind action .... 946 Summary and conclusions 976 Biogeography 948 References cited 000 Distribution of Pleistocene periglacial features, and reconstruction of Pleistocene climatic Figure zones 949 1. Climatic regions of the world according to their Reconstructions by Poser 949 physiographic effects 939 Climatic zones 949 2. Climatic provinces of central and western Summer temperature 951 Europe during the Wiirm glacial phase, Winter temperature 952 according to Poser 950 Atmospheric pressure 953 3. Depth of summer thaw in central Europe Wind and pressure systems 953 during the Wurm 951 Reconstructions by Biidel 954 4. Winter frost cracking of soil in Europe during Snow line and tree line 954 the Wiirm 952 Climatic zones 957 5. Summer atmospheric pressure and wind direc- Sequence of climatic change 959 tions in Europe during the Wurm . . . 953 Introduction 959 6. Summer atmospheric pressure and winds during Penck classification 959 the Late-glacial phase of the Wiirm . . . 954 Soergel classification 959 7. Vegetational zones of Europe during the Wiirm, Climatic curve of Gross 961 according to Biidel 956 General 961 8. Latitudinal displacement of terrestrial climatic Loess stratigraphy 961 zones during the Wiirm 958 Loesses and terraces of the Alpine foreland . 962 9. Climatic curves of the late Pleistocene .... 960 Conclusions 965 Pleistocene climatology 966 Table General 966 1. Forms of solifluction 944 Pleistocene temperature depression 966 2. Correlation of soils 963 Snow-line depression 966 3. Temperature and snow line in the Alps .... 967
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severe climatic conditions marked by intensive INTRODUCTION frost action and wind action occasioned by the During the cold phases of the Pleistocene, presence of the Scandinavian ice sheet to the glaciers covered much of northern Europe and north and the Alpine ice cap to the south. the Alps. The climate of the intervening area, Various manifestations of a periglacial climate generally called the periglacial region, was had long been known, and the history of this colder than the present, but the details of the concept has been reviewed by Zeuner (1945, climatic relations and the sequence of climatic Ch. 1), Troll (1947), and Smith (1949). changes depend on careful analysis of the Long ago Fisher (1866) attributed the un- geologic and biogeographic evidence and on consolidated debris known as "head" or "warp" the application of climatologic theory. The in parts of England to Pleistocene frost action, geologic features useful in such studies include and Blanckenhorn (1896) did the same for the principally frost-disturbed soils and wind- "pseudo-moraines" in Germany. After Lozin- formed deposits. Biogeographic relations in- ski's summary appeared, interest in periglacial volve pollen analyses and the interpretation of morphology increased, and observations were mammalian and molluscan faunas. On the basis recorded by many writers from localities all of these studies, maps have been drawn to show over those portions of Europe not covered by the major vegetational and climatic regions of the last ice sheet. Soergel (1919) and others Europe. The maps of climatic zones in turn showed the relation of the loess of central lead to maps of Pleistocene atmospheric- Europe to Pleistocene frost features and climate. pressure systems. Finally, the paleoclimatic During the same period, modern frost features maps and curves provide the basis of discussion were being studied and classified, impelled of the possible causes of Pleistocene climatic particularly by the work of Bertil Hogbom change. (1914). All these studies led to the preparation It is the purpose of the present paper to of a book by Kessler (1925) on the Pleistocene examine critically the several steps in this climate and its geologic effects in unglaciated pyramid of compilation. Consideration is con- regions. fined almost entirely to the last major cold Some of the enthusiasm for the periglacial phase of the Pleistocene (Wiirm, Weichsel), was opposed by the influential German geo- because so much more information is available morphologist W. Penck (1924) and others, who concerning this epoch than for the rest of the ascribed block fields and such "periglacial Pleistocene. Eastern Europe is not included in features" to modern processes. Soergel (1936) much of the discussion. continued to emphasize the effects of climate The writer is obliged to the following persons on surface features through his work on fossil for arranging excursions to important Pleisto- ice wedges, and since 1940 a new generation of cene localities in Europe so that he might have workers have refined the concepts of frost soils at least some familiarity with the field relations: and periglacial climate. Troll (1944) prepared a Karl Adam, Alfred Diicker, Julius Fink, Hans comprehensive analysis of the relationship of Graul, Karl Gripp, H. Jackli, Sheldon Judson, modern frost features to climate in different Rene Tavernier, Fritz Weidenbach, and Lud- parts of the world. He followed it with papers wig Weinberger. He is also grateful to Ward devoted more specifically to the geomorphic Barrett, P. E. Cloud, C. S. Denny, S. S. processes active in the periglacial environment Goldich, John T. Hack, J. Hoover Mackin, (Troll, 1947; 1948) and prepared a map of the and Paul Woldstedt for critically reading all Old World showing the distribution of modern or parts of drafts of this paper, and to H. T. frost soils with respect to altitude. Biidel Mantis for discussion of some of the climato- (1951; 1953) and Poser (1948; 1950) prepared logical problems. maps of Pleistocene vegetational and climatic zones of Europe, and Frenzel and Troll (1952) GEOLOGIC RELATIONS extended the surveys to Asia. This more recent work of the German ge- The Periglacial Region ographers has been supplemented by observa- Introduction. The term "periglacial" was tions from France (Tricart, 1956a; 1956b), and introduced by the Polish geographer W. von from Poland, where the tradition of von Lozin- Lozinski (1912, p. 1039) to emphasize the con- ski is revived by the work of Dylik (1956), Jahn cept that central Europe during the cold (1956), and others with the publication of a phases of the Pleistocene was subjected to very journal devoted exclusively to the periglacial.
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Investigations in Germany continue at a high environments, which are not necessarily any- rate (e.g., Klimaszewski, 1959). In the United where near a glacier. Actually, Lozinski (1912) States, studies have not progressed far beyond had introduced this term to substitute for the the stage of infancy assigned them in 1949 by word subglacial, which Andersson (1906) had H. T. U. Smith, who prepared an excellent used for ancient solifluction deposits in the summary of the status of both the European Falkland Islands and which today is used in a and American work up to that time. The small completely different sense. It has generally been volume of observations on periglacial features assumed that the severe frost action and other may not be a result solely of American disin- indicators of cold climate in central Europe in terest in this field, for it is probable that the the Pleistocene were a direct effect of the periglacial zone in this country was much smaller and less pronounced than it was in Europe. At any rate, the possibilities for a broad synthesis in this country such as has been prepared for Europe are greatly restricted by the paucity of data on Pleistocene climate and vegetation. The review by Smith (1949) appeared when the summary studies in Europe were just being started. So many amplifications, refinements, and paleoclimatic maps have been presented in the last decade that a more detailed discus- sion of this later work is here presented. Periglacial processes. Troll (1948, p. 13 if.) identifies the following processes as being domi- nant in the periglacial environment: (1) Congelifraction—breakage of rocks by frost action (2) Congeliturbation (cryoturbation)—dis- SOLIFLUIDAL OR SOLIGELID turbance of the ground by frost action, Figure 1. Climatic regions of the world ac- as in the formation of stone nets cording to their physiographic effects, ex- (3) Solifluction—flow of saturated soil on pressed diagrammatically (Troll, 1948, Fig. slopes in cold regions 1) (4) River-gravel deposition—caused by ex- cessive supply of rubble by congelifrac- tion and solifluction proximity of the expanded north European and (5) Wind deflation (gelideflation) and depo- Alpine ice sheets, and that the environment is sition therefore "periglacial." Biidel, however, be- (6) Cryoplanation—final smoothing of the lieves that the basic climatic change of the terrain as an end product of the activity Pleistocene involved the general shifting of of all these processes. climatic zones toward the equator, that the Troll (1947; 1948) proposes that the peri- frost soils and related cold-climate features glacial (subnival, solifluidal, soligelid) cycle of were a direct manifestation of this shift re- denudation should take its place along with the gardless of glaciation, and that the presence of fluvial, marine, eolian, glacial, and karst cycles the ice sheets (themselves of course also a as a major geomorphic phenomenon, for it now product of climatic change) only locally affects a large part of the continental surface served to intensify the cold climate. and during the Pleistocene it affected an addi- Evidence for Biidel's point is difficult to tional area to such an extent that fluvial adduce, although it is not hard to show that processes have subsequently produced very the retreating ice sheet had only a limited effect little modification. A triangular diagram (Fig. on the climate of the surrounding region. 1) designed by Troll emphasizes the relations Biidel, for example, emphasizes that the frost among humid, arid, and "nival" climatic features in question are largely confined to the regions. area beyond the deposits of the last glaciation Biidel (1953, p. 251) has questioned the in northern Europe, and that these deposits propriety of the term periglacial for such themselves do not in general bear such features.
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During the approximate 10,000 years of ice under the snow; cf. subglacial, under the ice). retreat from the outermost Weichsel moraine Perhaps straightforward terms like tundra zone (Brandenburg) in northern Germany to the or frost-soil zone or permafrost zone would be Central Swedish moraine, the climate was more satisfactory, depending upon the nature sufficiently mild that no extensive frost features of the criteria by which the zone is identified. were formed on the deglaciated terrain (Johns- In the present review these designations will be son, 1958, p. 338). Although some frost action used where appropriate, but the generalized may have been important during the early term periglacial will be retained for the Pleisto- stages of retreat, the terrain was probably cene relations to describe in a broad sense the stabilized with vegetation shortly after the ice middle-latitude zone of that time characterized started its retreat across the Baltic Sea from the by extensive frost action, wind action, and/or Pomeranian moraine; organic sediments began tundra vegetation, with the understanding that to accumulate in lakes, and the pollen content the climatic characteristics of this zone might shows that forest began to invade the tundra not necessarily arise from the presence of the as early as ca. 13,000 years ago (Boiling zone). ice itself. Modern polar, subpolar, and alpine Even though two later phases of forest recession zones may have comparable environments. are recorded in pollen diagrams (Older and Frost soils. Whenever frost action occurs in Younger Tundra zones) there is little evidence the soil, some disturbance may be expected. In for frost-soil formation in southern Sweden, temperate climates such disturbances are not which should technically have a periglacial sufficiently frequent to permit a recognizable climate because the ice sheet was so close. The pattern to develop, or the change is masked by climatic amelioration for the late-glacial was other processes that occur during the balance thus rapid. It may have been even more rapid of the year. In the high mountains of the than the changes recorded either by moraines tropics, however, where there is nightly frost or by pollen sequences, because of retardation but no annual temperature cycle, repeated factors, i.e. the time required to melt the great needle-ice formation can produce miniature mass of ice stored from the earlier climatic stone nets, stone stripes, etc., a few inches broad regime, and the time required for invasion and (Troll, 1944, Ch. 5, 7). Permafrost is clearly not succession of plants on bare terrain. a requisite for the miniature forms, and in fact On the other hand, involutions (Poser, 1948, is not a requisite for such moderate-sized stone p. 56) and ice-wedge casts (Johnsson, 1958) rings as are found on favorable sites in certain have been found under drift of the Weichsel subpolar areas (Hopkins et al., 1955, p. 138). ice advance, implying that the climatic change The very large boulder rings several meters that brought down the ice sheets had already across, however, are probably restricted to produced a belt of permafrost. Such buried permafrost areas, and the frost heaving of such permafrost may then have thawed under the large stones must be favored by the presence of insulating cover of glacier ice and may never a permafrost layer beneath that provides not have formed during or after ice retreat. Biidel, only a cold reservoir for freezing of the active therefore, believes that the Pleistocene "peri- layer upward from below (Taylor, 1956, Ph.D glacial" climatic zones of Europe were a direct thesis, Univ. Minn., p. 110) but also an im- effect of world-wide cooling and not primarily permeable barrier to downward infiltration of a secondary effect of glacier expansion. He thus thaw water. holds that the term periglacial, as applied to the Frost soils have been classified in numerous Pleistocene environment of central Europe, is ways, and there are as many terms as there are misleading. He now uses the term only with stones in a stone ring—and almost as many hy- quotation marks for this situation, and deplores potheses for the formation of the different its use for alpine or other environments clearly features. Recent summaries of all or some types far from the ice. of frost soil have been prepared by Troll (1944), Unfortunately, there seems to be no single Cailleux and Taylor (1954), and Washburn satisfactory substitute term. Troll (1947, p. (1956). For our purposes, in which we wish to 169) has discussed the terminology particularly identify areas of Pleistocene permafrost as well with respect to alpine environments and sug- as areas where frost but not permafrost was gests that the term solifluidal (or soli-gelid) be present, it is useful to emphasize certain types. applied to those areas characterized by frost- One group of frost features with surface ex- soil formation and solifluction. He also uses pression is termed patterned ground (Wash- subnivalfar such a zone below the snow line (not burn, 1956). We may consider this in three
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categories: (1) ice-wedge polygons and related often distinguished. Black's (1954, p. 841) map fracture features, generally confined to perma- of modern permafrost shows zones of discon- frost; (2) stone nets, stone stripes, and related tinuous and sporadic permafrost 500-1000 miles forms in which there is internal sorting of broad south of continuous permafrost. The particles in the soil but for which permafrost is occurrence of permafrost depends on a variety not a necessary prerequisite; and (3) other pat- of geologic and hydrologic requirements. The terns that involve only vegetation or unsorted mean annual temperature required for perma- fine soils (peat rings, earth hummocks, vege- frost is variously estimated to be below — 5°C tation stripes); these are rarely preserved from (Mortensen, 1952, p. 153) or below 0°C (for the Pleistocene and will not be further con- sporadic permafrost, Black, 1954, p. 839); in sidered. addition, other climatic elements such as conti- Involutions, another type of frost feature, nentality also affect its distribution (Brown, are formed beneath the surface by freezing of 1960). the summer thaw layer of permafrost. They Numerous frost features are described or may have no surface expression but are easily mentioned below, with special comment con- preserved (although not so easily discovered). cerning the preservability of the features after This type is believed to require permafrost a climatic change, and thus their utility as generally for its formation and so is particu- indicators of past permafrost or of specific past larly useful in reconstruction of Pleistocene climatic characteristics. Many frost features climate. are not mentioned because the}' are not com- A third group of frost features may be as- mon or do not have a particular value in signed to the process of solifluction, which in- Pleistocene studies. volves downslope movements of water-satu- ICE-WEDGE POLYGONS: Ice-wedge polygons rated debris (Andersson, 1906). The water are found today in regions of continuous saturation in solifluction materials comes about permafrost in favorable terrain. They were first generally not only through infiltration of snow- made famous by the work of Leffingwell (1919, melt, as Andersson thought, but also through p. 179-243) on the coastal plain of northern ice segregation during the freezing cycle Alaska, and the mechanics of their formation (Taber, 1943, p. 1457). If a base of frozen have been more recently discussed by Black ground (permanent or long-seasonal) is present (1954, p. 846). it prevents infiltration of thaw water and pro- Although Taber (1943, p. 1524) and Diicker vides a gliding surface as well. Solifluction is (1951) have made a case for the origin of ice thus favored in regions of permafrost or long- wedges merely by ice segregation in fine- lasting seasonal frost, but may occur in fine- grained soils, the prevailing theory for their grained debris in front of springs or other formation (Leffingwell, 1919, p. 205; Black, localized sources of moisture. In consideration 1954, p. 844) calls upon ground contraction for its original definition by Andersson, it seems resulting from severe winter cold. In winter the best to confine the term to those situations in wedge-shaped contraction cracks become filled which frost has at least some part to play. with snow and hoar frost and in the summer by As far as climatic reconstructions are con- meltwater and slump material. If the ice wedge cerned, the location of the Pleistocene perma- does not melt out in summer it forms a plane of frost boundary is critical, because this line gives weakness for cracking the following year, and a clue to the mean annual temperature of the the feature may thus widen. The width of the region (Black, 1954, p. 843), although the clue wedge thus reflects the frequency of cracking, may not be very precise (Brown, 1960, p. 171 and depth reflects the severity of the winter ff.). Therefore, frost features that require cold. Secondary factors are the materials, permafrost for their formation should be moisture content, topography, etc. distinguished from those that do not. Thus Repeated contraction and expansion produces Troll (1947, p. 165) distinguishes three zones: deformation structures in the ice crystals within (1) polar zone of permafrost, (2) subpolar (and the wedge (Black, 1954, p. 845) and in the ma- alpine) zone of long-lasting seasonal frost, and terial of the adjoining walls. The walls may thus (3) tropical high-mountain zone of daily frost be pushed up into a pair of narrow ridges (also found on certain high-latitude oceanic bounding the wedge, and the resultant land areas like Iceland). forms in many regions consist of a polygonal As a matter of fact, the modern permafrost network of narrow troughs, locally marked by limit is so difficult to map that three zones are long ponds; in other areas larger lakes or bogs
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occupy the low areas in the centers of polygons Dakota have been ascribed to periglacial frost (Hopkins et al., 1955, p. 138). cracking (Horberg, 1951), but other explana- Ice-wedge casts of Pleistocene age have long tions have also been proposed (Nikiforoff, been identified in Europe as a record of Pleisto- 1952; Mollard, 1957). cene permafrost (Soergel, 1936). They are Even where properly identified as ice-wedge recognized in the field by the wedge form, the casts the structures may not record continuous sharp discordance of unsorted debris against the permafrost, for Hopkins et al. (1955, p. 138) wall, the vertical orientation of elongate stones, found modern ice-wedge polygons in Alaska in and the distortion of the layers adjacent to the regions of discontinuous permafrost. wedge. The filling generally consists of hetero- STONE RINGS AND RELATED FEATURES: StOHC geneous material that sloughs off the walls or is rings, nets, garlands, stripes, and other sorted washed in or blown in at the top when the ice frost features are perhaps the best-known melts. In many places the wedge filling and the manifestation of extensive frost action and are adjacent ground are truncated by a solifluction found widely distributed in suitable sites in deposit in which the postglacial soil is devel- polar, subpolar, and alpine regions. Miniature oped. Such overburden may obscure the forms may be found on tropical high mountains polygonal pattern in plan view, and wedges and in certain regions with oceanic climate are commonly observable only in cross-section (e.g., Iceland). Extinct forms have been re- exposures. Miniature ice-wedge fillings only a ported from a great many areas in Europe and few inches deep are found in deposits of loess elsewhere; they indicate the distribution of past reworked by solifluction. These do not require intensified frost action probably under condi- permafrost for their formation, for simple tions of sparse cover of vegetation. Although it single winter freezing might produce the is commonly stated that such features require requisite contraction. permafrost for their formation, Troll (1944, p. Certain other soil structures may be confused 566) points out that the miniature forms pro- with ice-wedge casts, and it is necessary to ob- duced by daily frost alternation do not involve serve all or most of the criteria listed above to permafrost, and other forms may result from a be certain of the genesis of the feature (Johns- condition of long-lasting seasonal frost rather son, 1959). For example, soil pendants have a than permafrost. Recent studies in Alaska by similar form in cross section but are conical and Hopkins et al. (1955, p. 138) show that stone are isolated in distribution rather than occur- polygons may be formed in areas without ring in a polygonal network (Yehle, 1954). Soil permafrost, although they are more common pendants are particularly well developed in where permafrost is present. calcareous gravels. They consist of the localized Stone rings and related features, where downward extension of the B horizon of the stabilized by vegetation cover and modified by soil and represent spots in which the descending chemical weathering, suggest past conditions of weathering solutions become concentrated. Un- colder climate and presumed absence of forest like wedge casts, these features do not show cover. Probably only the larger features require oriented stones or deformation of the walls. permafrost for their formation. When covered Polygonal wedge patterns can also be formed with heavy vegetation such features are rarely by alternate cracking and swelling of the soil of detected, and they are often so shallow that desert playas as a result of wetting and drying even in vertical cuts any patterns have been (Knechtel, 1952, p. 694-698). Other desert obscured by later soil formation. Occasionally, polygonal patterns reflect lateral expansion re- however, one can sight local concentrations of sulting from growth of salt crystals (Bobek, vertically oriented stones, an almost certain 1959, p. 22). The gilgai soils of Australia result clue to upward frost heaving of individual from swelling of clayey subsoils; furthermore, stones. Sometimes these may be related to weak they are not restricted to desert regions but are involution patterns or to ice-wedge structures, also found in temperate forest areas (Halls- but in other areas they may record the remnants worth, 1955). Some of the so-called mima of a stone ring or other sorted feature. mounds of western United States form a poly- INVOLUTIONS: Involutions are tightly folded gonal pattern that has been attributed, most laminae of silt and fine sand generally believed recently by Pewe (1948), to Pleistocene frost to be produced near the base of the "active action, but other authors defend origins unre- layer" of permafrost during fall freezing. The lated to frost processes. Polygonal patterns on recognition of these features in the surficial the floor of Glacial Lake Agassiz in North deposits of Europe as records of Pleistocene peri-
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glacial climate dates from the work of Steeger ence of permafrost may not be essential where in 1925 (Steeger, 1944). They are generally poor drainage, growth of vegetation tussocks, called in German Wiirgeboden (literally strangle and other conditions may be favorable for soil) or Taschenboden (pocket soil). They have differential freezing and ice segregation. In also been termed Brodelboden (bubble soil), but Massachusetts, where no permafrost is now- this implies acceptance of the hypothesis that present, modern soil deformation resulting from the features are formed by convection in super- frost action around tussocks has produced saturated soil during the freezing process, as structures suggestive of involutions (Sigafoos thought by Gripp (1927), who introduced the and Hopkins, 1951). The writer has recently term. The word "involution" was introduced studied exposures in a shallow depression near by Sharp (1942), at the suggestion of Kirk North Branch in eastern Minnesota in which an Bryan, and has been widely accepted as a involution layer is present that is typical of the descriptive term noncommittal as to the exact fcrm so widely described from the permafrost mechanism of formation. The French equiva- region of Europe (Fries, Wright, and Rubin, lent is plication. 1961). The organic material incorporated into Involutions consist of highly crumpled the involutions, however, is 2520 years old, laminae with balloonlike upfolds or downfolds according to a radiocarbon date, so the feature that in cases are pinched off at the neck to form is much too young to permit the assumption of isolated pockets. The vertical axes of the struc- permafrost. The mean annual temperature in tures indicate that the lateral pressure was uni- the region at present is about 6°C. form, and in this respect they may differ from SOLIFLUCTION FEATURES, ASYMMETRIC VAL- crumpled beds produced by subaqueous slump, LEYS: Troll (1947, p. 166) has suggested that dragging of icebergs, solifluction, or comparable the term solifluction be broadened to include processes that involve a shearing force. The internal movement of soil particles as in stone pressure is believed to be caused in the autumn rings (microsolifluction) as well as downslope during final inhomogeneous freezing of the movement (macrosolifluction). This internal lower part of the active layer of permafrost, movement, which may result in the formation i.e., the surficial layers of annual freeze and of patterned ground, is included in the terms thaw (Bahr, 1932, p. 29 if.). The active layer cryoturbation of Edelman et al. (1936) or con- freezes from the surface downward and to a geliturbation of Bryan (1946). Troll's concept lesser extent from the base up (Taylor, 1956, of solifluction is expressed in Table 1, which Ph.D. thesis, Univ. Minn., p. 110). Although shows different bases of classification. the permeable laminae may be dehydrated in For our purposes we shall not include in the the process of segregation of ice into freezing term solifluction any internal movements centers, the finer-grained laminae retain water (microsolifluction of Troll) unless some slope and become mobile under the freezing pres- movement is involved as well (as in stone sures from above, below, and within. The con- stripes). tained water cannot infiltrate downward be- Some solifluction deposits have distinctive cause of the impervious character of the under- form or surface expression as well as content, lying frozen ground. Expansion downward is and where preserved may be useful as paleo- inhibited by the resistance of the permafrost, climatic indicators. On slopes they may take and expansion upward by the thickening crust. the form of lobes or steps or stripes, in which The deformation is therefore confined to the case some sorting of particles may be involved. lower part of the active layer. More often they have no surface micro-expres- The requirements of permafrost below the sion but impart smooth contours to the terrain involution zone are inferred from the belief by inhibiting the development of stream gullies. that involutions do not form in cool-temperate Solifluction may supply streams with so much regions, which may have deep winter freezing debris that extensive gravel deposition results; but do not have permafrost. Soil distortions river terraces in the Pleistocene periglacial area similar to involutions have been studied in the of Europe may have this origin. process of formation in Alaska (Hopkins and Solifluction materials usually consist of locally Sigafoos, 1950, p. 98), where they are related to derived angular rock debris unsorted and un- differential freezing of shallow vegetation poly- stratified in a matrix of fine particles. As such gons that have developed in a permafrost they may be confused with local till or land- region from frost scars in a once-continuous slide debris, and inference as to the role of frost vegetation mat. Certainly, however, the pres- is often difficult when the solifluction debris
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may be derived from upslope till and may thus steeper slope is aided by long-lasting solifluction include erratic and nonangular rock fragments. on the gentle slope, as the ground slowly thaws Stone-orientation analysis may be used to re- during the summer; the accumulating solifluc- late solifluction deposits to the slopes on which tion debris at the base of the gentle slope helps they occur (Lundqvist, 1948), and stone- to push the stream to the opposite side. In many roundness analyses have been used for a similar cases the valleys are now dry; it is reasoned that purpose (Cailleux, 1951). the runoff now infiltrates, whereas during the The very upper ends of valleys in regions of cold period the permafrost prevented infiltra- present or past intensive frost action are tion. marked by smooth slopes and gentle trough- Other workers, notably Biidel (1953, p. 255), like cross sections as a result of movement of attribute valley asymmetry in this region to saturated surface soil. Exposures show thick preferred accumulation of loess and drifting accumulation of debris at the base of the side snow on the east-facing (lee) slopes by westerly
TABLE 1. FORMS OF SOLIFLUCTION (Troll, 1947, p. 168)
Basis of classification Types of solifluction
Form Differential solifluc- Amorphous solifluc- tion (with sorting) tion (no sorting) Terrain and movement Macrosolifluction Microsolifluction (downslope) (radial) Time interval Seasonal solifluction Daily or short-term Kind of ice Frozen-ground solifluction Needle-ice solifluc- (seasonal or perennially frozen tion (substratum ground beneath solifluction layer) not frozen)
slopes and along the valley floors. Such small winds. In the summer the mixture of loess and smooth valleys in central Germany are called snowmelt flows as a solifluction sheet down the Dellen (dells) or Muldentalchen. Their slopes slope over permafrost and crowds the river to are now stabilized, and their floors dry, for the the opposite (west-facing) slope, which gen- forces of solifluction which are believed to have erally has no mantle of solifluction debris. molded them are no longer active. Even at the The relative importance of insolation or wind time of formation there was probably little in the formation of periglacial asymmetric val- stream action, as is indicated by the nature of leys is difficult to evaluate. Troll (1948, p. 18) the deposits in the valley bottoms. suggests that a combination of the two factors Some solifluction valleys show a distinct may produce the observed relations. asymmetry in cross section. Asymmetric valleys Excellent examples of valley asymmetry can have been noted in many parts of the world and be found in the terrace landscape of the Alpine have been attributed to a variety of factors. foreland of southern Germany and adjacent Although some asymmetric valleys are certainly Austria, where the higher Pleistocene terraces controlled by rock structure, regional tilting, have all been eroded on the east sides of the or possibly the deflection of running water by valleys and preserved on the west sides, be- the Coriolis force, most types are attributed to cause the axial rivers migrated eastward during some kind of climatic control. Those found in successive cold phases of the Pleistocene (Biidel, central and western Europe are generally 1944, p. 494 ff). steeper on the south- or west-facing slopes. The dells at the very heads of the valleys, Poser (1948, p. 54; Poser and Muller, 1951) with their solifluction fill on rounded bottoms attributes the asymmetry to greater insolation and with no axial streams, are generally sym- on these slopes. Early summer melting of metrical, but Poser (1948, p. 54; Poser and permafrost on these exposures allows the stream Muller, 1951) describes some that have steep to erode them more rapidly and thereby steepen slopes facing east or north, in contrast to the them. The migration of the stream to the sense of asymmetry downstream. Poser reasons
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that with no axial stream the sunny slopes are slopes, at least until some equilibrium condition not subject to undercutting, and apparently is reached, and differential exposure to the sun that more rapid flattening is caused in the (or some other climatic control) may result in spring by solifluction on the sunny slope rather a different equilibrium angle on opposite than by summer solifluction on the shaded slopes. slope. Analogy is made with the asymmetric Regardless of the details of the causes for valleys described in the permafrost region of climatically controlled asymmetry in small val- Siberia (Schostakovitch, 1927, p. 420), which leys, it has been assumed that the features indi- have steep north-facing slopes. It should be cate the existence of permafrost at the time of noted, however, that the widely quoted Rus- formation (Poser, 1948, p. 54; Troll, 1948, p. sian author did little more than represent the 18). This assumption is based on the fact that valleys in a diagram and did not explain their extensive solifluction requires supersaturation genesis. of soil on gentle slopes, and such a condition is Most of the climatically controlled asym- favored by the presence of frozen ground that metrical valleys described in Europe appear to prevents infiltration of the thaw water from have differential accumulation of solifluction the active layer. Actually, however, this im- deposits on opposite slopes. A recent soils study permeable substratum may merely be the in the Chiltern Hills of southern England, how- lower part of the layer of seasonal frost, which ever, showed that asymmetry of dry valleys may not thaw until very late in the season after occurs in the bedrock itself, with the solifluc- several months of solifluction. Certainly soli- tion deposit being thin and irregular (Oilier fluction deposits can be found forming today and Thomassin, 1957). Here we have an in- in regions without permafrost. Poser states that stance of river migration during downcutting, such asymmetrical valleys must be criteria of rather than lateral crowding by differential ancient permafrost because similar valleys are deposition. The authors still reason, however, found forming today only in regions of modern that differential insolation must affect the slope permafrost (i.e., Siberia). In view of the fact creep and thus the rapidity of slope retreat. that only this one Siberian study of modern The gentle slope is interpreted as simply the asymmetrical valleys can be cited and in view slipoff slope left by the stream as it cut down- of the variable factors and hypotheses for their ward and laterally. formation, those asymmetric valleys involving Alexandre (1958, p. 264 ff.) found valleys in differential solifluction should not be empha- the Ardennes that had been modified in two sized as reliable criteria for past permafrost, but ways—a solifluction phase in the upstream seg- merely as criteria for a past cold climate. ment, in many cases with asymmetry, and a Soil creep, of course, is not confined to rainwash phase downstream, marked by gully- permafrost regions and in fact is not confined ing by the axial stream. The junction between to areas of frost action (solifluction). The the two zones shifted up- and down-stream burden of proof must rest with each investi- during the Pleistocene, as recorded by com- gator to demonstrate that frost action and par- bination cross sections—the upper part of the ticularly permafrost are involved in the soil section may show an asymmetrical solifluction creep. This may be easier to do if creep de- slope, and the lower part a gullied slope. posits in the valley bottoms are closely associ- Alexandre attributes the solifluction phase to ated with periglacial loess or other deposits the time of permafrost and the gullying phase that are themselves demonstrably Pleistocene. to the time of maximum snowfall and runoff Measurements of particle shape, sorting, and (time of maximum glaciation in northern other characteristics of these deposits have been Europe). made by Cailleux (1951) in efforts to identify Most authors feel that solifluction valleys diagnostic criteria. and in particular asymmetric valleys reflect a BLOCK FIELDS, TALUS, ROCK GLACIERS: Block periglacial climate, but Oilier and Thomassin fields and block streams may be produced by (1957) admit that there is no proof that some frost breakage of rock outcrops accompanied by of the valleys in question in western Europe gradual downslope migration of the fragments. may not reflect interglacial conditions rather Where later stabilized by vegetation, soil forma- than periglacial. This disarming statement tion, or bog development they may be pre- causes one to be skeptical of immediate ac- served with little modification. Because block ceptance of asymmetric valleys as indicators of fields readily form today in polar, subpolar, and permafrost. Soil creep apparently flattens alpine regions, the stabilized block fields of
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central Europe and similar temperate regions frost soils bears a close relation to the snow line are generally attributed to Pleistocene climatic but is many hundreds of feet lower. Thus it conditions, although the cessation of block-field rises from sea level in the polar regions toward development may locally result from factors the equator, reaching an elevation of about other than climatic change. Biidel (1944, p. 5000 feet in the mountains of central Germany, 487), who proved the antiquity of some block 6500 feet in the Alps, and 11,000 feet in the fields in the Erzgebirge by the identification of Atlas Mountains. The line reachest its highest the entire postglacial pollen sequence in an position in the dry mountains of the subtropics overlying bog, studied the distribution of (about 15,000 feet in Tibet and the Andes) and stabilized block fields in mountains of central is lower in the more humid tropics of East Europe in an early effort to determine the Africa (13,500 feet), although the observations altitudinal limits of Pleistocene frost climate. in low latitudes are scattered. The effect of pre- Rock accumulations on steep slopes below cipitation on the climatic structure-soil bound- bare cliffs form readily in regions with frost ary can be seen in local areas, e.g., a rise from weathering. In alpine regions well above the 6000 feet on the outer Alps to over 7000 feet in tree line the talus cones below cliffs give way to the drier central Alps, or a rise from 9000 feet rock glaciers, which extend along the valley in the Caucasus and West Pamir to 15,000 feet floor much like true glaciers. Measurements in the drier East Pamir. So also goes the present of rock glaciers in the Alaska Range imply snow line, at higher elevations (Klute, 1928). that the thickness of a rock glacier, and pre- Wind action. The importance of wind ac- sumably of the source talus, must generally be tion in the Pleistocene periglacial regions of at least 100 feet before movement can take Europe has been realized for many years and place (Wahrhaftig and Cox, 1959, p. 426). has recently been discussed by Smith (1949). Where stabilized and no longer apparently SAND DUNES : Morphology of sand dunes may forming, these features are attributed by many reveal the direction of dune-forming winds and to past climates more severe than the present. thereby some of the characteristics of Pleisto- Forested talus accumulations on slopes in the cene climate. An early comprehensive study on mountains of central Germany and France may this problem was presented by Ivar Hogbom fall in such a category, as do beds of angular (1923), who inferred westerly winds for north- rubble derived from roofs of limestone caves ern and central Europe on the basis of dune and rock glaciers. The paleoclimatic significance form and structure. Enquist (1932), however, of these features is difficult to evaluate, and inferred easterly winds from the same data, and each occurrence must be considered on its this conclusion was used in support of the fa- merits. Local conditions unrelated to climate mous Hobbs theory of the glacial anticyclonic may cause a given talus to become stabilized— wind circulation in the periglacial region. More perhaps a change in the extent of exposure of recently Poser (1950) prepared a map of late the cliff that serves as the source. As far as caves Pleistocene pressure patterns and wind systems are concerned, however, Movius (1960, p. 380, on the basis of dune form in northern and east- 384) points to impressive evidence in the caves ern Europe. This work is discussed in the sec- of southern France that the roof-rock debris tion concerned with Poser's reconstruction of was produced by frost action: the Upper Paleo- Pleistocene climatic zones. 14 lithic culture layers, dated by C in the range SAND GRAINS AND VENTIFACTS: CailleUX 30,000 to 13,000 years ago, are filled with rock (1942) considered that wind action was so prev- chips, whereas the post-Paleolithic contain few. alent in the Pleistocene periglacial regions that It can be safely inferred that the process re- individual sand grains carried by the wind ac- sponsible for loosening the roof rock is not so quired a rounded form and pitted (frosted) sur- active now as it was during the late Wiirm. face—features that were retained even though DISTRIBUTION OF MODERN FROST SOILS: the sand grains were incorporated in other de- Troll's (1944; 1948) studies of the distribution posits like till, glaciofluvial sediments, frost of modern structure soils (patterned ground) soils, etc. This assumption was based on his and solifluction deposits with relation to lati- study of about 3000 samples of Pleistocene de- tude over the various climatic zones of the posits over Europe, in which he determined the world have given a broad basis for his presenta- percentage of frosted grains in the coarse sand tion of a map of the Old World showing the fraction. A map (Cailleux, 1942, Fig. 16, or altitudinal limit of these features (Troll, 1947, Wright, 1946, Fig. 1) shows that the belt of Fig. 1). Troll's map shows that the limit of high percentage of frosted sand grains lies just
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outside the drift border for the last glacial Studies have shown consistent changes in the phase of the Pleistocene in northern Europe, direction away from such sources, such as de- and that the highest values are in the eastern crease in thickness and grain size (Schonhals, part of this belt in Poland (80-100 per cent 1953). To the extent that such sources are re- frosted grains), where the continental climate quired, the formation of loess is thereby re- apparently permitted more extensive wind ab- stricted and is a secondary result of glaciation. rasion. Cailleux's study has been discussed at Weidenbach (1952), in emphasizing this view, length by Wright (1946). The method has been points out that most of the loess deposits of recently applied in less elaborate fashion in the central Europe were blown from the glacial United States, but with no results concerning meltwater trains of the Rhine and Danube riv- Pleistocene climate (Schneider and Cailleux, ers and their major tributaries, and that the 1959). loess of Belgium and northern France had a The association of ventifacts with frost soils source in the coastal meltwater plains since sub- has been observed in many Pleistocene peri- merged by the rise in sea level. He further glacial regions (Smith, 1949), but the only re- stresses that each loess deposit in a stratigraphic gional study has been that of Cailleux (1942), succession may therefore be correlated with a who noted the more common occurrences of glacial terrace and thence with a moraine. The large multifaceted ventifacts in northern -nd problems of regional correlation and chronol- eastern Europe in the area of frost soils and ogy thus become involved. frosted sand grains. He reasoned that in these Others believe that much of the loess of peri- regions such stones (larger than 10 cm) could glacial Europe is derived instead from the bar- be overturned by frost action and thus cut by ren terrain with sparse vegetation cover away wind abrasion on more than one side. from major meltwater valleys, particularly LOESS: The eohan origin for the upland loess terrain that is subject to intensive frost action of central and western Europe is now almost which not only produces silt-sized particles by universally accepted. In many areas each layer frost breakage but keeps the soil stirred up of homogeneous loess is underlain by a layer of enough to reduce the plant cover and to inhibit solifluction loess that includes soil and other the formation of the kind of stone pavement materials transported downslope. Three cli- that prevents deflation (Diicker, 1937). matic phases are generally recognized for each A third view about loess genesis is expressed loess layer: (1) a cold and moist phase involving by Biidel (1953), who considers that loess for- some loess deposition but accompanied by mation requires not only a suitable source re- much soil erosion and solifluction, (2) a cold gion, whether meltwater plains or frost-stirred dry phase with dominant deposition of homo- uplands, but also suitable conditions for ac- geneous loess, and (3) a warmer interstadial or cumulation. He points out that loess does not interglacial phase with formation of soil in situ occur in northern Europe immediately adjacent and without loess deposition. to the drift border of the last glaciation, but is Detailed stratigraphic studies, especially in found instead in regions somewhat farther from Austria and southern Germany, have shown the ice front. It might be suggested that lack of the relatively delicate regional climatic control loess close to the ice is rather a reflection of the on loess deposition and modification (Fink, fact that glaciofluvial sources are sandy rather 1956; Brunnacker, 1956). In the drier eastern than silty in this province and that sand dunes portion of Austria and parts of adjacent Czecho- are more common than loess deposits, at least slovakia the loess is little disturbed by solifluc- in northern Europe. Biidel, on the other hand, tion or frost cracking, except in favorable lo- points out that loess is absent also at higher ele- calities. In the more humid regions of western vations in the German Mittelgebirge. He be- Austria and southern Germany, however, the lieves that frost action close to the ice sheet and basal solifluction loess is more conspicuous, and at high elevations was too intensive to permit in many areas most of the soil formed during the accumulation of loess—this is his frost- the preceding climatic phase has been eroded rubble tundra zone where the vegetation cover and transported downslope by solifluction to be was sparse and where the loess falling on the redeposited along with fresh loess or other ma- surface was churned into the soil by frost proc- terials. esses. In regions where frost action was less pro- The source of the loess is predominantly nounced, the loess would be entrapped by the glacial meltwater plains or valley trains, which heavier cover of grass or herbs and would not dry periodically to permit deflation of the silt. be so readilv stirred into the soil beneath.
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Hack (1953, p. 184) has taken issue with Pollen occurs in sufficient quantities that it Biidel's requirement of tundra or grassland for may be treated statistically—a modern detailed loess deposition, pointing out that Pewe (1951) pollen analysis of a single sample involves the noted modern loess deposits in Alaskan forests enumeration of 300-1000 grains, and thus the near important glaciofluvial plains. If an ade- preparation of a pollen diagram through a pol- quate source is present there seems to be no len-bearing Pleistocene sediment may involve reason why loess could not accumulate under the counting of 20,000-100,000 grains. Numer- forest as easily as under herbaceous vegetation. ous potential sources of error are involved in It must be admitted that the gross distribution the method, such as differential pollen produc- of Pleistocene loess in Europe suggests that the tion and dissemination for different plant types, continental interior (Lower Austria to Ukraine) differential sedimentation and preservation of provided the principal depositional sites, and pollen grains, redeposition of older pollen here the forest cover was restricted, according grains, and other factors reviewed by Faegri to pollen analysis. The relations among vegeta- and Iversen (1950, p. 90 ff.). Despite these tion, loess deposition, and source area might difficulties the method has been remarkably better be tested in the United States, where successful in elucidating particularly the Late- there are thick loess deposits in the Middle glacial and Post-glacial vegetational succession West but less evidence for the type of broad of Europe, as seen from the almost identical dia- tundra zone that characterized most of Europe grams from hundreds of localities (Firbas, 1949). during the Wiirm. As a quantitative or detailed representation of vegetation or ecological relations surrounding BIOGEOGRAPHY the site of deposition, the standard pollen dia- The shifting distributions of plants and ani- gram is somewhat less successful, for there is mals during the Pleistocene provide a record of only a gross similarity between the modern climatic change, and Deevey (1949) has dis- vegetation and the modern pollen precipita- cussed at length some of the ecological factors tion at those sites at which comparisons have as well as the sequence as it was understood at been made (Faegri and Iversen, 1950, p. 82 ff.; the time. Davis and Goodlett, 1960). Nonetheless, pollen Plant macrofossils are not commonly pre- analyses furnish by far the most abundant data served in Pleistocene sediments, and where pre- for paleoclimatic reconstruction for continental served they often record microclimatic environ- regions. ments like swamps that might not be particu- The Late-glacial and especially the Post- larly diagnostic of the regional vegetation. It glacial pollen sequence has been worked out may be mentioned, however, that the impor- with great precision for most of central and tant Allerod oscillation of the late glacial of northern Europe (e.g., Firbas, 1949, p. 294- Denmark was established on the basis of plant 346; Godwin, 1956, Ch. 3; Iversen, 1954). Cor- macrofossils (Hartz and Milthers, 1901), and relation of the Late-glacial pollen zones with that Nathorst (1914, p. 290 ff.) long ago em- moraines is possible by the varve and radio- phasized the frigid character of the Pleistocene carbon chronologies. The Late-glacial diagrams climate of central Europe on the basis of macro- record a tundra or at most a park-tundra land- fossils of Dryas and other arctic plants in south- scape that reflects much lower temperatures eastern Europe. Iversen (1954) has discussed than today. The absence of pollen in underlying the value of macrofossils, especially of aquatic sediments is generally interpreted as a result of plants, in climatic reconstructions. Macrofossils climatic conditions so severe that the vegeta- have certain advantages over pollen grains in tion was indeed sparse, or that organic sedi- that specific rather than only generic determi- ments were thoroughly mixed and diluted with nations can often be made, and further that the mineral sediment contributed by solifluction fragments are too large to have been brought to (Gross, 1954, p. 198). Consequently there are the site of deposition from another environment few pollen diagrams recording the main part far away (Godwin, 1956, p. 6-10). They rarely (Full-glacial, Pleni-glacial) of the last (Wiirm, can be analyzed statistically like pollen grains, Weichsel) cold phase. In Holland the first part however. of the Pleni-glacial is recorded by tundra pollen Plant microfossils, especially pollen, have been spectra, but the latter part is barren (Van der much more useful in delineating regional vege- Hammen, 1952). For Denmark the mean July tation because, although the pollen is commonly temperature for the Pleniglacial is estimated as preserved only in lakes or bogs, it is blown to 10°C below the present on the basis of the these sites from the surrounding upland terrain. modern distribution of the plants represented
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by pollen in the sediments (Andersen, De Vries, logic data for reconstruction of European peri- and Zagwijn, 1960). This phase was preceded glacial climate, the first detailed map and dis- in both Holland and Denmark by an Early- cussion of the distribution of Pleistocene frost glacial phase with at least two interstadial in- soils and the reconstruction of climatic zones tervals (Amersfoort and Br0rup) during which did not come until Poser (1948). Using what he subarctic and even north-temperate forests considered to be three reliable indicators of temporarily invaded (Andersen et al., 1960); permafrost, namely ice-wedge fillings, involu- the pollen studies here are supplemented by tions, and asymmetric valleys, he mapped the radiocarbon datings that control the correla- distribution of Pleistocene permafrost and de- tions. lineated several subzones within the entire peri- For central and southern Europe the pollen glacial area of central and western Europe. investigations of the Pleni-glacial are less ad- The northern limit of Pleistocene permafrost vanced, partly because of the lack of good pol- was clearly the ice sheet. It has been mentioned leniferous sediments dated by radiocarbon anal- that the ice apparently overrode already exist- ysis. The Alpine glaciated region has yielded ing permafrost occasioned by the cooling cli- no good organic deposits for this phase. The mate, as indicated by the occurrence of involu- best stratigraphic records of the last cold phase tions in sediments buried by young drift. But are found in the loess region of Austria and once this ground was insulated from the cold adjacent areas, but unfortunately neither the air by a thick ice cover, it is likely that it loess itself nor the intercalated soils contain pol- thawed by heat flow from the earth's interior, len in sufficient quantities for analysis. and when the ice withdrew from its maximum Paleozoological studies yield climatic in- moraine the climate was not sufficiently severe formation primarily through their implications to freeze the ground again, or in fact to cause about vegetation—forest forms vs. steppe enough solifluction to modify the deglaciated forms vs. tundra forms. Animals are therefore terrain very much. Consequently, the drift one step further removed from a direct record limit of the last glaciation in northern Germany of climate. Many of the important fossil mam- and Denmark, as also in the Alpine foreland, is mal occurrences are at archeological sites (e.g., a striking geomorphic boundary between a caves), and there is the danger that some of the relatively fresh glacial topography and a rolling animals may have been imported by man from topography on older drift molded by periglacial different vegetational zones and thus record solifluction during the last glaciation (Gripp, hunting or dietary habits rather than the local 1932). climatic environment. Small mammals like The southern boundary of permafrost during rodents, which are ordinarily not dietary items, the last glaciation was likewise inferred by may be more significant in this respect. Other Poser (1948) from the limit of occurrence of occurrences involve so few specimens that no the indicators used (Fig. 2). It extended from reliable percentage analysis can be made. Never- the Atlantic Ocean in Brittany eastward to the theless, mammal studies have their place in Seine Valley, thence south down the Rhone paleoclimatic analyses because, unlike pollen, Valley and around the south base of the Alps. the fossils may be found in loess, river terraces, Throughout this course it followed just north and other sediments of glacial correlation. of the northern forest limit that was inferred by Similarly the mollusk faunas yield informa- Firbas (1939, Fig. 6) from the basal zone of tion about vegetation and thus climate, and are Late-glacial pollen diagrams. The northerly po- particularly useful in the loess region where pol- sition of both this forest line and the permafrost len analysis fails. line in France is attributed to the ameliorating Some important mammal and molluscan effect of the oceanic climate, even as their faunas are enumerated in the section describing modern counterparts are in Scandinavia and the climatic sequence for the late Pleistocene. Alaska. The strong southward bend of the lines DISTRIBUTION OF PLEISTOCENE around the Alps reflects the combined cooling PERIGLACIAL FEATURES, AND influences of the northern and Alpine ice RECONSTRUCTION OF PLEISTOCENE masses. East of the Alps the permafrost line re- mained south of the Hungarian lowland be- CLIMATIC ZONES cause of the effect of the colder continental Reconstructions by Poser winters, but the forest line went north around Climatic zones. Although Soergel (1919) the Carpathian Mountains because of the warm was probably the first to exploit in toto the summers characteristic of the continental possibilities of utilizing geomorphic and bio- climate.
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Three major climatic-vegetational zones are places the permafrost boundary actually in the thus delineated by the relations of the southern south of France rather than in the north, so limit of permafrost to the northern limit of the the concept of a broad zone of maritime forest forest, as shown on Poser's map: (I) permafrost- climate without permafrost over all but south- tundra climate north of both lines, (II) con- ernmost France is not justified if these indicat- tinental permafrost-forest climate in the Hun- ors are accepted. The use of the basal zones of pollen diagrams to characterize the vegetation at the time of the maximum of the last glacial period requires many assumptions and involves a consideration of the time factor, which seems to have been neglected in most of the climatic reconstruc- tions based on frost features. The Allerod zone is the oldest zone that can be identified in Late- glacial pollen diagrams in most of the parts of Europe that were not covered by ice during the last glaciation, and it is claimed to be identified over hundreds of localities (Gross, 1954, Fig. 1; Wright, 1957, p. 449 ff.). Although there is al- ways the problem of the proper correlation of the substratum beneath a pollen-bearing de- posit, especially in unglaciated regions, one is impressed by the similarities among the very large number of available detailed diagrams for localities distributed across several vegetational Figure 2. Climatic provinces of central and provinces from the young glaciated region in western Europe during the Wiirm glacial the north (where the substratum is known) to phase, according to Poser (1948, Fig. 6). The heavy solid line and its dotted continua- the Alps and southeastern Europe (Firbas, tion delimits the ice sheets, the dashed line 1949, p. 301). The fact remains, however, that shows the southern limit of permafrost, and the pollen zone below the Allerod usually rep- the line of circles indicates the northern resents an unknown time interval prior to forest border. The following provinces are about 12,000 years ago. It may not reflect the identified: vegetational conditions at the time of maxi- I Permafrost-tundra climate mum cold of the last glacial period, but rather la Glacial-maritime province some time later when the climate had already lb Province between the ice sheets improved considerably. The 16 localities Ic Glacial-continental province spotted on Poser's map in central and western II Continental permafrost-forest climate Europe presumably represent those in which III Maritime tundra climate without perm- the basal pollen zones show treeless vegetation. afrost If the region was treeless at this time during the IV Maritime forest climate without warming stages near the end of the last cold permafrost phase, it must have been treeless during the maximum cold, so Poser is on safe ground as far as the mapped treeless area is concerned. But garian lowland, and (IV) maritime forest cli- for the areas mapped as wooded, synchroneity mate without permafrost for the southern half is not assured unless independent dating is pos- of France. A fourth zone (III), a narrow strip sible. across France, is problematical. Poser's data Furthermore, it is not known when the frost map (1948, Fig. 1) shows no basis for the de- features were formed within the last cold period, marcation of this zone. The pollen analyses used which started perhaps 70,000 years ago and was show treeless vegetation in Germany and Po- marked by climatic oscillations of varying mag- land and birch-pine in Hungary; there is noth- nitudes (see p. 959). In the loess region of ing indicated for France, so it is not clear why Austria and adjacent areas a phase of solifluc- the forest limit was extended across northern tion preceded each phase of loess deposition, France. More recent identification of the same and small ice-wedge fillings and involutions permafrost indicators in France (Tricart, 1956a) have been found in the solifluction loess of the
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more humid parts of southern Czechoslovakia, mean — 2°C, the January mean must have but not the drier (Brandtner, 1956, p. 132). been — 13°C. For northern France, where the Adequate moisture as well as cold was appar- two Wiirm lines are supposedly close together, ently necessary for the development of frost Poser made similar calculations to show that features. The paleontological studies and the the seasonal contrast was not so great (just as correlation of loesses with glacial deposits indi- today) because of the maritime influence. He cate that the coldest time came with the maxi- also attempted to show that in France the sum- mum of glaciation near the end of the last cold mer temperatures were lowered in the Pleisto-
10-
Figure 3. Depth of summer thaw in central Europe during the Wiirm glacial phase as based on the depth of involutions (Poser, 1948, Fig. 2). Figures in decimeters. Ruled area shows extent of ice sheets. Dashed line indicates southern limit of permafrost, and line of circles shows northern limit of forest.
phase, whereas the solifluction loess was formed cene (with respect to today) more than the during the early part. These relations imply winter temperatures, whereas in Hungary, that frost features may have formed through- which has a continental climate, the winter de- out the cold period, but that during the time pression was much greater than the summer of maximum cold they were restricted to depression. Also, the Pleistocene depression of relatively moist sites. the annual mean temperature was much greater In order to characterize further the climate in Hungary (by 10°C) than in France (by of Europe during the last glaciation, Poser as- 4°C) because of this same effect. Although the sumed that the southern limit of permafrost at new data on the position of the Wiirm perma- that time corresponded to the — 2°C annual frost line in France (Tricart, 1956a) will re- isotherm, and that the northern forest border quire recalculation of the temperatures, the fell along the 10° July isotherm. Where the contrasts between the maritime and continen- two lines cross at the east end of the Alps, the tal climates will probably still be noticeable. entire annual temperature cycle can be cal- Summer temperature. Poser (1948) also plot- culated from these two control temperatures. ted from the literature the depths at which in- Thus if the July mean was 10°C and the annual volutions have been found at 71 localities
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throughout Europe (Fig. 3). The map is in- thaw depths were also found in the Hungarian tended to show the range of thaw depth and lowland. Here the location deep within the con- thus the summer temperature, on the theory tinent brought strongly contrasting seasons. that involutions are formed near the base of Winters were cold enough for permafrost even the active layer of permafrost. Shallow thaw at this low latitude, but the correspondingly depths, less than 15 decimeters, occur in south- warmer summers resulted in relatively deep ern England and in Denmark and northwest summer thaw as well as the development of
Figure 4. Winter frost cracking of soil in Europe during the Wiirm (Poser, 1948, Fig. 4). Ruled area represents ice sheets. Dashed line shows southern limit of permafrost, and line of circles shows northern limit of forest. Small triangles show occurrences of loam wedges, and small black rectangles indicate frost cracks. Pairs of numbers show the breadth (above) and depth (below) of wedges and cracks, in decimeters.
Germany because of the cooling effect of the forest—the one area in Europe characterized ice sheet in summer. Shallow thaw depths are by both permafrost and forest, according to also found through central Germany to the Poser. Alps, presumably because of the combined cool- Winter temperature. Another map by Poser ing effects of the northern and Alpine ice sheets, shows the distribution of about 25 localities of which actually were no more than 300 miles ice-wedge casts that have been reported in the apart at their closest points. Cold summers in literature for Europe (Fig. 4). The depth and this region are further implied by the absence the width (at the top) are noted for each lo- of a forest cover, as indicated by pollen cality and are believed to be directly propor- analyses. tional to the severity of the winter cold. Al- Thaw depths greater than 20 dcm are in- though the observations are not sufficiently ferred farther west in northern France. Here numerous to justify lines of equal wedge depth the maritime influence brought summers that and width, Poser infers several regions of dif- were longer (although perhaps cooler) than the fering winter climate. more continental regions to the east. Greater Close to the North Sea the wedges are less
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than 30 dcm deep and only a few decimeters forest zone without permafrost in France- wide at the top. Here the maritime influence Poser (1948) mapped the summer and winter kept the winters relatively mild and short, de- atmospheric pressure patterns and wind direc- spite the fact that the ice sheet was not far to tions for the Pleistocene (Fig. 5). A high-pres- the north. Thus the penetration of winter cold sure area is located over the Scandinavian ice and the consequent ground contraction was not sheet, with a ridge connection across Germany great. The total depth of permafrost itself is to another high over the Alps, especially in unknown; it is determined by summer as well winter. A low-pressure cell is postulated for the as winter temperature. warm Hungarian lowland in the summer, but In central Europe the depths of the wedges displaced eastward to the Black Sea region in winter by the expansion of the Asiatic con- tinental high. This low would have pulled dry air from the northeast and east off the con- tinent—incompatible with the presence of forest in the Hungarian lowland. Perhaps, how- ever, the same low-pressure cell attracted cy- clonic storms which must have entered the Mediterranean from the west much more fre- quently in the Pleistocene than today, and moisture could have been brought to the Hun- garian lowland by this route. On the other hand, vegetational reconstructions by Biidel (1951) and Frenzel and Troll (1952) show the Hungarian lowland as nonforested, so perhaps there is no problem to explain here. Other low-pressure cells are postulated over the western Mediterranean and over France. The former would have attracted Atlantic Figure 5. Summer atmospheric pressure and storms along the Mediterranean, and the wind directions in Europe during the Wiirm French low would have allowed moist air and glacial phase, as determined from the clima- storminess to invade northwestern Europe for tic and vegetational provinces (Poser, 1948, Fig. 3). H = high-pressure area, T = low- ice-sheet nourishment in both winter and pressure area summer. The moisture must not have pene- trated far eastward into central Europe, how- ever, for the climate was distinctly dry in lower are generally 50-80 dcm, and the widths are up Austria and adjacent areas during the principal to 30 dcm. Such large features are believed to times of loess deposition, although important reflect severe winter cold with deep and fre- regional fluctuations of precipitation are sug- quent cracking of the ground. The lower winter gested by the humid-type solifluction phase at temperatures were caused by the junction of the base of each loess bed. cold air masses over the north European and Wind and pressure systems, A further analy- Alpine ice sheets, intensifying the increasing sis of Pleistocene atmospheric pressure patterns continentality of the interior region. and wind directions was made by Poser (1950) In the Hungarian lowland a third climatic on the basis of the study of Late-glacial sand region is identified, although on the basis of dunes in northern and eastern Europe. Al- only three observations. Here the wedges are though this study does not give a picture of the deep (22, 35, 50 dcm) but relatively narrow (4, wind systems at the time of maximum frost 8, 10 dcm respectively). Severe winter cold is activity, maximum loess deposition, or even consistent in a continental climate, but in- maximum glaciation, it reveals the situation frequent cracking may reflect the insulation when the Scandinavian ice sheet was still in provided by a forest cover. northern Germany or in southern Scandinavia, Atmospheric pressure. On the basis of the lo- prior to reforestation. cation of the three major Pleistocene climatic Wind directions at the time of dune forma- zones thus delineated in Europe—permafrost- tion are inferred by Poser from dune form and tundra zone across the north, permafrost- internal structure. It is important whether the forest zone around the Hungarian lowland, and dune-forming winds represent prevailing sum-
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mer winds or merely storm winds caused by day, the high over Europe is considered to be passing cyclones. In the former case the pattern more stable because of local cooling influence of dunes and thus prevailing winds might re- by the ice sheet and by cool glacial meltwater veal the mean summer pressure pattern for in the sea off the coast of western Europe. That Late-glacial time. Poser determined that the the region was still cool cannot be doubted from sand of the mean size grade concerned could the occurrence of tundra plants in the Late- all be moved with a wind strength of 8-12 glacial pollen zones. The extent to which de- miles per hour for variable wind, which pre- pressions moved along the North Sea and vents packing of the surface layer of sand Baltic Sea at this time cannot be easily deter- grains, or 25-31 miles per hour in the case of mined, but the persistence of a European high constant winds and a packed surface. The presence of a few coarse laminae in the dunes indicates that strong winds did blow from time to time, and brought coarse sand from available outwash plains or other sources. The dominance of smaller grains and the uniformity of particle size, however, suggest that the dune-forming winds were relatively constant, and a threshold of 19-24 miles per hour is taken for the velocity required for transport. The dunes in northeastern Germany and Poland are primarily bow-shaped dunes; those of Hungary are linear dunes. The uniformity and simplicity of form and structure suggest to Poser that the winds were not the change- able winds associated with passing cyclonic depressions, but rather a wind system around an anticyclone. A further indication of this Figure 6. Summer atmospheric pressure and relation comes from the fact that modern dune- winds during the Late-glacial phase of the Wurm, based on morphology of ancient forming winds in northern and eastern Europe sand dunes (Poser, 1950, Fig. 1). Hachured have precisely the same mean directions as the lines show the positions of the north prevailing wind directions inferred for the European ice sheet at the times of the Late-glacial dunes, namely southwesterly winds beginning and the end of the Late-glacial. in Belgium, westerly in eastern Germany, northwesterly in Poland, and northerly in Hungary. Examination of modern weather into the Late-glacial implies that some storms maps showed that on most of the days during still took the Mediterranean route south of which the wind blew 19-24 miles per hour at this air mass. one or another of the regions under considera- tion the map showed a high-pressure area over Reconstructions by Biidel Europe, as an eastward extension from the Snow line and tree line. Climatic zones of Azores High. In fact the most favorable circum- Europe for the last cold period were recon- stances seemed to involve some storminess structed with a different basis and with more around the high, to produce moderately detail by Biidel (1951). He assumed that the variable winds more conducive to sand trans- polar forest border in the Pleistocene followed port. the 10.5°C July isotherm just as it does today The pressure map (Fig. 6) reconstructed by and that this isotherm in turn followed the Poser for the Late-glacial summer resembles 1000-m level of snow line in the Pleistocene the summer map he had drawn for the Wurm just as it does today in the polar regions. The maximum on the basis of the distribution of level of Pleistocene snow line was determined certain frost soils (Fig. 5), except that in the in the mountains of Europe by Brusch and Late-glacial the Scandinavian high-pressure Biidel on the basis of published field data area has retreated to the north along with the (Biidel, 1949, Fig. 3). The 1000-m Pleistocene ice sheet that produced it, and is no longer con- snow line thus reconstructed extends across the nected with the high over central Europe. Central Plateau of France and across the Compared to the summer pressure map of to- German Mittelgebirge. The position of the
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Wtirm 10.5°C July isotherm in the nonmoun- regions in which the forest gives way to tundra. tainous areas was then determined by applying Further, the forest limit is not a line but a zone. the topographic correction factor (lapse rate) Biidel offers a compromise about France by of 0.5°C/100 m of elevation. The corrected line inserting a transitional zone of forest tundra or extends from the mouth of the Garonne River shrub tundra north of the "forest limit." This in France southeast to the French Riviera, pro- zone would presumably resemble the park trudes up the Rhone Valley, and then extends tundra of the pollen analysts. On Biidel's map to the south base of the Alps. The Pyrenees are it includes the southwestern quarter of France surrounded by a separate line (Biidel, 1951, and all of the Iberian Peninsula north of the Fig. 2). forest limit (except the high Pyrenees). Having made the assumption that the Pleis- One of Poser's major climatic provinces was tocene polar forest line followed the 10.5°C based on a forest-covered France. Poser advo- July isotherm and the 1000-m snow line, Biidel cated strong climatic influence exerted by the then points out that Pleistocene tree-line de- two ice sheets and attributed the forest zone pression in southern Europe in fact exceeded on the west to the ameliorating effect of the the snow-line depression (Firbas, 1939, p. 105), maritime climate. Biidel, on the other hand, and he thereupon proceeds to make exceptions uses the absence of forest in France to support to his assumption. Thus he draws the polar his thesis that the Pleistocene climate of forest limit, as determined from ' 'paleobotani- Europe was little affected by the ice sheets, cal investigations," south of the Pyrenees and pointing out that the tree line in western other mountains of the Iberian Peninsula, Europe was 700 miles from the ice sheet, but in thence along the French Riviera to the Alps. Russia was less than 100 miles away. Studies of This line thus essentially excludes the Garonne- the form, distribution, and other character- Aquitanian Basin and the lower Rhone from istics of sand dunes and loess deposits all indi- the Wiirm forested region despite the fact that cate that the dune-forming winds in western these areas were south of the reconstructed Europe came from the ocean on the west, not 10.5°C July isotherm. A later map by Bvidel, from the ice sheet, which therefore could not here reproduced as Figure 7, contains slight have influenced the climate appreciably except revisions in southern France, with the omission in the frost-rubble tundra zone immediately of the line for the 10.5°C isotherm. bordering the ice. This is a forceful argument, Inasmuch as the polar forest limit in the west and further field evidence on the Wiirm climate was placed by Poser in northern France but by and vegetation of France will be welcome. The Biidel in central Spain, 400 miles away, both recent map of Wiirm frost features by Tricart on the basis of "paleobotanical investigations," (1956a, PL 3) is impressive in its detail; one one wonders how accurately the Pleistocene cannot be too skeptical about climatic interpre- forest limit can be determined. Neither author tation of stabilized eboulis (talus and slope shows the distribution of the paleobotanical rubble) when there are so many occurrences of sites which guided the mapping. Firbas's similar concentrated rock debris in demon- (1939, Fig. 6) boundaries of Wiirm forest zones strably Pleistocene archeological levels in caves, do not extend westward from Germany and and when there are so many occurrences of ice- Italy. Gross's (1954, Fig. 1) map of Allerod wedge fillings and involutions in the same localities shows very few in France, and if the area (unless one is skeptical about the signifi- Allerod cannot be identified in a pollen diagram cance of these features as well). One must ac- there is generally no datable base, at least in cept the French map on its face value, for there diagrams for sites in unglaciated regions. It ap- is no way to evaluate the correctness of the pears that Biidel shifted the forest limit to the interpretation of individual exposures. The south because of Cailleux's discovery of frost French workers are enthusiasts for the peri- features, including ice-wedge fillings and in- glacial because of their experience with frost volutions, in central and even southern France, features in Greenland and Iceland. One comes in the belief that these features could not have away with the conclusion that France may have formed under continuous forest cover. This is been quite cold during certain parts of the not paleobotanical evidence for a forest limit, Wiirm, with sporadic permafrost possibly ex- but geomorphic. Actually, as Hopkins et al. tending far to the south. The faunal evidence (1955) have shown in Alaska, such frost features from some of the Upper Paleolithic sites in can be found today in regions of sporadic and southern France certainly suggests severe cli- discontinuous permafrost, which are also the mate. We must await further pollen studies,
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Ice-sheet border
Polar tree line M Frost-rubble tundra
1 Loess tundra
Forest tundra
Loess steppe
Loess-forest steppe n % Undifferentiated loess O and steppe o PI 2 Nontropical forest W O Nontropical deciduous forest S, > Mediterranean vegetation
Figure 7. Vegetational zones of Europe during the Wiirm glacial phase, according to Biidel (Woldstedt, 1958, Fig. 129, slightly modified by Biidel from his 1951 map)
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however, before drawing the position of the east, with interruptions caused by highlands. polar forest limit or tree line with more As far east as the Russian border it is placed by certainty. Biidel in a loess tundra zone, but here north- Eastward from France, Biidel draws the polar east of the Carpathian Mountains it is crossed forest line as coincident with the 10.5°C July obliquely by the Pleistocene polar forest line. isotherm. The line skirts the Apennines, The forest line in this continental region, how- Dinaric-Balkan Range, and the Carpathian arc ever, was probably a rather indefinite feature. and then extends northeastward into Russia. Certainly a distinction between loess tundra Its position in the mountains of southern and west of this line and loess steppe (with a few southeastern Europe was delineated by to- trees) east of this line must be tenuous. pography rather than by field evidence. Biidel distinguishes between tree line and Another possible discrepancy in the location forest line. These two boundaries were con- of the Pleistocene forest line comes in the sidered continuous across southern Europe to Hungarian lowland. Poser mapped the region the Carpathians, but in the east, where the dry as forested and showed six localities in which continental climate prevailed, the continuous birch-pine characterizes presumably the lowest forest was confined to the south, in fact south parts of the pollen diagrams. Biidel, on the of the Black Sea. Northward was the broad other hand, maps the area as steppe, referring loess steppe, which had an increasing number of only to paleobotanical investigations in support. trees northward (less evaporation) until a nar- Climatic zones. The polar forest line is the row loess-forest steppe zone was reached just key to the climatic zonation of Biidel. In west- south of the tree line. The tree line here in ern and central Europe he recognizes a transi- Russia was scarcely 100 miles from the ice tion zone on the north (shrub tundra), but to sheet, presumably because of the warmth of the the east across most of Europe the tundra zone continental summer and despite the proximity is separated into loess tundra and frost-rubble of the ice sheet, which Biidel, of course, be- tundra. The frost-rubble tundra zone occurs as lieves had relatively little effect on the "pen- a continuous belt close to the ice sheet from glacial" climate. Ireland to Russia, and also occurs on the upper Thus in Russia the climatic-vegetation zones parts of the highlands of western and central and boundaries from north to south were frost- Europe. In this zone some loess may have been rubble tundra, shrub tundra, tree line, loess- deflated, for certainly winds and suitable source forest steppe, loess steppe, forest line, and areas were present, but continual frost disturb- forest. In western Europe they were frost- ance of the soil did not permit its accumulation rubble tundra, loess tundra, tree line, shrub as pure loess, according to Biidel. tundra, forest line, and forest. The loess belt occurs at lower elevations The Wiirm vegetation zones in European through western and central Europe. Its upper Russia were sketched largely in speculation by limit climbs from 1000 feet above sea level in Biidel, but were drawn with more refinement the north-northwest to 2000 feet at the north by Frenzel and Troll (1952) on the basis of base of the Alps, rising just like any other more extensive Russian pollen-analytical, plant- boundary controlled by vegetation. The vege- geographical, and geomorphic studies and were tation is considered to be tundra, with a mat extended far eastward to incorporate the earlier thick enough to inhibit continual frost dis- work of von Wissman (1938) for China. On this turbance of the loess that was deposited. A map the vegetation zones for central and west- transitional zone occurs near the upper bound- ern Europe were prepared in consultation with ary in the mountains, where loess and solifluc- Biidel and differ little from those of the earlier tion deposits are interbedded. Fluctuations of Biidel (1951) map. the geographic boundary may also be inferred In eastern Europe, however, the following from loess sections, in which each loess cycle, zones were differentiated by Frenzel and Troll at least in the more humid parts of Europe, (1952). Close to the ice was a narrow frost- commonly starts with a solifluction loess, grades rubble tundra zone with dwarf birch and wil- into pure loess, and terminates with a soil. Thus low, increasing southward. Then came the the frost-rubble tundra zone with a cooler, "tree line," south of which was a mixed zone more humid climate moved down even into the of forest-tundra herbs but also chenopods, lowlands at the beginning of a cold phase. Artemisia, and other steppe elements. Biidel had The loess belt as a whole broadens from termed this zone simply a forest steppe and Brittanv on the west to the Ukraine on the considered it much narrower. Ice wedges are
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described from this zone, as well as solifluction throughout. During the glacial period the features, and the southern limit of sporadic winters were colder, as indicated by the south- permafrost is put at a line extending through ward extension of permafrost. Also, the sum- Stalingrad. mers were not so warm, and thus tree line was South of the mixed tundra-forest-steppe also pushed somewhat to the south. But the
TODAY WURM O O O O O 0 Polar sea ice o o o o o Polar sea ice O O O O O O Land ice Frost rubble Tundra zone
Land ice
Nontropical forest Frost-rubble 51« zone Tundra zone
Mediterranean Nontropical vegetation forest Mediterranean vegetation
Desert and Desert and desert steppe desert steppe
1 1 uYi'i', ,',',', ,', Savanna Savanna
Tropical forest Tropical forest
0' 15° E. 0" 15° E. Figure 8. Latitudinal displacement of terrestrial climatic zones during the Wiirm in the meridional belt 0°-15° East Longitude (Budel, 1951, Fig. 1)
zone is the steppe proper (loess steppe of summer change in particular was probably not Budel), with pine and spruce on favorable sites. so pronounced as it was in central Europe, In the northern part of the zone there are be- where the combined effect of the Scandinavian lieved to have been outliers of birch and willow and Alpine ice sheets intensified the refrigera- shrubs and, in the south, long extensions of tion. deciduous bushes up the river lowlands. The equatorward displacement of the vege- The vegetational zonation in Russia during tational zones of western Europe during the the last glaciation does not differ from that of Wiirm are represented diagrammatically by today to the extent that it does farther west Budel (Fig. 8). The most pronounced changes because the climate has been continental were in the zone of land ice, which was in-
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creased from a breadth of 4° to 27° of latitude, Penck (1922, p. 240) later reassigned these to and the zone of nontropical forest, which de- the preceding interglacial, and thereafter con- creased from a breadth of 24° of latitude today sidered the Wiirm to represent essentially a to 10° in the Pleistocene. The zone of Mediter- single climatic cycle, with minor fluctuations ranean vegetation was pushed to the south and during ice retreat. The Wiirm outside the was narrowed, and the subtropical desert and moraines is represented by the Niederterrasse desert steppe were decreased in breadth from in the Alpine valleys that lead to the Rhine and 20° of latitude to 13° not only because of re- Danube rivers. In northern Europe the equiva- striction from the north but also because of the lent Weichsel moraines include several ridges expansion of the zones of tropical forest and across northern Germany and southern Scandi- savanna on the south, according to Biidel. The navia, e.g. Brandenburg, Frankfort, Pomer- extent of the changes postulated for lower anian, Langeland, Salpausselka moraines. The latitudes are based on Biidel's work (1952) in later fluctuations in the sequence are correlated northern Africa and are beyond the area under with the three tundra ("Dryas") phases bound- consideration. ing the Boiling and Allerod oscillations recorded in Late-glacial pollen diagrams (Wright, 1957). SEQUENCE OF CLIMATIC CHANGE Penck's single-cycle classification of the Wiirm has been defended repeatedly by Biidel Introduction (1953, p. 264), who believes further that super- In the last decade there has been renewed imposed on the single temperature cycle there interest in problems of the classification of the was a precipitation change—from humid in the last (Wiirm, Weichsel) cold phase of the Pleisto- first part of the cold phase to dry in the second cene and in the correlation of sequences among —as indicated especially by the solifluction the three principal areas concerned—the north loess and the overlying pure loess of the peri- European glaciated area, the Alpine area, and glacial area (Fig. 9). Other champions of this the periglacial area between. The interest has basic classification are Weidenbach (1953) and been impelled by new field studies particularly Graul (1952), who have mapped the moraines in the loess region, and by the application of the and terraces of the Alpine foreland, and Freis- radiocarbon dating technique to carefully col- ing (1957), Brunnacker (1956), and Fink lected samples from sections that are significant (1959), who have studied the loess stratigraphy stratigraphically or geomorphically. This gives of southern Germany and Austria. the possibility for the first time of correlation on an absolute time scale. Particularly im- Soergel Classification portant here are the analyses from the Gronin- A different scheme of subdivision of the last gen radiocarbon laboratory in the time range major cold phase came into use following the 30,000-65,000 years ago, made possible by iso- studies of Soergel (1919) of the loesses, buried topic enrichment of samples before counting, soils, terraces, frost soils, and mammal faunas for these analyses bring within dating range the of central Europe. The classification was based materials from the early part of the last cold primarily on the loess stratigraphy, in which the phase of the Pleistocene. The new work, how- so-called Older Loess was correlated with Riss ever, has not yet settled a long-standing contro- glaciation of Penck and the Younger Loess with versy about the basic classification of the last the Wiirm glaciation. The Younger Loess was cold phase, whether it be single or multiple, subdivided into Y.L. I and Y.L. II by a buried and in fact the controversy seems to have be- soil, and Y.L. II into Ha and lib in turn by a come more vigorous than ever. Not until this less well developed soil. The terms Wiirm I, II, question is settled will a general climatic curve and III were soon applied to the three units of for the late Pleistocene be accepted. Younger Loess in the belief that Y.L. II (Wiirm II) correlated with the main outer young Pencfy Classification moraine loops of the Alpine glaciers. In this In the Alps, Penck and Bruckner (1909) scheme Wiirm I was represented in the Alps by mapped the Wiirm moraines generally as three a moraine in front of the main Wiirm II outer loops at the base of the mountains plus moraines but inside the Riss moraines (Soergel, some smaller moraines far up the valleys. Al- 1919, p. 89), and in the north the Wiirm I was though they recognized possible interstadial represented by the Warthe drift in front of the fluctuations (Laufen and Achen) in the Wiirm Brandenburg moraine. Weidenbach (1937, p. prior to the construction of the main moraines, 87), however, has shown that the so-called
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IQPOO 0 B.P
WOLDSTEDT 1958
Weichsel
Denmark -16
ANDERSEN 1-12 ET AL. I960 Pollen \f\ -8 Analyses 4
-31 -29 -27 -25 -23 21
-ZEUNER 45-59\ Wurm II -EMILIANI '58- \Wurm" I
* Before Present IOOPOO 90 Figure 9. Climatic curves of the late Pleistocene according to various authors. Amer. = Amersfoort, Br0. = Br0rup, Got. = Gottweig, Pau. = Paudorf, Bra.= Brandenburg, B61. = Boiling, All. = Allerod
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Wiirm I moraine of Soergel relates to the Riss Older Loess/Younger Loess, Y.L. I/II, Y.L. II complex and the Riss terrace (Hochterrasse) a/b respectively in the Soergel classification. rather than to the Wiirm, so should be termed The three terms Krems, Gottweig, and Paudorf Young Riss rather than Wiirm I, and in the have since been widely adopted (Woldstedt, north Woldstedt (1958, p. 20) demonstrated 1958, Fig. 86; Movius, 1960) and assigned the that the Warthe belongs to the Saale glaciation following dates on the radiocarbon time scale: rather than to the Weichsel. These denials of a Krems (Eem) > 65,000 years ago, Gottweig Wiirm I moraine in front of the Wiirm II 44,000-28,000, and Paudorf 25,000. The cli- moraines left the Wiirm I without a representa- matic curve of Gross (Fig. 9) and the similar tion in the glaciated areas, so an alternative curve of Woldstedt (Fig. 9) for the entire proposal was made that the innermost of the Wiirm, as based on these subdivisions and this three Wiirm moraine loops in several Alpine C14 time scale, assumes temperature values de- valleys is actually a Wiirm I moraine overridden rived in part from molluscan and mammalian during later readvance of the ice in the Wiirm faunal studies largely summarized from the II stadial (Eberl, 1930; Weinberger, 1955, p. Czech loess studies by Brandtner (1956, p. 155). 15). The interstadial significance of this feature Loess stratigraphy. The loess stratigraphy of is problematical, however, because of the lack four general areas in western and central Europe of any weathering horizon or other evidence of has been studied intensively in recent years: (1) appreciable ice retreat between the time of northern France (Bordes, 1954) and Belgium formation of the "overridden" moraine and the (Tavernier and deHeinzelin, 1957); (2) south- main Wiirm outer moraines. ern Germany (Schonhals, 1951; Freising, 1957; The Soergel classification, that the Wiirm is Brunnacker, 1956); (3) Lower Austria (Brandt- bipartite or tripartite, was adopted by Zeuner ner, 1954; 1956; Fink, 1956; 1959); and (4) (1945-1959), who related the inferred Wiirm Czechoslovakia (Prosek and Lozek, 1957). temperature curve to the Milankovitch radia- Correlations have been attempted between tion curve for the last 100,000 years. It has been France and Germany (Bordes and Miiller- utilized almost exclusively by archeologists in Beck, 1956), France and Czechoslovakia (Va- the interpretation of cave deposits. It recently loch and Bordes, 1957), and Austria and has been greatly elaborated by detailed studies Czechoslovakia (Brandtner, 1956). The follow- in the loess region of Czechoslovakia (Prosek ing review is confined largely to the loess and Lozek, 1957) and Austria (Brandtner, stratigraphy of Czechoslovakia and Lower 1956), and has been adopted in the voluminous Austria because the sequence is more com- reviews of Gross (1956; 1958; 1959) and in the pletely developed, radiocarbon dating and authoritative book of Woldstedt (1958, p. 243- molluscan studies have been applied, and part 248) as the key to the climatic sequence of the of the area shows connection with the Alpine late Pleistocene. The matter is by no means glaciofluvial features. settled, however, despite the vituperation of The Czech loesses are related to periglacial Gross (see comments to Movius, 1960, p. 378) terraces of river systems draining to the area of and warrants review herewith because it con- north European glaciation via the Elbe River cerns the form of the climatic curve for the late (Prosek and Lozek, 1957, p. 43-54), and the Pleistocene. correlation of loesses and soils depends to a great extent upon the proper correlation of Climatic Curve of Gross these terraces with glacial events in the north. General. In his attempt to summarize, Connections southward with the Danube ter- generalize, and correlate, Gross adapted the races are unfortunately either not well de- terms Krems, Gottweig, and Paudorf that had veloped or are controversial (Brandtner, 1956, been used by Gotzinger (1936) with different p. 166-172). correlation for three soils in the loess sequence On the Moldau River near Prague the ter- in a small area about 30 miles west of Vienna race and loess that are correlated as Saale (Riss) close to the Danube River. Whereas Gotzinger bear a soil (termed Riss/Wiirm and correlated had used the correlations Mindel/Riss, with Krems) with which is associated a snail Riss/Wiirm, and Wiirm I/II respectively, fauna characterized by Helicigona banatica and Gross (1956), basically following Brandtner other forms now found in warmer regions—the (1956), moved the sequence up one step to so-called "Banatica" fauna, believed to be the Riss/Wiirm, Wiirm I/II, and Wiirm II/III by last true interglacial molluscan fauna of the assigning the soils to the weathering intervals loess region (Prosek and Lozek, 1957, p. 74-
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86). The overlying Younger Loess I contains temperate steppe fauna of mammoth (Elephas Helicella striata and other forms that indicate a primigenius), woolly rhinoceros, giant deer (the relatively warm steppe environment ("striata" so-called "Primigenius" fauna), along with fauna). hyena, bear, cat, wild horse, and bison. This The soil found on this younger Loess I is fauna thus parallels the "Striata" snail fauna. termed Wiirm I/II and correlated with the The Gottweig soil in the loess (and in cave Gottweig soil of Austria. It is considered to be layers correlated therewith) yields remains of as well developed as the "Krems" soil below, red deer, moose, red fox, brown bear, badger, according to Prosek and Lozek (1957, p. 77), marten, and lynx, which imply at least some but not so according to Brandtner (1956, p. woods on favorable sites. Ibex, chamois, pole- 136), who considers that the Czech workers cat, ermine, weasel, and ground squirrel, have misinterpreted the soil profiles. At any although associated with culture layers and rate, the soil differs from the "Krems" to the thus potentially imported, imply at least important extent that the associated snail fauna temperate climatic conditions. The presence as is not interglacial like the "Banatica" type but well of steppe elements of the "Primigenius" is essentially the "Striata" fauna as in the fauna suggests an overall park landscape for this underlying Younger Loess I. Also present, how- interval. Above the Paudorf soil, along with the ever, are a few forms suggesting open deciduous cold "Columella" snail fauna, such tundra woodland and thus somewhat moister condi- forms as reindeer, lemming, arctic fox, snow tions, with temperature much like the present hare, white grouse, wolf, and wolverine appear, (Prosek and Lozek, 1957, p. 77). Inferences and the steppe animals are absent. from snail faunas concerning the climate at the Pollen analyses, which generally give the time of soil formation must be limited because most direct and detailed data about con- of the fact that few fossils are preserved in the temporary vegetation, have provided little soil itself, especially under humid conditions. information about the relations in the loess Younger Loess Ila contains the "Striata" region because few satisfactory sites with well- fauna at the base; this is replaced upward preserved pollen have been found. Insufficient abruptly by the "Columella" fauna, character- quantities of pollen are present in the loess it- ized by Columella edentula columella and other self, and in the intervening soils the pollen is arctic-alpine forms, indicating a change to very poorly preserved. Schiitrumpf (Brandtner, cold steppe or tundra conditions. This fauna 1956, p. 150) reports an analysis from the humic persists through the slight interruption of the bands over the "Gottweig" soil at Unterwis- Wiirm II/III ("Paudorf") interstadial and ternitz in southern Czechoslovakia as contain- culminates in Younger Loess lib. ing mostly pine and nonarboreal elements. A The succession of mammal faunas in the loess buried peat in loess in the area of the Saale deposits is incomplete. Most of the material is (Riss) moraine at Cimoskowicze in central from archeological sites, many of which cannot Poland is considered by Woldstedt (1958, p. be related to units of the loess stratigraphy. The 130) as the best case for a complete "Gott- information is supplemented by evidence from weig" pollen profile. The loess here rests on a cave sites, but here the correlation must be lower peat that shows an Eem-type (last inter- based on independent methods, such as the glacial) pollen diagram, so the upper peat, physical stratigraphy of cave deposits—frost- which reveals vegetation of dominant pine, is rubble layers, weathering horizons, carbonate correlated as Wiirm interstadial (i.e., Gott- zones, analyses of particle size and shape, and weig). certain other features often of problematical Loesses and terraces of the Alpine foreland. significance. Correlation is also made (pre- The critical area for the Wiirm classification is cariously) by means of the cultural stages them- the Alpine foreland, where the glaciofluvial ter- selves. The possible errors that creep into mam- races are related to the loess stratigraphy. In malian faunal lists for the various units of the southern Germany and western Austria the late Pleistocene are therefore many. The ex- Alpine glaciers came down to the Rhine and tended table produced by Mottl (1953; see also Danube River valleys, and the moraines and Flint, 1957, p. 451-455), for example, is dim- terraces are well developed. The loesses in this cult to evaluate. region are thin, however, and the soils have A generalized succession of mammal faunas been deeply truncated or transported by applicable for the Austrian area (Brandtner, processes of solifluction during the beginning 1956, p. 164-165) shows for the Early Wiirm a of the cold phase following their formation, for
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the climate here in the west was relatively caused the older sediments to be buried succes- humid, as it is today. The modern soil in this sively by younger sediments, thus preventing region is a Parabraunerde, similar to the Gray- their being preserved as terraces. In this drier Brown Podzolic soil of American terminology. region east and north of Vienna and in adjacent The annual precipitation is now 30-35 inches parts of Czechoslovakia and Hungary, how- (Fink, 1956, p. 52). Few stratigraphic sections ever, the loess stratigraphy shows soils that are contain more than one buried soil. Although relatively well preserved because of less solifluc- the loess stratigraphy has been carefully studied tion, and the loesses are less leached of carbon- in the upland area north of the Danube River ate and contain more snails and mammal fossils. (Freising, 1957; Brunnacker, 1956; Schonhals, The modern soil in the dry landscape is
TABLE 2. CORRELATION OF SOILS (Adapted from Fink, 1959, p. 55)
Dry loess landscape Transition region Humid loess north and east (Krems-Gottweig landscape of Vienna area) (Linz area and west)
Recent Soil: Chernozem Braunerde Parabraunerde Loess Loess Loess "Stillfried B" "Paudorf" "Nassboden" (Tundra soil) Loess Loess Loess Fellabrunn (Stillfried complex): Interbedded loess Humic solinuc- Weakly humic soh- and humic zones tion loess fluction loess B horizon of soil "Gottweig" B hor- B horizon of Para- izon of soil braunerde Loess Loess Loess "Krems" B horizon Pseudogley of soil Loess Loess
1951), correlations with the Alpine terrace se- Chernozem or (above altitudes of 200 m) quence and with the Austrian loess soils are Braunerde. Precipitation is less than 25 inches speculative. (Fink, 1956, p. 52). Farther east the Alpine glaciers did not reach The fades of buried soils in the arid region the foreland, but the main terraces may be differ sufficiently from those in the humid and traced along the Danube as far as the Vienna transitional regions that correlation is not Woods, and may be matched by climatic (peri- always easy. It must be based on general pedo- glacial) terraces on the nonglacial tributaries logic comparisons, such as color, structure, (Fink, 1956, p. 71; 1959, p. 50). Here the loess degree of carbonate leaching and precipitation, cover on the terraces is thicker, and the soils preservation of original loess character, etc. are less affected by the solifluction of a transi- Correlation by terrace relations, paleontologic tional or subhumid climate. Archeological sites studies, or similar means among the three are richest in this region, so that many sections regions is either not possible or not sufficiently have been studied in detail. The modern soil on well worked out. The modern and buried soils loess is Parabraunerde or Braunerde. Precipi- of the three climatic regions are listed in Table tation is 25-30 inches (Fink, 1956, p. 52). 2. East of the Vienna Gates the Danube terrace KREMS SOIL: The Krems soil at Krems, 30 system breaks down as the river enters the east miles west of Vienna, is found in a thick loess end of the Alps because tectonic subsidence in section in which the Gottweig soil occurs at a the Vienna Basin throughout the Pleistocene higher level. The Krems soil here is a complex
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of several soils. At the top is a red-brown chernozemic layers in the upper part of the soil truncated B horizon of a forest soil about 50 cm complex at Oberfellabrunn are in the range thick with a secondary carbonate horizon at the 37,000-43,000 years old. These dates fit those base. This is underlain by several meters of loess set for the Gottweig interstadial by Gross with weak red-brown layers that record other (1958, p. 166), but they apply only to the weak phases of soil formation. It was originally cor- chernozemic humic layers and not necessarily related by Gotzinger (1936, p. 55) as Mindel/ to the main forest soil which unconformably Riss but is considered by Brandtner (1956, p. underlies them and which is presumably the 151) to be Riss/Wiirm. The complex cannot be equivalent of the Gottweig soil at the type certainly identified within the loess section in locality. The age of the Gottweig itself is thus southern Czechoslovakia where the important not necessarily measured. In fact, Gross's dating mollusc sites are located. The soil called Krems of the Gottweig interstadial is based on a series in Czechoslovakia by Brandtner (1956, p. 134- of samples that are not well located with respect 142) is a complex consisting of a reddish-brown to the Gottweig soil. The charcoal sample from clayey B horizon of a forest soil that is trun- Senftenberg, a loess site near Krems, is stated to cated at the top and is overlain by two or three mark the beginning of the Gottweig interstadial chernozemic A horizons intercalated with at 48,000 years ago (Gross, 1958, p. 166); calcareous loess layers and modified by ice- actually, however, the sample came from on wedge casts. Whereas the truncated forest soil top of the soil rather than at the base, and is considered by Brandtner to represent the furthermore has been redated > 54,000 rather R/W interglacial, the overlying parautoch- than at 48,000 (Fink, 1959, p. 58). Other dates thonous chernozemic layers are referred to used by Gross for the Gottweig interstadial climatic oscillations at the onset of the Wiirm come from the Upper Paleolithic Willendorf cold phase. The complex is developed on peri- site in loess on the Danube River west of Gott- glacial terrace deposits considered to be Saale weig, from the Upper Paleolithic Istall6sko (Riss) by Prosek and Lozek (1957, p. 75), but cave in Hungary, from England (Upton War- the correlation with the type locality of Krems ren terrace site), and from Holland. The three seems by no means certain without additional samples dated at Willendorf (about 30,000- evidence. 32,000) come from humic solifluction layers, GOTTWEIG SOIL: The Gottweig soil at the and the relation to possible buried soils is un- type locality is a truncated B horizon of a clear (Fink, 1959, p. 58). The sample from forest soil. It is correlated by Brandtner (1956, Istallosko (30,670) came from a level assigned p. 143; 1954, p. 57) with the Fellabrunn (Still- to the end of the W I/II interstadial on the fried) soil complex found in the drier region of basis of particle-size analysis of the cave sedi- Austria, which consists of a leached and trun- ments. This cave date and the dates from cated B horizon of a forest soil with a secondary England (about 42,000) and Holland (28,500- carbonate zone at the base, overlain uncon- 32,000) are not the most appropriate for con- formably by several thin layers of calcareous firming the stratigraphy of buried soils in loess intercalated with partially leached humic Austrian loess. soil (chernozemic A horizon) with crotovinas. Just as important as radiocarbon dating in In the humid and transition climatic regions the the correlation of the Gottweig soil are the humic soil layers of the Fellabrunn complex are relations to the Alpine glacial-terrace sequence. represented by humic solifluction loess found at In the critical region near Krems and Gott- some localities on top of the B horizon. weig, along the Danube River valley and its The description of the Fellabrunn complex tributaries where the Alpine glaciofluvial ter- resembles closely that stated above for the races are preserved, new field work being com- R/W soil ("Krems") of southern Czechoslo- pleted by Fink and collaborators (kindly vakia (e.g., at Briinn), and in fact it was so cor- demonstrated to the writer in July 1960, to be related by Pelesik, who worked intensively on published in Archaeologia Austriaca 1961) both types (Brandtner, 1956, p. 146). Brandt- shows stratigraphic and geomorphic relations ner himself, however, believes that the two that require different correlation for the Gott- should not be correlated because they are in weig soil. different climatic provinces, and their re- Gotzinger (1936, p. 55) believed that the semblances are accidental—Briinn is in the Gottweig soil at the type locality was the humid region, and Oberfellabriinn in the arid. Riss-Wiirm interglacial soil developed partly Three radiocarbon dates for humus from the on "Late Riss" loess, which rests directly on
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Riss gravels. Gross reinterpreted the "Late Paudorf soil is well developed, and here it has Riss" loess as Wiirm I and thus correlated the been dated by C14 analyses at Stillfried (about Gottweig as Wiirm I/II interstadial. The new 28,000 years old) and at Unterwisternitz (about field studies, however, show that gravel beneath 26,000 years just above the Paudorf soil). In the pre-G6ttweig loess in question is not Riss the transitional climatic region a new date for but older, and that laterally in the exposure the the Paudorf near the type locality is 27,570 Gottweig soil is formed directly on the true (Fink, oral communication). Other possible or Hochterrasse (Riss) and is buried by Wiirm minimum Paudorf dates from less sure strati- loess, which contains the Paudorf soil in its graphic positions (because of solifluction) in- upper portion. The Hochterrasse in this region clude those from Pollau (25,000), Aggsbach may be distinguished from the Niederterrasse (25,000), and Willendorf (30,000-32,000). (Wiirm) below and the Deckenschotter (Mindel Conclusions. The case for marked tripartite and older) terraces above not only by altimetry subdivisions of the Wiirm in the Austrian loess but also by the degree of erosional modification, area is not strong. The so-called Gottweig or and it may be mapped westward up the Danube early Wiirm interstadial is founded on the to the moraines of the Salzach and other Pleis- existence of a buried soil. At the type locality tocene glaciers. It is a single unit: it does not this soil may be the Riss/Wiirm interglacial have the subdivisinos (Young, Middle, and Old soil. In the drier region of Austria it is a soil Riss) present in the Wurttemberg area of complex of which the upper part may represent southern Germany studied by Weidenbach weak climatic oscillations about 40,000 years (1953), where the complications about Jung- ago and the lower part may be Riss/Wiirm. riss and Mittelriss have arisen. The Paudorf soil seems to be correctly inter- The relations at Gottweig therefore suggest preted, but it admittedly represents a weak that the Gottweig soil on the Hochterrasse climatic oscillation and may be somewhat older gravels is R/W interglacial, as Gotzinger than believed by Gross (28,000 rather than originally believed. Where it is traced laterally 25,000 years old), thus bringing it so close to onto a loess parent material in the adjacent up- the range assigned by Gross to the Gottweig land, it is a truncated forest soil at least as well (44,000 to 28,000) that the two would be coinci- developed as the modern loess soil of the region dent even if his Gottweig interval were cor- and thus deserves an interglacial designation rectly dated. rather than interstadial (Fink, 1959, p. 48). The The loess sections seem to be more nearly Niederterrasse bears no buried soils or for that complete in Czechoslovakia than in Austria, matter no true loess, implying that the Wiirm because the drier climate inhibited contempo- meltwater flood plains served as a source for the raneous solifluction. For the same reason, the Wiirm loess, which covers the Gottweig soil on mollusk remains are more abundant and provide higher ground, and at the same time the flood independent climatic information as well as a plains were active enough to prevent loess correlatable datum in the last interglacial. The accumulation on their own surfaces. Austrian area, however, has the advantage of PAUDORF SOIL: If Gross's correlation of the proximity to the Alpine glaciofluvial terraces to Gottweig soil at the type locality as an early which the loess chronology must eventually be Wiirm interstadial is thus jeopardized by both related. It further has had the benefit of radio- radiocarbon dating and field relations, what carbon dating of some important sites. Eventu- about the subdivision of the Wiirm? The Pau- ally radiocarbon dating of mollusc or humus dorf soil has generally been considered to repre- horizons in the Czech loesses may confirm the sent a minor warm oscillation during the epoch chronology of Prosek and Lozek (1957), and of loess deposition. It still shows much of the tracing of the loess-terrace sequence may be original loess structure (Brandtner, 1956, p. possible southward to the Danube glaciofluvial 145) and is distinguished in the region of the terrace system above the Vienna Gates. Until type locality by a flecked reddish-brown and such time as the Czech chronology is confirmed gray and by a crumby structure (Brandtner, by radiocarbon dating and the correlation of 1954, p. 68-73). In the humid region, the the soils of the Krems-Gottweig area is more Paudorf soil presumably is represented by a firmly established, the writer believes that the "Nassboden," considered to be a type of wet recognition of a major early Wiirm interstadial tundra soil characterized by gray mottling and in the loess region and in the Alps, and its a platy structure (Fink, 1956, p. 58-60; Brun- identification as "Gottweig," should be con- nacker. 1956, p. 44-45). In the dry region the sidered tentative.
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These conclusions bring us to a brief con- Emiliani's correlation of the radiation curve sideration of climatic curves for the Late Pleis- with European climatic chronology (Wiirm I tocene based on other regions. New pollen and II cold phases at 72,000 and 23,000 years analyses and radiocarbon dates from deposits ago respectively) seems to be more satisfactory of the Weichsel glacial stage in Denmark and than that of Zeuner (WI, II, and III cold Holland give curves (Fig. 9) that show two phases at 115,000, 72,000, and 25,000 respec- Early-glacial interstadial fluctuations in which tively). birch-pine forests temporarily returned to that region (Andersen et al., 1960). The base of the PLEISTOCENE CLIMATOLOGY sections is firmly founded on Eem (last inter- glacial) deposits which have characteristic pol- General len diagrams. The two interstadials are dated at The efforts of Poser (1948; 1950), as reviewed about 64,000 (Amersfoort) and 59,000 (Br0rup) above, to produce maps of Pleistocene winds years old on the basis of five radiocarbon analy- and mean atmospheric pressure systems for ses. No correlative of the Gottweig of Gross is summer and winter represent an approach to recorded in this region; the time range from Pleistocene climatology that is based on the 50,000 to 32,000 is assigned in Holland to the interpretation of geomorphic features. A more Pleni-glacial A, marked by tundra vegetation common (but probably more tenuous) ap- at best. A possibility of a correlative of the proach is based on extrapolation of present-day Paudorf interval at about 30,000 years ago is climatic anomalies under the postulate that suggested. The following phase, Pleni-glacial B, mean temperatures were appreciably lower from about 28,000 years ago until the start of during the Pleistocene. the Late-glacial 14,000 years ago, remains unre- Attempts at such reconstructions go back at corded by organic sediments in this region. least to Penck (1913) and were quite general- The curves of late Pleistocene ocean-surface ized for many years (Brooks, 1926; Simpson, temperatures based on study of planktonic 1934). They became more detailed after studies Forarninifera fossils from deep-sea sediments on Pleistocene snow line indicated the extent of must be mentioned as another measure of depression of the temperature, and after studies European climatic change inasmuch as they of the Pleistocene frost features delineated the presumably reflect the planetary situation. periglacial zone. Particularly important have Two types of curve have been constructed, one been the recent analyses of modern circulation (Fig. 9) based on oxygen-isotope analysis patterns that prevail when Europe experiences (Emiliani, 1955), the other on frequency of anomalous heavy snowfalls, cool summers, or certain cold- or warm-loving Forarninifera other conditions suggestive of Pleistocene (Ericson and Wollin, 1956). The time scale is climate. based on radiocarbon and ionium dates in the upper part of the cores and extrapolation to Pleistocene Temperature Depression depth by estimation of rates of sedimentation Snow-line depression. The most common (Emiliani, 1955, p. 556 if.). If the slightly modi- method of calculating the Pleistocene depres- fied time scale of Broecker et al. (1958, p. 513) sion of temperature comes from the determi- is used, the oxygen-isotope curve shows two nation of the difference in elevation between cold maxima in the last 70,000 years—the time the present and the Pleistocene snow lines and assigned to the Weichsel by Andersen et al. the multiplication of this figure by a vertical (1960). These two maxima support the bipartite temperature gradient (normal lapse rate). Pleis- division of the last glaciation used by Gross and tocene snow line is generally determined from others for Europe. the lowest elevations of small cirques of firn The Milankovitch curve of variations in the basins, although an alternative and less accurate intensity of summer solar radiation at 65° N. method involves the calculation of the eleva- latitude is also presented as a possible key to tion midway between the glacier terminus and late Pleistocene climatic change in Europe the crest of the cirque wall. (Fig. 9). Such a curve is adopted by Zeuner The history of investigations on Pleistocene (1945-1959) as an explanation for the geologic snow line has recently been summarized by and biogeographic sequence in Europe and is Morawetz (1955, p. 192-194). Seventy years also used by Emiliani (1955, p. 567 ff.) and ago Penck calculated the Pleistocene snow-line Fairbridge (1960) for correlation of ocean depression for the Alps at 1000 m and, assuming temperatures and sea-level changes respectively. a lapse rate of 0.5°C/100 m, arrived at a figure
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of 5°C for Pleistocene depression of the mean western United States by Baker (1944, p. 225) annual temperature. Klute (1928, p. 80) used gives the following average figures. this figure in his comprehensive study of snow- line depression in different climatic zones over January mean January mean maximum .54 the whole world. Later, however, it was deter- January mean minimum .54 mined from weather stations in the western July mean .65 British Isles that the difference between the July mean maximum .74 temperature at the modern snow line (esti- July mean minimum .54 mated to exist just above Ben Nevis in Scot- Annual mean ca,. 6 land) and the present temperature at the eleva- tion of the Pleistocene snow line is 8°C, and Large variations from these figures, especially Penck (1938, p. 87 ff.) accepted this figure as in winter, are attributed by Baker in part to more reliable for Pleistocene temperature de- pronounced temperature inversions. The gradi- pression, at least for humid regions. ent depends on many factors such as frequency
TABLE 3. TEMPERATURE AND SNOW LINE IN THE ALPS (Weischet, 1954, Fig. 1)
Mean annual temperature At elevation At elevation Vertical of present of Pleistocene Pleistocene depression temperature snow line snow line Temp. Snow line gradient
°c °C °c m °C/100m Santis — 2.5 + 4 6.5 1000 .65 Zugspitze -2.5 + 5 7.5 1100 .68 Sonnblick -5 + 2 7.0 1100 .70
From more recent studies in the Alps we can and character of invading air masses, the local identify the following facts and opinions. topography, radiational cooling, snow cover, Present snow line ranges from 2500 to 3350 m etc., as well as the season. Any major middle- (Klebelsberg, 1949, p. 660); it is relatively low latitude mountain mass such as the Alps can on the north flank because of the higher lati- certainly experience a variety of synoptic situa- tude, and in the western Alps because of tions over the years, and the effect of each type greater snowfall. Pleistocene snow-line de- on the glacial, geomorphic, and biogeographic pression was 1000-1200 m. Vertical temperature relations is perhaps not most reliably expressed gradients more realistic for ground stations by a consideration of annual means or any other than the traditional free-air rate of 0.5°C/100 means. It is not easy to analyze these climatic m can be determined from a few high-mountain elements statistically without the use of means, weather stations, so that only slight extrapola- however, and at present the mean values are tions are required to calculate the temperature the most practical representation of the climatic at the elevation of both the Pleistocene and the factors for the discussion at hand. modern snow line (Table 3). In a recent thorough study of the climatic Table 3 shows that the mean annual tempera- conditions in the Sierra Nevada of California, ture at the modern snow line is not 0°C, as is Miller (1955, p. 15-20) discusses the effects of often assumed, but is generally lower. For the local factors on the vertical temperature gradi- Alps the vertical temperature gradient for ent, which in this region ranges from .46 to ground stations is closer to 0.7°C/100 m than .61°C/100 m for ground stations. Miller also to 0.5°, and in fact this gradient had been used made a comparison of the relation of ground by Koppen (1920) in an early study of Pleisto- temperatures at high mountain stations in 17 cene depression of tree line. Morawetz (1955, mountain ranges throughout the world to the p. 195) cites other modern gradients in the Alps free-air temperature at the 700-millibar level ranging from 0.8°C/100 m at high elevations to (elevation ca. 10,000 feet) directly above the 0.3° in the lower valleys. Measurements of the range. For the comparison, the ground tem- surface lapse rates in 28 mountain areas in the peratures had been adjusted to a uniform lati-
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tude (40°) by the use of sea-level latitudinal just east of the Alps. The position of the Pleis- temperature gradients, and adjusted to a uni- tocene forest boundary was based on the pollen form elevation (7000 feet) by the use of the analyses of Firbas and others. Mortensen (1952, local surface lapse rate extrapolated to 7000 p. 153) states that on the basis of more recent feet from the closest mountain station available studies the permafrost limit marks the — 5°C in the particular mountain range. If the normal isotherm rather than —2°C, implying a tem- free-air lapse rate between the 7000-foot perature depression in central Germany of ground station and the 10,000-foot air station 15°C rather than the 12°C calculated by applies, namely .55°C/100 m (3°F/1000 feet), Poser. As stated above, however, sporadic it would be expected that the temperature at permafrost can occur in regions with mean 10,000 feet should be 5°C colder than at 7000 annual temperatures as high as 0°C (Black, feet. The figures show that the mean tempera- 1954, p. 839). Inasmuch as the Pleistocene frost ture for the 3 winter months at 10,000 feet features recorded by Poser may represent only ranges from only 2°C colder to as much as 3°C sporadic permafrost (or even deep seasonal warmer than at 7,000 feet, and that for spring it frost), the temperature requirement may thus ranges from 7°C colder to 1°C warmer. The have been close to 0°C. This possibility, specific figures for the Alps are a 0°C difference coupled with the potential unreliability of some for winter (i.e., isothermal from 7000 feet on of the so-called permafrost indicators (as dis- the ground to 10,000 feet in the air above) and cussed above), makes the precise calculation of 3°C colder for spring. The figures illustrate that temperature depression difficult on the basis of temperatures at mountain stations probably do frost features. not accurately reflect temperatures in the free It seems probable, however, that the Pleisto- air beside the mountains at the same elevations, cene temperature depression close to the and therefore that the use of free-air lapse rates ground in the periglacial region of central in predicting ground temperatures in mountain Europe amounted to at least 10°C, or 2-5°C regions is subject to large errors. Use of any greater than the figures usually given for the lapse rate, whether accurate or not, in the con- Alps on the basis of snow-line depression. Mor- version of Pleistocene snow-line depression to tensen (1952, p. 154 ff.) explains the discrep- temperature depression depends on the further ancy by the proposition that the vertical tem- assumption that the Pleistocene lapse rate of perature gradient was actually inverted in the the region was the same as the modern. Such an Pleistocene at low altitudes, so that the inferred assumption may not be justified in view of the temperature depression was great in the low- possibly different air-mass and precipitation lands (where frost features and plant fossils are conditions during the Pleistocene. found) but small in the mountains near the Inverted lapse rates. Besides the snow-line snow line. His evidence for a strong and deep studies, other methods of calculating Pleisto- inversion comes from Penck's observation that cene temperature depression have been used— the Alpine firn basins near the present snow line studies of the distribution of fossil plants, (2600 m elevation) had no more ice in them animals, and frost features in regions beyond during the glacial period than they do at pres- their modern ranges. Iversen (1954, p. 89 ff.), ent. He infers from this observation that the for example, calculated mean July temperatures temperature and snowfall in the Pleistocene at and other climatic elements on the basis of the this elevation must have been the same as to- modern distribution of plants found as pollen day, and that the Pleistocene temperature de- in Late-glacial sediments of Denmark, and pression was thus confined to lower elevations. Firbas (1949, p. 286-294) made similar esti- His curve of vertical temperature gradient for mates for central Europe. Soergel (1936) was the winter shows a sharp inversion below 2500 the first to calculate temperature depression m. Although his summer curve shows no in- from frost features (ice wedges) and from the version, a distinct change in gradient at 2500 m distribution of certain vertebrates. His studies is postulated. In this view the Pleistocene de- on frost features were followed by those of pression of the mean annual temperature thus Poser (1948) as enumerated in the earlier pages increased from zero at 2500 m to high values of this review. (10-15°C) at lower elevations and accounts for The basis of Poser's calculation of tempera- the discrepancies between the calculations in ture depression was the intersection of the the mountains based on snow line and calcula- Pleistocene polar forest border (July 10°C tions in lowlands based on frost features and on isotherm) and the equatorial limit of perma- plants and animals. frost ( —2°C annual isotherm) near Zagreb Mortensen's (1952) inversion hypothesis re-
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quires generation of cold air masses over the tudes might introduce enough turbulence to re- north European ice sheet and adjacent snow- tard the development of intense inversions. On covered central Europe during the winter, with the other hand, the pollen-analytical evidence frequent outbreaks of this cold air to underride for tundra in central Europe during the glacial warmer air masses and produce the postulated period implies that the albedo of the ground inversion. The atmospheric-pressure pattern surface in winter would be high, because the worked out by Poser (1950, Fig. 1) on the basis low vegetation would not project far above the of sand dunes suggests a semipermanent high- snow cover. The effect discovered by Miller pressure system over Europe even in summer— (1955, p. 120-122) in the Sierra Nevada of a condition that might be produced by such a California would not apply—that even with cold air mass. Some confirmation of the hypoth- one of the heaviest snowpacks on the continent esis comes from studies of modern lapse rates in the albedo in winter and spring is low because arctic continental regions. At Yakutsk in north- the cover of coniferous trees adsorbs so much eastern Siberia the ground inversion in winter of the radiation. depresses the basal air temperature as much as Mortensen's hypothesis, however, assumes 45°, with the thickness of the inversion layer that at a station in the Alpine foreland the free- being about 2500 m (Flohn, 1952b, Fig. 1). air lapse rate up to a height of 2500 m is the The mean January gradients at several North same as the lapse rate on ground stations up the American arctic stations show an inversion up north flank of the adjacent Alps. To the extent to about 1500 m, which produces surface tem- that the inversion in the lapse rate is a result of peratures 10°-20°C lower than they would be local radiational cooling, however, the height if the upper-air rate continued down to the of the inversion would be determined by height surface (Meteorological Div., 1944). Although above the ground level rather than the height July lapse rates show no inversion, there is a de- above sea level. Furthermore, the hypothesis pression of a few degrees. The gradient above assumes that the Alpine mass itself had no effect the inversion at these North American stations, on the character of the polar air which moved incidentally, is about 0.6°C/100 m up to about against it. The contrasts between ground tem- 5000 m altitude, and about 0.7°C from here up perature and free-air temperature at the same to the tropopause at 10,000 m (8000 m in sum- altitude, as cited above from the work of Miller mer). (1955, p. 19), indicate that large mountain The winter inversions at high latitudes are a masses have important local effects on the tem- result of radiational cooling under cloudless, perature of the air nearby. windless conditions during the long polar night Flohn (1953, Fig. 2) accepts the probability when the sea is frozen or the ground is snow- of a mean inversion caused by ground cooling covered (Flohn, 1952b, p. 81-82). The winter up to a height of 1000 m. He does not accept inversion at Eismitte is 20°C greater in clear Mortensen's hypothesis of no temperature de- weather than in overcast. That daily heating in pression above 2500 m, however, but adopts winter has a negligible effect on the basal tem- the general figure of 4°C for elevations above perature is seen from a comparison of noon and the inversion level. This appears to be a more midnight readings at Fairbanks, Alaska. The realistic hypothesis, and accounts for the evi- effect of snow cover is shown by the stronger dence for particularly low temperatures at inversion at Eismitte, a station in the middle of ground level in the periglacial region of central the Greenland ice sheet, than at Thule, which Europe. is located on exposed la'nd 400 miles to the Effects of precipitation. The effects of pre- north. cipitation on snow-line depression have been The cold air generated by ground radiation examined in different ways. Klute (1928, p. 82 spreads laterally out from its source region by ff.) attributed excessive snow-line depression in subsidence (Wexler, 1936, p. 135), and from certain climatic zones to the lowering of snow time to time the entire polar mass breaks out- line down into the elevation zone of maximum ward. Thus a much larger area may be affected Pleistocene snowfall. The existence of such a by the cold basal air. Pleistocene conditions zone of maximum snow fall at intermediate ele- over the north European ice sheet and a snow- vations at the present time in the Alps, how- covered central Europe might be less favorable ever, has never been proven, according to for the development of these conditions in Klebelsberg (1949, p. 664), and in the northern winter than the present arctic regions are, be- Sierra Nevada of California, where it has long cause of the lower latitude involved—the oc- been assumed on the basis of earlier work, its currence of daily solar heating in middle lati- existence has recently been challenged by
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Miller (1955, p. 22) on the basis of new 10.6°C, which should be equivalent to a snow- analyses of the snow records. line depression of 1514 m if the humid-region Firbas (1950, p. 163 fF.), on the other hand, gradient of 0.7°C/100 m is used. The difference found from pollen studies that the Pleistocene 1514-1200 m = 314 m is attributed to an in- tree line in the Alps was depressed more than crease in precipitation of (150 mm per 100 m) the snow line, and attributed this relation to X 314 m = 470 mm. After computing such aridity. Vegetation at this time was marked by discrepancies all over Europe, Klein prepared a Artemisia, generally considered as an indication map to show the Wiirm precipitation, and from of relative aridity. Mortensen (1952, p. 154) this Klute (1951, Fig. 3) constructed a map to points out, however, that these paleobotanical show the percentage of precipitation during the studies were necessarily all from sites at rela- Wiirm compared to the present. The percent- tively low elevations, where the temperature age map shows high values on the west coasts of inversion would have greater effect than at ele- the British Isles and in France, Spain, Italy, vations near the snow line. Actually, the loess and the Balkans (60% of today), drier in the stratigraphy shows both humid and dry phases, Scandinavian mountains and in the Alps, and with the climate at the time of maximum glaci- still drier farther east so that Russia had only ation being relatively dry. Weischet (1954, p. 20 per cent of the modern precipitation. 112) suggests the precipitation was 20% less Ingenious though these cartographic repre- and therefore that the modern temperature sentations appear to be, the data on which they gradient in the Alps should not be used without are based do not justify their detail. The pos- a correction. He therefore believes that a tem- sible errors in the basic Poser-Budel maps are perature depression of 8-10°C is more accurate great, as are also the difficulties in choosing the for the Alps than 7°C. correct vertical gradients for conversion of In another study of the precipitation effect, snow-line depression to temperature depression Klute (1951) prepared January and July tem- or precipitation change. Errors may thus be perature maps for Europe with attention to the compounded. This is not to deny that it was local effects of the ice sheet and oceanic in- probably drier in eastern than in western fluence on the west. The maps are based on Europe during the Wiirm, but the degree of Poser's (1948, p. 57) figures for summer and differentiation cannot be expressed so pre- winter mean temperature at the point east of cisely. the Alps where the Pleistocene forest border Mediterranean region. For the subtropical and the permafrost border cross, and contain mountains a Pleistocene temperature depres- the revisions of Biidel (1951). sion of 4°C has usually been assumed on the Klein (1953, p. 101-104) has taken the next basis of a snow-line depression of 800 m. Recent step. Using Poser's map of climatic zones based studies in these latitudes, however, suggest that largely on frost features (Fig. 3), she estimated the snow-line depression may have rivaled or the temperature depression for various parts of even exceeded that of the middle latitudes. Europe, ranging from about 9°C in Greece, Thus Mortensen (1957, p. 44-46) refers to Italy, and Spain, to 13°C in central Europe and studies by different workers that indicate snow- 16°C at the very edge of the ice sheet. Then, line depression ranging from 1400 to 2100 m in using figures for snow-line depression in various the Sierra Nevada of Spain, the Atlas Moun- mountain regions of Europe, and applying a tains, Abyssinia, and the Himalaya, and the vertical temperature gradient of 0.9°C/100 m present writer has calculated a figure of 1800 m for the continental climatic zone and 0.7°C/100 for the Zagros Mountains of Iraq (Wright, m for the oceanic zone, she calculated what the 1960, p. 89). These values, when converted to temperature depression should be in the several temperature depression on the basis of the gen- areas under consideration. The discrepancy be- erally assumed lapse rate of 0.5°C/100 m, yield tween this calculated value and the value de- figures of 7°-10°C temperature depression for rived from Poser's frost map is attributed to low latitudes—considered to be much too great differences in precipitation with the use of a when applied to low elevations. Mortensen factor of 150 mm precipitation for each 100 m (1957, p. 49) shows that for mountains near Rio of extra snow-line depression. For example, the de Janiero, where Pleistocene snow line had an snow-line depression in the Spanish Pyrenees is elevation of 2300 m, the use of this gradient stated to be 1200 m. The temperature depres- yields a summer temperature depression of as sion for nearby southern France, as inferred much as 14°C at sea level. from the Poser-Budel maps, is calculated as One way of overcoming this difficulty is by
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the postulate that the excess amount of snow- ture of the air over subtropical oceans applies to line depression beyond that resulting from the Mediterranean area is problematical. Riehl temperature may be explained by increased (1954, p. 50) shows that lapse rates range from snowfall. Antevs (1954, p. 187) uses such reason- stable to inverted for different air masses over ing in a study of Pleistocene snow-line depres- Khartoum, Sudan. The continental polar air sion in east-central New Mexico (latitude 37°), that occasionally reaches the region in winter and a parallel study of a nearby lowland Pleisto- has an inversion layer at moderate elevation, cene lake indicates an increase in precipitation attributed to subsidence with divergence. as well as a decrease in temperature (Leopold, In view of the extreme variability in sub- 1951, p. 166). tropical temperature lapse rates thus summar- An alternate explanation for the high values ized by Riehl, Mortensen's (1957, p. 50) par- of snow-line depression in low latitudes is the ticular quotations of steep low-altitude lapse suggestion by Mortensen (1957, p. 50) that the rates from isolated localities (e.g., Samoa) are vertical temperature gradient below the Pleis- insufficient to support by themselves his ex- tocene snow line is closer to 1.0°C/100 m (the planation for the high values of Pleistocene dry-adiabatic rate) than 0.5°C because of the snow-line depression. His main conclusion, how- complete instability consequent on the great ever, seems valid—that snow-line depression radiational heating from the ground in sub- alone is insufficient grounds for inferring Pleis- tropical regions. Use of a gradient of about tocene temperature depression at sea level. His 0.9°C for the Brazilian case would yield a tem- opinion that biologic evidence and frost features perature depression of only a few degrees at sea are more reliable temperature indicators than level rather than 14°, permitting the continu- snow line cannot be tested for the Mediter- ance of tropical conditions, whereas at high ranean region until more fossil materials have elevations the temperature was lowered enough been found and studied. Although it is hard to to bring about extreme depression of the snow believe that all the life zones on subtropical line. mountains (like the Atlas or Zagros ranges) Studies of temperature lapse rates over sub- were depressed an amount comparable to the tropical continental areas have not been under- depression of the snow line, without some tele- taken, but for ocean areas Riehl (1954, p. 52- scoping, it will be some time before the pollen- 71) has summarized the observations of the analytical and other paleontological studies of German METEOR expedition in the Atlantic subtropical lowlands and foothills allow the and his own studies in the subtropical eastern kind of climatic reconstructions that have been Pacific. The most striking feature of these re- made for central Europe. gions is the persistent trade-wind inversion, Despite the conflicting evidence concerning which is produced by the subsidence of warm the specific amount of temperature depression dry air from the subtropical high-pressure re- in continental regions of the temperate and sub- gions over the cool moist air that originates tropical latitudes, there seems to be general over the upwelling cold water along the west agreement that the temperature was lowered coasts of Africa and America. In the case of the at least 4°C. Perhaps the most reliable indica- Atlantic the inversion layer near the source tions for Pleistocene planetary temperatures at region has a base at an elevation of about 500 m. sea level, however, may come not from conti- Its thickness is highly variable but averages nents but from the study of ocean-bottom sedi- about 400 m. The temperature increase up- ments. Changes in the frequency of certain ward within the layer amounts to 5°-8°C. The temperature-sensitive planktonic Foraminifera layer persists as the air masses move westward in cores of abyssal sediments give a record of in the tradewind belt, but its intensity weakens, temperature fluctuations during sedimentation and its base rises to about 1500 m. (Ericson and Wollin, 1956). Changes in the As a result of these conditions, lapse rates ratio of oxygen isotopes in certain planktonic below the inversion range from superadiabatic Foraminifera also reflect surface-water tempera- (e.g., 1.0° - 1.4°C/100 m) in the lowest few tures, and Emiliani (1955, Fig. 15) has con- hundred meters to stable (0.5° - 0.7°C) as the structed a paleotemperature curve that sug- inversion base is approached. The counter rate gests an ocean-water temperature for the gla- within the inversion is moderate to steep. Above cial phases 6°C lower than the present. the inversion the air is very dry, and the lapse Assuming a general temperature reduction of rate steepens to 0.8° - 0.9°C/100 m. 4°C and disregarding for the moment the pos- The extent to which this temperature struc- sibility of a general precipitation increase as a
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cause for the glacial period, Flohn (1953, p. winter intervals of severe storms, and he would 270) attempts to calculate the secondary effects therefore postulate that these anomalous condi- of temperature reduction on precipitation. tions of today were prevalent during the Pleis- Pointing out that most of the atmospheric tocene. This type of comparison seems to be a moisture comes from the oceans, presumably at reasonable approach to the reconstruction of low latitudes, Flohn calculates that a 4°C tem- Pleistocene atmospheric circulation, and as perature reduction in the Pleistocene should further analyses of the weather record are made bring about a 20 per cent reduction in evapora- new opportunities become available for the tion and a comparable decrease in precipitation study of conditions favorable to glaciation. in this region (0°-5° latitude). For latitudes Thus Lysgaard's (1949) maps on recent climatic 20°-25° the reduction of precipitation is pos- fluctuations and similar studies have been tulated as 40 per cent because the evaporation adapted to show that the time of maximum is further decreased by weakening of the trade recent glacial advance in 1600-1850 was marked winds and by southward migration of the trade- by a pattern of meridional circulation and was wind belt. In central Europe close to the ice preceded and followed by "nonglacial" times sheets the evaporation was especially low be- with dominant zonal circulation (Willett, 1953, cause of the even greater depression of the p. 56). Correlation of the "glacial" pattern temperature (12°C); relative aridity is sug- with times of maximum variations in solar ac- gested for this region during at least part of the tivity is suggestive although not certain, and glacial phases by the prevalence of loess deposi- the resemblance (or lack thereof) between tion. Only in the Mediterranean region should Willett's Pleistocene winter pressure map and the frequency and probably the quantity of Wexler's (1956, Fig. 3) map of January pressure precipitation have been greater—the increased anomalies during sunspot maxima may be frontal activity in this zone could compensate noted. for the effect of lower temperature on evapora- Leighly (1949, p. 141-145) showed that the tion. synoptic situations during the period 1916- 1938 that produced heavy snows in interior General Atmospheric Circulation North America are those with low-index (me- Willett (1950) prepared a mean-pressure map ridional) circulation. Such a pattern is provided for the northern-hemisphere winter during the by a high-pressure cell over western or central last cold period; it shows a dominant meridional Canada that not only served as a block to circulation, with low-pressure cells over the At- westerly winds but permitted flow of moist air lantic and Pacific oceans and glacial anticyclones into the Canadian prairie from the southeast. over southwestern Canada and northeastern Once the North American ice sheet became ex- Europe. The anticyclones served as blocking tensive it increased the occurrence of these highs for the eastward movement of cyclones favorable anticyclonic conditions. Leighly ad- that originated in the Aleutian and Icelandic mits that the same reasoning cannot be applied lows respectively to the west, and many storms to the nourishment of the north European ice were therefore forced to skirt the highs on the sheet, because the moisture source is clearly to southeast, bringing heavy precipitation to the the west rather than to the southeast as in Gulf of Mexico and the Mediterranean in the North America. winter, and in the summer nourishing the south- Another example of the application of re- ern margin of the ice sheets. South of the low corded weather anomalies to Pleistocene clima- cells the intensified subtropical high-pressure tology is Mather's (1954) study of the mean belt, which today controls the aridity of the January pressure, temperature, and precipita- Mediterranean, was therefore pushed to the tion of the 1930's compared to the preceding south and restricted, allowing the development 30 years. Assuming that the recent warming of of pluvial conditions in North Africa. The the North Atlantic region and the consequent tropical easterlies were strengthened as a reflec- recession of glaciers are related to measurable tion of the intensified zonal (latitudinal) circu- changes in the general circulation similar in lation, and precipitation increased here as well, nature to those prevalent in the Pleistocene bringing pluvial conditions to East Africa south interglacial phases, Mather's maps of anomalies of the subtropical dry belt. suggest the following conditions favorable for Willett based his map in part on the anoma- Pleistocene glaciation: lous pressure patterns that prevail today during (1) The Icelandic and Aleutian lows were
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displaced to the west compared to today, and currence of heavy precipitation in the dry the Siberian high to the east. For Europe this Great Basin of the western United States in meant reduction of the latitudinal pressure January 1949, related to this pattern (Flohn, gradient and consequently a decrease in the in- 1952a, p. 169). tensity of southerly air flow over northwest For the Mediterranean, Flohn (1952a, p. Europe. 162) shows that in modern severe winters the (2) Areas of past continental glaciation were cold air penetrates gaps in the weakened sub- colder and drier than today, and unglaciated tropical high, bringing rains to the dry belt and continental areas were warmer and wetter. The increased cyclogenesis in the tropics. A similar different patterns require that the cause for the condition may have dominated the Pleistocene, climatic changes have effects on continents dif- although Flohn differs from Willett in presum- ferent from effects on oceans. Changes in solar ing that the subtropical high was weakened radiation, either in intensity or in wave length, rather than intensified, allowing pluvial condi- produce differential effects of this type. tions here that are certainly correlated with the (3) Storm track were shifted southward in glacial phases of Europe (Biidel, 1952). North America as a result of the westward shift In a further attempt to relate the present to of the Aleutian low. Temperature was lower the past for the Mediterranean climate, Butzer in western Canada, and precipitation was (1957a) has found that the mean atmospheric higher in the Great Basin of the United States. pressure was appreciably higher during 1921- Still another study of recent weather changes 1940 for both winter and summer in the Medi- (Lamb and Johnson, 1959) is the analysis of terranean than in preceding decades, and that January atmospheric pressure (the key to the the precipitation (years 1911-1940) was lower. general circulation) the world over for each This relation is explained in terms of Flohn's decade from 1760, and the statistical correlation (1953, p. 267) concept of intensification and of pressure changes with temperature, precipi- northward shift of the subtropical high during tation, ocean currents, and the extent of the an interpluvial (interglacial) phase, northward arctic ice pack. The authors do not speculate on shift of the ploar front and storm tracks in the the extrapolation of these recent changes to the belt of westerlies, and decreased cyclogenesis in Pleistocene but suggest that Willett's concept the tropical convergence zone resulting in of expansion and contraction of the circumpolar fewer outbreaks of polar air through the vortex is oversimplified because it ignores the strengthened subtropical high. asymmetry of development of the two principal In another analysis Butzer (1957b) points ice sheets and other geographic factors. out that according to the geologic evidence the Flohn (1952a; 1953) essentially follows Wil- temperature fluctuations of the Pleistocene in lett's picture of Pleistocene circulation and Europe were not necessarily in phase with the adds more details based in part on analogies changes in precipitation, and therefore that the with selected anomalous patterns of modern changes in the general circulation of the at- severe winters. The mean upper-air maps for mosphere may have been more complicated the present show cold poles over northern than generally assumed in the reconstructions Elurasia and northern Canada. The Canadian of Willett and Flohn. The loess sequence, for center provides a frontal zone against moist air example, indicates that each cold phase started masses of the western Atlantic, resulting in with a relatively moist interval, marked by the nourishment for the Greenland ice sheet. Dur- formation of solifluction loess, and then con- ing severe winters these cold centers and their tinued with a relatively dry interval character- associated upper-air troughs shift to the west ized by deposition of pure loess without soli- and become stronger. If in the Pleistocene this fluction (Biidel, 1951, p. 25). Whether this se- were the prevailing condition, along with in- quence marks each of the major subdivisions creased meridional circulation, then the polar of the Pleistocene (e.g., Wiirm), as Biidel as- front would be expanded in activity, and the sumed, or the minor subdivisions as well (e.g., Labradorean and Scandinavian ice sheets Early Wiirm, Main Wiirm), as is implied by would grow to the southwest as well as to the the climatic curves of Gross, Woldstedt, and southeast of the respective cold centers. That others, depends on one's concept of the scope the southward and westward expansion of these and correlation of the Wiirm. Butzer has at- high-latitude blocking highs can have effects tempted to cite geologic evidence from the even on the lower latitudes is seen from the oc- Mediterranean region pointing to a similar kind
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of precipitation fluctuation for each phase of raphy on ocean currents and other elements of the Wiirm as well as a general decrease in climatic control (Brooks, 1926, p. 302-320) humidity throughout the Wiirm, but the paleo- cannot be ignored and in fact have recently climatic interpretation of cave deposits and been re-emphasized in a new theory of climatic other geologic and paleontologic features of the change by Ewing and Donn (1956). The diffi- Mediterranean region, as well as their detailed culties in accounting for the cyclic nature of correlation with possible subdivisions of the Pleistocene and Recent climatic changes by Wiirm sequence of Europe, is still uncertain cyclic changes in topography, however, have (Wright, 1960, p. 87). made the solar hypotheses more popular in re- With this assumption that each cold phase cent years (e.g., Flint, 1957, p. 495-503). Cyclic (both major and minor) was marked by a cool solar changes of short duration are known from moist portion (pluvial in the Mediterranean) the occurrence of Sunspots, and much interest followed by a cold dry portion (with maximum has been directed toward determining the re- of glaciation), Butzer (1957b) proposes that the lations of the weather and climate to sunspot principal change in the general circulation to cycles and possible longer cycles of similar bring about a glacial-pluvial epoch involved character. primarily precipitation rather than tempera- It might be assumed that a decrease in in- ture. He therefore supports the Simpson-Wil- coming solar radiation should result in cooling lett hypothesis that Pleistocene circulation of the atmosphere and bringing about glacia- changes were not the secondary effect either of tion. Simpson (1934, p. 428 ff.), however, a change in planetary temperature or of the pointed out that decrease in radiation would spread of ice sheets, but instead were a result of have the greatest effect in tropical latitudes and changes in solar radiation as they affected the therefore would result in a decrease in the pole- intensity of the circulation. The moist cool ward temperature gradient. As a result, the in- phase that was brought about by an increase in tensity of circulation and the consequent solar radiation was marked by increased me- storminess that he believed necessary for glacier ridional circulation, southward shift of the growth would be lessened. Increased radiation, polar front, and weakening and penetration of on the other hand, would increase the circula- the subtropical high and trade-wind belt as ex- tion and storminess; the intensified cloud cover plained by Flohn (1952a, p. 161). The second in the middle latitude storm belts would in- phase, with a cold-continental aspect and sulate the ground from the increased solar heat- maximum glaciation in Europe, is not explained ing, and the additional snowfall would build the in detail by Butzer, who appeals to complica- ice sheets at favorable latitudes. The ice sheets tions brought about by the presence of the ice would grow, favored by the self-intensifying sheets. On this basis one could make a case that effects of the glacial anticyclone, until such the maximum coldness of the second part of time as the equatorward extent of the ice and each phase was in fact a secondary result of the still increasing solar radiation caused excess growth of the ice sheets, which had been melting of the ice and thus retreat. Then would started in their advance by the humidity of the follow a warm wet interglacial phase during the first part of the phase and reached their maxi- maximum of the radiation curve. The next mum only after long retardation. Butzer con- glaciation should occur during the descent of cluded that the reversal from dominant me- the radiation curve, leading to a cool dry inter- ridional circulation to latitudinal circulation glacial. was synchronous with the change from moist Simpson's (1957, p. 469 ff.) latest defense of to dry climate within the cold period, rather his hypothesis presents a correlation of the in- than with the temperature change that brought terglacial phases of middle latitudes with the about the end of the cold period. pluvial phases of South Africa rather than with interpluvials, as is usually assumed. His calcula- Change in Solar Radiation as a Cause for tions suggest temperatures in the periglacial Climatic Change region only 1°-3°C lower than the present Possible causes for Pleistocene climatic fluc- rather than the 8°-12°C usually inferred from tuations may be grouped as terrestrial causes geomorphic and paleontologic evidence. For related to changes in topography, volcanic ac- latitudes below 45°, the temperatures calcu- tivity, etc., or as solar causes related to changes lated are as much as 6°C higher than today. in the amount of solar radiation received by the Simpson must therefore depend on the sec- earth at various latitudes. The effects of topog- ondary refrigeration effects of the ice sheets to
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produce the actual cooling recorded in the peri- regions, however, and Wexler's maps show con- glacial region. Geologic support for the hypoth- ditions unfavorable for glaciation in south- esis is generally weak, and its principal merits eastern Alaska during these same times of sun- seem to be in climatologic theory. spot maxima. This serves as an explanation for Willett (1953, p. 61 ff.) accepts the Simpson Lawrence's (1950, p. 217) discovery that recent hypothesis with enthusiasm but has modified minor moraines formed by slight glacier ad- it by suggesting that the radiation changes vances during the past 200 years may be cor- were confined to the ultra-violet range or to related with sunspot minima rather than the emission of charged particles (as in the sun- maxima, as worked out on the basis of tree-ring spot cycle), permitting changes in circulation studies. Extension of this explanation to the intensity without necessarily much heating. Pleistocene may not be justified, however, for Willett (1949) believes that the short-period all the evidence from elsewhere in Alaska sug- changes in circulation patterns as analyzed from gests general synchroneity of glacier advances weather records show relations to changes in here with those of the ice sheets in the Great solar activity as recorded by sunspot cycles; he Lakes region and northern Europe (Karlstrom, suggests that long-period climatic fluctuations 1957). (including those of the Pleistocene) resemble Although the relations between Sunspots and the short-period changes so much that they also large-scale pressure and weather patterns may must be controlled by radiation cycles, of hardly be considered established, ''there are larger magnitude and duration. He shows a enough promising clues, both theoretical and correlation between storminess, glacier reces- observational, to keep at a high pitch the in- sion, and groups of four consecutive active sun- terest of the investigator" (Wexler, 1953, p. spot maxima—the time from 1917 to the pres- 84). One of the weak points of the correlation ent is an example (Willett, 1951). This correla- is that the summer anomalies for sunspot max- tion, however, seems to controvert rather than ima show conditions favorable for ablation support the hypothesis, which requires de- (warm, dry, cloudless) rather than for preserva- creased rather than increased solar radiation for tion of the winter snow cover. Wexler (1956, p. glacier recession. Wexler (1953, p. 84), on the 488-494) therefore essentially discards the high- other hand, shows a correlation between indi- radiation hypothesis and instead favors a low- vidual sunspot maxima and the winter develop- radiation hypothesis based on the periodic ment of high-pressure cells and of excess pre- blanketing of the atmosphere by volcanic dust. cipitation at high latitudes—conditions con- Postulating a 20 per cent reduction in radiation, ducive to glacier expansion. He then makes the which would have a greater effect over con- following disarming statement, however (p. 84): tinents than over oceans, Wexler calculates that a pressure pattern in winter would develop "There is as yet no proof that these particular that would result in heavy snowfall in north- pressure, temperature, and precipitation difference patterns are uniquely determined by taking dif- eastern North America and in Norway. Equally ferences between sunspot maxima and minima. Nor important, the summer temperatures would be can such proof be readily produced. It may well be cool enough to inhibit melting of winter snow. that similar patterns can be obtained by selecting Wexler points out that both this hypothesis comparable groups of years at random, without and the sunspot hypothesis depend on ocean- any regard to sunspot number, and taking differ- continent contrasts, so the situation should be ences. When, in fact, such a random experiment different in the southern hemisphere. was performed for the winter and summer half- The natural skepticism about the paradox years, the patterns and magnitudes of pressure "hotter sun, cooler earth", inherent in the differences were remarkably similar to those of sun- spot maxima minus sunspot minima found earlier." Simpson-Willett correlation of glaciation with increased solar radiation, has led to the re- In a later paper, however, Wexler (1956, p. presentation of the opposite view by Viete 481-488) abandons the caution and makes a (1950), who makes a case for the correlation of stronger case for the relation between sunspot glaciation with decreased solar radiation. This number and glacier expansion. His maps of hypothesis assumes that the initial reduction in winter pressure, temperature, and precipita- temperature at high latitudes would result in tion anomalies during the maxima of the last expansion of the polar sea ice and sufficient four sunspot cycles show conditions favorable cooling of adjacent land areas so that ice sheets for glacier growth in eastern Canada and would grow. Despite the initial decrease in northern Europe. The case is different for other poleward temperature gradient, circulation in-
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tensity, and storminess—resulting from the fact Klein (1953), and Weischet (1954), among that radiation changes have greater eifect at others, are inconsistent and range from 3° to low latitudes than at high—Viete suggests that 15° for different climatic regions according to the increased latitudinal circulation resulting different authors. The wide range of variation from greater contrasts in land and sea tem- may be attributed primarily to insufficient peratures would produce the local storminess knowledge of the vertical temperature gradient necessary for glacier nourishment. Once the (lapse rate) for the Pleistocene by which the glaciers reached a moderate size they would snow-line depression is converted to tempera- have secondary cooling effects for further ex- ture depression. pansion, in the manner emphasized by Brooks Frost soils and vegetation, two criteria for (1926, Ch. 1). As the ice sheets reached lower regional temperature, may actually be con- latitudes the meridional temperature gradient trolled in part by limiting conditions of severe would eventually be increased to the point at winter temperatures or length of the frost which the increased circulation and storminess season, and these in turn may be controlled by would bring about the precipitation required ground cooling of sufficient magnitude and for further ice-sheet nourishment. frequency that a mean inversion in the lapse The case for strong secondary cooling and rate, at least in winter, might result. Frost soils self-accelerating glacier growth, however, is not may well be confined to certain favorable lo- so strong as when it was emphasized by Brooks calities and geologic conditions, and their dis- (1926, Ch. 1) as the major control on glacier ex- tribution may be the result simply of more tension. Pertinent here are the arguments of frequent occurrence of ground-radiational cool- Budel (1953, p. 251) in his opposition to the ing. They may thus give a distorted picture of use of the term "periglacial", that the Euro- the temperature depression that actually pre- pean Wiirm climate was not a reflection of vailed at a greater distance above the ground. glacier expansion but was instead a direct re- The distribution of tundra plants is perhaps a sult of the basic climatic change and was only more reliable criterion of regional climate if it locally modified by the presence of the ice is based on numerous pollen diagrams that re- sheet. The most recent climatic curves for the flect upland as well as local lowland vegetation, Wiirm of Europe, as based on the geologic and but again it gives information only about the paleontologic evidence (Woldstedt, 1958, p. climate near the ground. Lapse rates in the 244; Andersen et al., 1960), indicate a long era lower few thousand meters of the present at- of low temperature before the glaciers reached mosphere range from 1.0°C/100 m down to their maximum extent near the end of the even a negative gradient (inversion) during Wiirm, but cause and effect cannot easily be winter. The relations between the observed separated here. Willett (1950, p. 179) in fact gradient and the climatic region, season, eleva- admits that "the question is left unanswered tion zone, and other elements are poorly as to the degree to which the major climatic known, however, and a careful study of these changes of an ice age are a preceding or a con- factors should be made before the appropriate temporary cause of glaciation on the one hand, gradient might be postulated for Pleistocene or a contemporary or subsequent result, on the conditions. The effects of possible Pleistocene other hand". He is therefore forced to base his changes in precipitation on snow line are also opposition to Viete's argument on climatologi- difficult to evaluate. cal theory by insisting that ice-sheet growth Use of the distribution of biologic and mor- can come about only through the vigor of an phologic features as Pleistocene temperature increased meridional circulation and of an ac- indicators is also subject to error. The best celerated condensation cycle as impelled by in- Pleistocene vegetational reconstructions are creased solar radiation. based on pollen analyses (Firbas, 1939; 1950), but frequently the pollen diagrams cannot be Summary and Conclusions dated well enough to establish correlation with Lowering of temperature during the last gla- a particular climatic phase under consideration. cial phase of the Pleistocene has been calculated Also, precipitation changes may have an added from the depression of the snow line in moun- effect on plant distribution. Animals reflect the tains or from the distribution of cold-loving vegetational types and are thus one step further plants and animals and of frost-disturbed soils. removed from the climatic factors. Frost fea- The results, as reviewed in part in recent years tures, as mapped especially by Poser (1948), by Morawetz (1955), Mortensen (1952; 1957), point to low temperature, but only a few are
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reliable indicators of permafrost, and observa- sions do not support the Simpson-Willett tions of present permafrost show that the tem- hypothesis that calls for increase in tempera- perature required for permafrost is controlled ture and precipitation to bring about glaciation as well by many factors of topography, geology, in the middle latitudes. and hydrology. Where not controlled by such geologic or This situation leaves the skeptic with grounds biologic evidence for Pleistocene temperature for ignoring the entire procedure of searching or precipitation values, climatologic theory as for a geologic measure of the extent of tem- applied to any detailed reconstructions be- perature depression in the Pleistocene. Yet the comes highly speculative. Potentially the most climatologist must depend on the best geologic productive approach is probably the study of judgment as a base for attempts to reconstruct those recent anomalies in pressure, tempera- the atmospheric circulation patterns for the ture, and precipitation that appear to be favor- Pleistocene. For example, one of the leading able for heavy winter snowfall and low summer solar-radiation theories for Pleistocene climatic temperatures in the once-glaciated regions. changes—that recently defended by Viete Four recent statistically based analyses of such (1950)—is based on an assumption of signifi- anomalies are cited in the present review as ex- cantly lower planetary temperatures during the amples—Leighly's (1949) study of recent glacial phases, whereas the other (Simpson- snowy winters in interior North America, Willett) depends on increase in precipitation Mather's (1954) study of the patterns of the despite slightly higher temperatures. The direc- 1930's, Butzer's (1957a) analysis of recent tion and magnitude of the Pleistocene tem- trends in the Mediterranean, and Wexler's perature changes should therefore be deter- (1956) investigation of the situation during re- mined from geologic evidence because they are cent sunspot maxima. Beyond these, the com- vital to these and other theories. prehensive discussions of Pleistocene climate by The geologic and biogeographic evidence re- Willett (1949; 1950; 1953), Flohn (1952a; viewed indicates that temperatures close to the 1953), Viete (1950), Simpson (1934; 1957), ground in Europe were much lower during the Brooks (1926-1949), and others are largely glacial phases than they are today, and that the theoretical and speculative, and it will be some climate of central Europe was probably drier, time before the cause and course of Pleistocene at least during parts of the glacial phases. The climatic change are established in their mam ocean-sediment studies suggest that planetary details. temperatures were also lower. These conclu-
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MANUSCRIPT RECEIVED BY THE SECRETARY OF THE SOCIETY, MARCH 22, 1960
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