Block fields in southern Norway: Significance for the Late Weichselian ice sheet

ATLE NESJE, SVEIN OLAF DAHL, EINAR ANDA & NORALF RYE

Nesje, A., Dahl, S. 0., Anda, E. & Rye, N.: Block fields in southern Norway: Significance for the Late Weichselian ice sheet. Norsk Geologisk Tidsskrift, Vol. 68, pp. 149-169. Oslo 1988. ISSN 0029-196X.

The geographical and altitudinal distribution of block fields and trimlines in southern Norway are discussed in relation to the vertical extent of the continental ice sheet during the Late Weichselian glacial maximum. Inferred from these considenitions and formerly presented ice-sheet phases for the last glaciation in southern Norway, a new model on the Late Weichselian ice sheet is presented. This mod el indicates a low-gradient, poly-centred ice sheet during maximum glaciation with the ice divide zone located eiose to the present main watershed. During the deglaciation, the margin of the ice sheet retreated to the coast and fjord areas of western Norway. This induced a backward lowering of the iee-sheet surface, and the culmination zones in areas with low pass-points between eastern and western parts of southem Norway thus migrated E/SE ofthe present main watershed. During maximum glaciation the are as of greatest relative ice thickness were located to the central lowland areas of eastern Norway, to the Trøndelag region, and along the deeper fjords of western Norway.

A. Nesje, S. O. Dahl & N. Rye, Department of Geology, Sec. B, University of Bergen, Allegt. 41, 5007 Bergen, Norway. E. Anda, Møre og Romsdal Fylkeskommune, Fylkeshuset, N-6400 Molde, Norway.

For decades there has been considerable cirque topography would have been smoothed discussion between botanists and geologists con­ out by the ice sheet(s). cerning whether parts of the highest mountain In addition, autochthonous block fields areas of southern Norway had been completely ('Felsenmeere') and deep weathering of rocks, ice-covered or, instead, if ice-free areas existed both supposed to require a long time for forma­ during the Late Weichselian glacial maximum tion, have been considered as other evidence of (Blytt 1876a, 1876b, 1882; Reusch 1901; Ser­ non-glaciation. The absence of erratics, glacial nander 1896; Kaldhol 1930, 1946, 1950; Nord­ striae and other positive evidence of ice moulding hagen 1933, 1936, 1963; Undås 1942; E. Dahl in the highest block-fieldareas supported the idea 1949, 1950, 1954, 1955, 1961, 1963, 1987; o. of unglaciated areas. However, observations of Holtedahl1953; H. Holtedahl1955; O. Holtedahl glacial striae and erratics in some 'refuge areas' & Rosenquist 1958; Knaben 1959a, 1959b; Gjæ­ were said to disprove the hypothesis in those revoll 1963, 1973; Hoppe 1963; Ives 1966; R. areas. In this paper, data on the geographical Dahl 1972; Mangerud 1973; Mangerud et al. 1979, and altitudinal distribution of block fields and 1981; Sollid & Sørbel 1979; Sollid & Reite 1983; trimlines in southern Norway are collected and Nordal 1985a, 1985b, 1987; Rye et al. 1987; Nesje discussed in a glacial geological/historical con­ et al. 1987; Nesje & Sejrup in press). text. The discussion of possible ice-free areas was initiated by the discoveries of errdemic plant and animal species in NW Scandinavia, which induced many botanists to postulate the existence The distribution of autochthonous of ice-free areas throughout the Weichselian block fields in southern Norway glaciation(s). In order to prove the existence of General description refugia, additional geological evidence was used: Areas characterized by glacial cirques surrounded The definition of block fields used here is that by pinnacle-like mountain peaks were regarded of Fairbridge (1968:351) termed 'felsenmeere'. as former nunataks. It was suggested that if an Block fields normally consist oi in situ angular ice sheet bad overridden these landforms, this boulders and stones. They can be from a few 150 A. Nesje et al. NORSK GEOLOGISK TIDSSKRIFT 68 (1988) to several metres thick and are formed through Table l. References used to map the block-field areas in dif­ mechanical and chemical weathering of the local ferent part� of southern Norway. bedrock (autochthonous block fields). However, Area References intermediate layers in the block fields, char­ acterized by organic and minerogenic fines may Møre-Nordfjord Kaldhol (1930, 1946, 1948) occur. Basal layers lying on bedrock may consist Undås (1942) either of blocks with an in-filling df silt and sand, E. Dahl (1949, 1954, 1955, 1961, 1966) or silt and/or sand without blocks. The fineinter­ Sørensen (1949) stitial material in the block fields may consist of Grønlie (1950, 1953) quartz, smectite and hydromicas. Occasionally P. Holmsen (1951) occurrences of kaolinite, siderite, aluminium and H. Holtedahl (1955) Sollid Sørbel ferroxide/hydroxide are also found. The latter & (1979) Mangerud et al. (1979, 1981) minerals are interpreted to be the result of a Sollid, Carlson & Torp (1980) preglacial weathering (e.g. Roaldset et al. 1982). Sulebak (1?82) A thin and in periods dynamically active ice- or Sollid & Reite (1983) snow cover (nivation), subsequent frost sorting Follestad & Henningsen, (1983) Follestad (1984) and slow downslope movement of the weathered Sollid & Kristiansen (1984) material may form para-autochthonous or alloch­ Klakegg & Nordahl-Olsen (1984) thonous block fields. Follestad (1986) Rye et al. (1987) Nesje et al. (1987) Hordaland Follestad (1972) Jotunheimen Tollan (1963) Geographical distribution Den Norske Turistforening From numerous descriptions of block fields in (1948, 1986) l) Dovre Hogbom (1914) southern Norway (Table and other available Den Norske Turistforening data (pictures, air photographs, oral and written (1952) communication), Nesje et al. (1987) presented a Rondane Barth (1971) map (Fig. l) showing the most extensive block­ Den Norske Turistforening field areas and regions dominated by alpine mor­ (1960, 1984) Østerdalen G. Holmsen (1958, 1960) phology in southern Norway. The geographical Sollid & Carlson (l980) distribution of block fields is independent of the Røros area G. Holmsen (1956) main bedrock regions in southern Norway (Figs. E. Dahl (1966) l & 2), and the most extensive areas of auto­ Hemsedal/Hallingdal Reusch (1901) chthonous block fieldsand alpine morphology are G. Holmsen (1955) S. O. Dahl (1987) located to the Jotunheimen, Rondane, Dovre, Telemark Jansen (1983) upper Hemsedal/Hallingdal, and to the inner Buskerud Kristiansen & Sollid (1985) Nordfjord-Møre regions. Along the Swedish bor­ der the block-field areas are distributed widely, while summits south of Hallingskarvet, with a few exceptions, are not covered by block fields. a.s.l.). Toward the Swedish border in the east, the lower limits of block fields are located c. 1000 m a.s.l. As the autochthonous block fields are situated above certain altitudinal levels, a Altitudinal distribution weathering boundary can be defined. Slope­ The lower limits of block fields in southern related processes have in places transported Norway display a geographically consistent block-field material below the actual weathering pattern. Their lower boundaries gradually slope limit. Where it is difficultto determine the weath­ from the central mountain range of southern ering boundary directly, we have found it most Norway (Jotunheimen c. 2000 m a.s.l.) toward convenient to map this limit by means of the the coastal areas of Nordfjord-Møre (500-600 m 'summit method' (cf. Nesje et al. 1987). Fig. 3

Fig. l. Map showing the most extensive block-field areas (black spots) in southern Norway (slightly modified from Nesje et al. 1987). The location of the profile in Figs. 4 and 14 is indicated. NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Block fields in southern Norway 151 152 A. Nesje et al. NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

Fig. 2. The main bedrock regions in southern Norway.

�---

' \ \ ' l l l

l l \ \ \

l l

l l

Late Precambrian rocks, mainly Permian rocks, mainly lavas sandstone and conglomerate Precambrian gneiss and gabbro Devonian sandstone and conglomerate in Caledonian nappes Precambrian rocks, mainly Cambro-silurian rocks, mainly gneisses and granites p hylli te and mica schist NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Block fields in southern Norway 153

LOCAL PLATE AU GLACIER/ SNO# FIELD WITHOlJT DVNAMIC CONNECTION Wl TH THE "THE SUMMIT METHOD" ICE SHEET Alpine rnorphology

� --- l � .....striated bed�k '-1 �: · · transitiono.l boundary

Fig. 3. Different criteria used for determining the upper ice limit during maximum glaciation(s).

shows the main criteria for determining the upper mountains (c. 2000 m a.s.l.). Deviations from the ice limit in southern Norway. altitude of the weathering boundary may be re­ By plotting summits with and without block" corded if moving too far aside from the profile. fields along a longitudinal profile from the Møre coast across the water divide at Lesja, and extended to the central area of eastern Norway, the lower summits covered by block fields show Possible relationships between a geographically consistent vertical distribution autochthonous block fields and the (Figs. l & 4). Similarly, a north-south profile Late Pleistocene glaciations along the central mountain range of southern Norway (Fig. 5) shows an undulating lower Several mechanisms for the formation of autoch­ boundary of block fields, with the highest elev­ thonous block fields with consequences for the ation of the weathering limit in the Jotunheimen age relationships have been proposed:

111.1.1.

2400 Done . . 2200

hfjordfjtlll • 2000 .. . Rond1nt . . . 1800

1600 Swedish border 1400

�/ / . 1200 / . .

. . 1000

100

·.. 600

400

200

Andtlsnu Oo111bas Ott1 lllleh1111atr HJISI

•- Summits with at*le morphoklgy A-SI.mmits iacking block tields, commonly showing or covered by !.l..§1!y bJock fields glacial sclJpttrtlg or covered by moraine o 50 100 1501\m

Fig. 4. Longitudinal profile showing the weathering limit from the Møre coast (NW) across Tafjordfjella-Dovre toward the Swedish border (SE). The altitude of the present (1931-1960) annua! O"C isotherm (0.65°C/100m) is indicated from meteorological stations along the profile. (Data from the Norwegian Meteorological Institute 1985.) 154 A. Nesje et al. NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

N TRONDHEIM (1) The block fields have no relationship to the - - - - "' "' "' "' Weichselian glaciation(s), and have been devel- o "' • Ol OD o "' • C1> o o o o o o o o o o o o o o o o o o oped during postglacial time in a high-altitude l l l l l l l l l climatic zone TROLLHEIMEN t> : � Snota 116681 : �BIAhe (16721 A possible connection between a high-altitude : �Gjevilvasskammene 116271 periglacial climate and the weathering boundaries SUNNDALSØRA ilotiornet�Trolla OPPDAL t>: 116051 118501 in Scandinavia has been suggested (e.g. Rudberg ' ' 1977). However, the sharp and regionally con­ t> ' sistent weathering boundary suggests that this postulate is unlikely. If postglacial high-altitude 122861 DOVRE � Snøhetta periglacial weathering had explained the occur­ rence and distribution of the block fields, a dose DOMBÅS \,.stygg­ relationship between modern temperature con­ KJØLEN/SKJÅK .,�•in 117251 ditions and the weathering limit should have been t> RONDANE \ �Rondeslottet 121781 expected. The altitude of the present (1931-1960) SKJÅK . annua! ooc isotherm descends gradually south­ OTTA . � . eastward from the coastal areas of Møre, across .... Dl • d e the central mountain range of southern Norway JOTUNHEIMEN �:�Fana- ��li\ te��\�� (2����9, 3·.' rAkenl20681 (Fig. 4). The serrate pattern of the ooc isotherm is HURRUNGANE �>store Skagaslllstind due to local climatic effects at the meteorological :.: 124031 BYGDIN �.: stations along the profile (e.g. different exposure and altitude above valley bottom). The altitudinal zonation of the annua! ooc isotherm, which shows FILLEFJELL t>; Berdalseken 118141 no correlation with the altitude of the weathering i� Ranastongi 119001 boundary, is in agreement with the altitudinal

! 118191 distribution of permafrost in southern Norway. HEMSEDAL t> � Raudebergskarvet � Reineskarvot 117911 King (1986) found that the lower limit of discon­ tinuous permafrost rises from about 1000 m a.s.l. in the Rondane mountains to c. 1200 m a.s.l. in \ Hallingskarvet 119331 t> � Jotunheimen, with the highest elevation approxi­ GEILO 118621 iHardanger jtkulen mately 1600 m a.s.l. at the coast of western Norway.

HARDANGERVIDDA Similarly, annua! mean temperature (1931- 1960) ftuctuationsat meteorological stations along t>HA,rteigen 116901 the profile from the Møre coast extended across the central mountain range of southern Norway, t>Solfonn 11674) do not show any significant relationship with the D>iSandfloegga 117f91 altitudinal distribution of the block fields along HAUKELI f �Gaustatoppen 11883 l the profile (Fig. 6). t>V�ssdalsegga (16581 Frost-shattering is considered the most impor­ tant mechanical weathering process in periglacial climates (e.g. Washburn 1973, 1979). The number l t>Sninuten 11606) of freeze/thaw cycles and the amplitude of the 80km temperature variations are important factors con­ trolling the effectiveness of various kinds of frost action. l In order to test the present intensity of frost­ shattering in periglacial climates and thereby the Fig. 5. A north-south longitudinal profile along the central mountain range of southern Norway. Summits characterized by block tields and/or alpine morphology are marked with black Fig. 6. Annua! mean temperature (1931-1960) fluctuations at triangles, while summits showing glacial moulding are shown several meteorological stations along a protile from the Møre with open triangles. coast extended to eastern Norway. NORSK GEOLOGISK TIDSSKR!Ff 68 (1988) Block fields in southern Norway 155

No. Met. station m a.s.l. -12 -10 -8 -6 -4 -2 o 2 4 6 8 10 12 14 16•c ! .--r--,- ---r-T-- 6099 Vigra 22 :

o : 608 Skodje 26 :

o

602C Stranda 84

606E Valldal 50

605C Tafjord 8

o

587( Oppstryn 201 o

o �572 Bråtå 712 :

552 Fanaråken 2068 o 536 Elveseter 674 506 Lom 382 655 Dombås 643 1660 Fokstua 952 LEGEND Mean annual temperature 1931-1960

miniml.m maximum : : 1367 Skåbu 865

o 1355 Vinstra 241 235( Løken (1/otku) 525 : o

1660. 1655•

1572 • 1506• 1536 • 1355• 5523• 1367• JOTUNHEIMEN

2350• 156 A. Nesje et al. NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

Fig. 7. Autochthonous block fields at the summit of Reineskarvet 1730 m a.s.l. (for location, see Fig. l).

possibility that the block fields may have been onstrate that Precambrian rocks are very resistant formed during the Late Weichselian deglaciation to weathering under existing climatic conditions. and the Holocene, laboratory experiments have From southern Norway, this is also supported by been carried out by several investigators to glacial striations on rock surfaces in high-altitude quantify frost-shattering as a function of climatic climatic zones above the reconstructed Younger conditions, bedrock and time (for references, see Dryas glacier surface, which are well preserved Lautridou & Sepplilli1986). The intensity of frost­ and have been exposed without significant alter­ shattering and the amount of debris produced ation for more than 10,000 years. This is also from quartzite, granulite and Rapakivi granite demonstrated by the extensive block fieldson top samples from northern Finland were studied with of Reineskarvet 1730 m a.s.l. (Fig. 7), which laboratory experiments run for 1103 temperature strongly contrast the well-preserved glacial stri­ cycles from + 1.5 to -8°C (Lautridou & Sepplilli, ations on bedrock exposures 1740 m a.s.l. west of op. cit.). Their laboratory experiments dem- Hallingskarvet (Fig. 8).

Fig. 8. Striated bedrock surface at the western part of Hallingskarvet 1740 m a.s.l. The crimpassand pens indicate differentice ftow directions (for location, see Fig. 1). NORSK GEOLOGISK TIDSSKRIFr 68 (1988) Block fields in southern Norway 157

Consequently, an actively occurring block-field these summit areas were covered by relatively formation related to the present climatic con­ thin ice sheets on! y. ditions would probably have created more uni­ Sollid & Sørbel (1979) and Sollid & Reite form altitudinal weathering levels all over (1983) argued that the block fields in Møre were southern Norway. In addition, the lowest-lying formed prior to the Late Weichselian and that weathering zonation would probably have been the lower limit of block fields could be used to located to the central and eastern mountains of delineate the upper limit of the Late Weichselian southern Norway, and not along the coast of ice sheet. Møre. (For further discussion concerning the From studies in the Møre-Nordfjord region,E. relationship between block fields and climate in Dahl (1961), Sollid & Sørbel (1979), Sollid & southern Norway, seeE. Dahl 1955, 1961; Nesje Reite (1983), Rye et al. (1987), and Nesje et al. et al. 1987). (1987) concluded that the regional weathering boundary is erosive and represents the upper limit (2) The biock fields are younger than the Late of one or more ice sheets. Weichselian glacial maximum, and are developed Inferred from the large altitudinal difference on nunataks that were deglaciated during the between the weathering boundary and the early Late Weichselian Y ounger Dry as lateral moraines in inner Nordfjord, Rye et al. (1987) and Nesje et al. P. Holmsen (1951) and H. Holtedahl (1955) (1987) concluded that the upper limit of the maxi­ argued for a post-Late Weichselian age for the mum Late Weichselian ice sheet, or a pre- Late weathering products at Gjevilvasskammene in Weichselian glaciation, was responsible for the Trollheimen (Fig. 1). However, N. A. Sørensen formati on of the weathering boundary. (1949), Grønlie (1950, 1953) andE. Dahl (1961) Nesje et al. (1987) suggested on the basis of the · suggested that these weathering products were present knowledge of glacial extent during the formed prior to the Late Weichselian. Weichselian glaciations in southern Norway (Bergersen & Garnes 1971, 1981, 1983; Miller & (3) The block fieldsare older than the Late Weich­ Mangerud 1980; Mangerud 1981, 1983; Miller et selian glaciation, but have been covered by a cold­ al. 1983; Sejrup 1987) that summits covered by based, non-erosive ice sheet autochthonous block fieldsbad proba bly not been overridden by an ice sheet since the Saalian glaci­ The sharp and regionally consistent altitudinal ation (before c. 130 ka). pattern of summits with and without block fields in southern Norway suggests that the summits (4) The block fieldsare older than the Pleistocene were not totally covered by a huge and glaciations, and have never been overridden by dynamically inactive frozen ice sheet. If the ice continental ice sheets. sheet had been frozen in the upper part, the regional weathering boundary would probably The areas of most extensive and deeply weathered not have been so regular over the mountain block fieldsin southern Norway are located to the regions. Gently undulating mountain plateaus paleic, pre-Quaternary land surface of Norway, as covered by block fields may, however, have been described by Reusch (1901) and Gjessing (1967). covered by dynamically inactive local snow fields Together with the clay mineral content, this indi­ or minor ice caps. These can have been either too cates that parts of the block fields may have been thin, or periodically/permanently frozen to the formed by chemical weathering and that they substratum, and therefore not able to erode the have been under formation since at !east Tertiary block fields already formed. In parts of central time, as also previously suggested by E. Dahl and eastern Norway, however, the existence of (1961, 1987). erratics above transitional weathering boundaries In its widest consequence, some of the highest indicates that these areas have been covered by and most extensive block fields may never have one or more temporarily/permanently cold based been eroded/covered by the Pleistocene ice ice sheet(s). Thus the upper boundary of erratics sheets. gives the minimum altitudinal extent of the ice Relatively fresh glacial striations of possibly sheets, and not necessarily the upper Late Weich­ Late Weichselian age are frequently recognized selian ice limit. It is, however, suggested that ·just below the weathering boundary, suggesting 158 A. Nesje et al. NORSK GEOLOGISK TIDSSKRIFT 68 (1988) that the Late Weichselian ice sheet may have Jotunheimen. Plant species located within both reached an altitude at, or just below the possible main regions are called bicentric, while species maximum Pleistocene ice limit. Therefore, the growing in only one of them are called northern/ weathering boundary in different parts of sou­ southern unicentric. In southern Norway they are thern Norway may not have been formed con­ restricted to the central mountain areas and the temporaneously, or by the same ice sheets since southern unicentric plants were therefore thought a similar state of dynamic stability may have been to have immigrated fromcoast-/foreland refuges achieved during previous maximum glaciations. along the western coast after the Weichselian East of the main water divide, horizontal glacier glaciation. The geographical distribution of block expansion was probably the main mechanism of fields (Fig. 1), however, indicate ice-free areas in equalization '}nd adaption of some significance the central mountain regions of southern Norway for the ice excess. The excess or deficiency of where the refuge plants are located. An immi­ glacier mass may therefore be recorded as dif­ gration of refuge plants from the coastal to the ferences in the ice-front position along the sou­ central mountain areas to explain the very restric­ thern and eastern margins of the Scandinavian ted distribution of the southern unicentric species, ice sheets. Off western Norway, however, the is therefore not necessary. margins of the ice sheets never reached outside Except the refugeplants found elsewhere in the the edge of the continental shelf due to calving world, some unique Scandinavian species exist. processes (Flint 1971; Weertman 1973; Schytt Knaben (1959a, 1959b) distinguished six endemic 1974). This may have induced more or less the poppy species (Papaver sp.) in southern Norway. same altitudinal distribution of the ice sheets The different species and subspecies were thought along the fjords in western Norway during periods to have developed their own characteristics in when the margin of the ice sheets reached the different isolated populations (Fig. 9). edge of the continental shelf. Nordhagen (1936) suggested that the mountain poppies are old species in the Norwegian flora because of the long time needed for separating reproductive species. Nordal (1985a, 1985b, l987) discussed evolution rates of endemic species, and Other evidences for the existence of nunataks during the Late Weichselian glacial maximum

Botanical evidence In Scandinavia, several plant species are located to isolated high-altitude mountain regions. Out­ side Scandinavia, man y of these plants are located to distant regions like the Alps, Kaukasus, Ural, Greenland and North America. To solve this plant geographical problem, botanists at the end of the nineteenth century introduced the 'refuge theory' (Blytt 1876a, 1876b, 1882; Sernander 1896). Instead of having immigrated after the Late Weichselian glacial maximum, several of these plant species were thought to have survived at localities ('refuges') not covered by the con­ tinental ice sheet along the western coast of Norway. The present dispersal was therefore explained by a preglacial distribution across the northern Hemisphere. The refuge plants in Scandinavia are located Fig. 9. The areal distribution of Papaver radicatum in southern within two main regions; North of the polar circle Norway. The area of each subspecies is indicated (numbered and in the southern Norway areas: Dovre, black spots); (slightly modified from Knaben 1959a). Knaben Trollheimen and the northern part of (1959b) postulated one refugium for each of the six subspecies. NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Block fields in southern Norway 159

suggested that the potential evolution rates may shows a strong concurrence with the mountain have been underestimated. In addition, she did regions covered by block fields. Alpine mor­ not tindthe prevailing arguments for rejecting the phology in southern Norway is mainly located to alternative 'tabula rasa' theory, involving total western Jotunheimen and Møre-Romsdal postglacial immigration, biological convincing. (Gjessing 1978). In the eastern part of southern Since it was first presented, the refuge theory Norway (eastern Hallingskarvet, Hemsedalsfjel­ has been strongly debated between biologists and lene, eastern Jotunheimen, Dovre, Rondane), geologists, the majority of the latter group being however, the cirques are commonly not so closely sceptical (e.g. Mangerud 1973). However, the spaced and the undulating and rounded pre-Quat­ strong overlap between the endemic species and ernary land surface is in general better preserved. the distribution of autochthonous block fields In areas with well-developed cirques, the sur­ does not need to imply immigration, neither from rounding mountain peaks are commonly covered coastal foreland refuges nor from areas outside by block fieldsand/or are dominated by pinnacle the marginal ice limit to the central mountain topography in a highly dissected landscape. The regions. This strongly supports the 'refuge theory' most extensive cirques are therefore suggested to as presented by botanists at the end of the nine­ have been formed where cirque glaciers were able teenth century, despite recent objections. to erode throughout the ice ages, and not only at the beginning and end of each glacial cycle. In areas totally overridden by the Pleistocene ice Geomorphological evidence sheets, however, extensive cirques are in contrast The geographical distribution of cirques and either lacking or poorly developed. alpine morphology in southern Norway (Fig. 10)

The continental ice sheet in southern Norway during the Late Weichselian glacial maximum Based on the previous considerations, we pos­ t tulate that autochthonous block fields without il \ erratics or other positive evidence of ice moulding \ on summit areas in southern Norway represent \ i i ice-freeare as during at !east the Late W eichselian glacial maximum. Inferred from the geographical . , and altitudinal distribution of autochthonous ' block fields, we have constructed a model for the \ l ! continental ice sheet covering southern Norway l (-·� during its maximum extent. The model is also l discussed against data concerning the lateral '· ' \ distribution of the ice sheet on the continental .i shelf off western Norway. OSLO The reconstructed ice-sheet model describes a relative! y thin ice sheet controlled by the regional topographical features within southern Norway (Fig. 11). During the maximum glaciation, the ice surface at the main ice divide zone, which was located approximately along the main water divide, formed several ice domes and saddles. The most prominent centre of ice dispersal existed in the 100km Jotunheimen mountains, where the ice sheet

reached its maximum altitude of about 2000 m Fig. JO. Map showing mountain regions in southern Norway dominated by cirques and alpine morphology (slightly modified a. s.l. (relative to present sea leve!). The ice dome from Gjessing 1978). over SW Hardangervidda is reconstructed 160 A. Nesje et al. NORSK GEOLOGISK TIDSSKRIFr 68 (1988)

Fig. 11. Main ftow lines and tentatively reconstructed contour lines (relative to present sea level) of the ice sheet during Late Weichselian glacial maximum in southem Norway. The reconstruction is based on weathering boundaries (suggested nunataks are indicated by black spots), glacial striation and reconstructed ice-sheet profiles. Local deviations from the contour lines may, however, be recorded. The main ftow lines of the supposed Late Weichselian maximum ice sheet in southem Norway are compiled from: G. Holmsen (1915); Gjessing (1960); O. Holtedahl & Andersen (1960); P. Holmsen (1964); Vorren (1973, 1977); Bergstrøm (1975); Vorren & Roaldset (1977); Garnes (1978, 1979); Carlson & Torp (1980); Andersen et al. (1981); Hamborg & Mangerud NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Block fields in southern Norway 161

indirectly from the eastward sloping weathering therefore seems likely. Furthermore, the maxi­ boundary along Hallingskarvet and from NE/ mum Late Weichselian ice limit along the shelf ENE ice movements on the eastern part of Har­ break outside Møre (Andersen 1979, 1981) has dangervidda (Rye & Follestad 1972; Vorren been generally accepted. Data from Scotland 1977). (summarized by Sutherland 1984) and the central The presented model' suggests that the ice sur­ North Sea (Sejrup et al. 1987) suggest that the face was considerably lowered along the deeper Scandinavian and the Scottish ice sheets did not fjords of western Norway. This is well coalesce in the central North Sea, indicating areas documented from autochthonous block fields at of dry land and an open embayment in this area. the southern part of Hardangerfjorden, and in These latter ideas of a smaller extent of the the ioner part of Sognefjorden. These obser­ Late Weichselian ice sheet in the North Sea, vations are also consistent with the strongly con­ compared to previous reconstructions, fit well vergent ice movements toward the ioner part of with our model for the ice sheet over the mainland Sognefjorden during the supposed Late Weich­ areas, and make it possible to tentatively recon­ selian glacial maximum (Vorren 1973, 1977; Aa struct the Scandinavian ice sheet over southern 1982). Effective ice drainage along the deeper Norway and in the North Sea (Fig. 12). main fjords is suggested to have prevented vertical Fig. 13 shows a tentative profile for the glacier build-up of greater ice thickness in the sur­ surface from central south Norway across Ska­ rounding mountain areas, as also concluded by gerrak toward the Main Stationary Line in Nesje et al. (1987) from studies in the Nordfjord­ Jylland, Denmark. The profilefollows the recon­ Møre region, structed flow lines from striations. In the Ska­ At Sørlandet (Fig. 1), divergent ice movements gerrak area, however, the flow lines of the ice parallel to the main valleys dominated. In the sheet are not evident. A relatively thin ice sheet central lowlimd areas of eastern Norway, above the overdeepened, glacially shaped however, an extremely flat ice sheet (approxi­ Norwegian Trench in the Skagerrak, with water mately 3-5 m/km), with convergent ice move­ depths exceeding 700 m (e.g. Thiede 1987), may ments toward the outer Oslofjord-Skagerrak suggest that the ice movements at !east in the area, are in agreement with the reconstructed ice basal parts of the ice sheet were defiected more movements (e.g. O. Holtedahl & Andersen 1960; or less parallel with the Norwegian Trench. For Sørensen 1983). this reason, the ice-sheet profile may have During the Late Weichselian glacial maximum, deviated from the supposed flowlines in the Ska­ the margin of the Fennoscandian ice sheet was gerrak, as indicated in Fig. 13. previously supposed to have been situated along Important causes for the shape of the ice-sheet the edge of the continental shelf seaward off surfaces may have been: western Norway and Britain (Boulton et al. 1977, 1985; Andersen 1979, 1981). This in contrast to (l) The growth of the Late Weichselian Scandi­ the proposal of Jansen (1976), who suggested ice­ navian ice sheet caused an increasing dominance freeareas in the southern North Sea, while areas of high atmospheric pressure zones above the north of 60°N were ice covered. central parts of the ice sheet. At the same time, Boulton et al. (1985) presented two alternative a more or less seasonally permanent sea-ice cover models for the ice extent. In their minimum model was probably established in the Norwegian Sea. of the Scandinavian and the British ice sheets As a result, the eastbound Atlantic cyclone tracks they took into consideration basal shear stress moved southward during expansion of the ice conditions according to the substratum. Their sheet (Liljequist 1974; Gates 1976). Thus defi­ resulting model implies two separate ice sheets ciency of precipitation in the central and north­ which did not coalesce in the central North Sea. eastern regions of southern Norway probably In Jutland, Denmark, the northern part of the prevented the ice sheet from achieving a 'steady Main Stationary Line has an east-west direction state' ice-sheet profile. (See also the recently and strikes toward the Lille Fiskebank Moraine developed model for the Late Weichselian ice (see Fig. 13). A correlation of the two moraines sheet in southern Sweden (Lagerlund 1987)).

(1981); Aa (1982); Bergersen & Garnes (1983); SoUid & Kristiansen (1984); Klakegg & Nordahl-Olsen (1985); Kristiansen & Sollid (1985); Fareth (1987); Rye et al. (1987); Nesje et al. (1987); S. O. Dahl (1987). 162 A. Nesje et al. NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

Fig. 12. Tentative contour-Iine reconstruction of the Late Weichselian ice sheet in southern Norway and in the North Sea during its maximum extent. The ice limits in the North Sea and Scotland are from Sutherland (1984) and Sejrup et al. (1987).

(2) Due to the short time span (c. 10,000 years) Consequences for the ice between the Ålesund Interstadial (Mangerud et movement phases in southern al. 1981; Larsen et al. 1987) and the Late Weich­ Norway inferred from the ice-sheet selian glacial maximum, the build-up of the ice sheet may have stagnated before its potential mod el vertical extent was attained. The presented model has important consequences for the age of the meltwater- and ice drainage (3) Effective icedrainage along the deeper valleys phases in southern Norway during the Late and fjords of western Norway prevented vertical Weichselian deglaciation. build-up of the ice sheet. So far, the prevailing model describes a main ice drainage phase prior to the Late Weichselian (4) Fast and low-friction glacier movement due maximum with ice movements controlled by an to low basal shear stress in areas covered by ice divide approximately along the main water sediments, and especially across deformable sedi­ divide of southern Norway (Phase Il of Vorren ments on the shelf. 1977; phase B of Garnes & Bergersen 1980). As the ice sheet expanded to its maximum extent, the ice divide migrated E/SE, accompanied by W /NW ice movements toward NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Block fields in southern Norway 163

�3 o� ou, z 1:- J l l \ l , l l l \

l l l From Sejrup et al. l .... � .... (1987) l l � l , .... l l \ l l l l l l l l l on fi ' l : Profile l : Fig. 15 l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l l '{li �l �l l l �� );l al The North Sea l l 'l �--� 1 ml l �3: l � � l �(/j J �-i � �· 15' "'Q4-�� ;"!.!f �� 1- l

Fig. 13. Tentatively reconstructed ice-sheet profile for the Late Weichselian glacial maximum from Hallingskarvet via Oslofjord and extended to the Main Stationary Line in Jylland, Denmark. The Younger Dryas glacier profile is also indicated (from S. O. Dahl 1987). If the main ice ftow from the Oslofjord area was conducted by the Norwegian Trench along the coast of Sørlandet, the Lille Fiske bank Moraine (Andersen 1979) should be regarded as a lateral moraine to this ice ftow. The location of the profile from the central North Sea to Hardangervidda in Fig. 15 is indicated. 164 A. Nesje et al. NORSK GEOLOGISK TIDSSKRIFf 68 (1988)

Late Weichselian maximum m a.s.J. ice di vide zone 2400 : :Dovre 2200 \ \: .. ;ratjordfjella: • 2000 • : Rondane ,. : ••.._ • • ... • ·l l ... 1800 • ... • : ... Romsdal • � _ � _ •• • • Late deglaciation ... 6 -a.._ _t...._ ice di vide 1600 • �___..-= __ - ... •� Swedish ... _.",",...�>- ... ---_.b..._t;--,.-_-:.� !... : .. '""'3m/km border • //'' .. 1400 -- A å6 å �---·- • / /< /";;. - =---= �� 1200 - :- � �?��- :-._ - - -- -:.._ _ ·�� - -:.._ -- . . --- :...::. ------� �-- -::--- - - ..... _...Theoretical- ...... gla cier profil e§.. -;:--- 6 :::::-...... �..,!� "':... -- ..t. � - 1000 • "'t:. 6 "åuring/ �he recession and - .... • .....,9m/km / / .... _ ..... / _ _ _ _ _ 6 ...-,downmeltmg of the .".." " _ .....:;_ ...... �.::_::::- 800 , .... 4 A Store _ - ...-sheet . . The MØ' re ) / le; / - ...... "'"' / "'/ ���!��!�' .... - -- 6 ...... 600 coast :"' / ,. / . -...... / / �/ passpomt .,. "'· 6 " .... / / .... "..... �' .. ' Lesjaskog .... / .... / 400 "' / ' / ,.. / ' .,...." / / / / ' / 200 ... ' / / Åndalsnes Dombås Otta Lillehammer Mjøsa' o .. Summits with alpine morphology A Summits lacking block fields, cornmonly showing coveredby in situ block fields glacial sculpturing or covered by fTlOfaine o 50 100m Of /

Fig. 14. Longitudinal profile from the Møre coast (NW ) across Tafjordfjella--Rondane and extended to the Swedish border (SE),

showing reconstructions of different/ tentative glacier profiles subseque.1t to the Late Weichselian glacial maximum. The profiles demonstrate the suggested displacement of the ice divide during the period of marginal retreat and backward lowering of the ice sheet.

the main water divide (Phase Ill of Vorren 1977; i987), but bad in this area less inftuence on the phase C of Garnes & Bergersen 1980). ice thickness in the central mountain regions of However, according to the presented model, southern Norway. This was mainly due to the west/northwesternice movements could not have longer distance between the accumulation zone been possible E/SE of the watershed before the and the calving area in Skagerrak and Oslofjord glacier surface at the present watershed bad been (e.g. Hogbom 1885). lowered relative to the areas in E/SE (Fig. 14). During phase D of Garnes & Bergersen (1980) As a result, phase II/B and not phase Ill/C may and Bergersen & Garnes (1983), a culmination represent the period of maximum glaciation. zone was located across Vinstra in Gudbrandsda­ Phase B, with great glaciogeological inftuence, is len (Fig. 1). During this phase, the ice divide zone also called the 'Main Phase' in the Gud­ sloped toward NE, most probably caused by a brandsdalen region (e.g. Garnes & Bergersen more rapid lowering of the ice surface in the 1980). eastern than in the western parts. In addition, The model presented in this paper therefore however, glacier supply from eastern Jotun­ suggests that phase Ill of Vorren (1977) is from heimen during this phase may have contributed a period of marginal retreat from the edge of the to this gently sloping glacier surface toward NE. continental shelf to the fjords and inner valley This is demonstrated by the ice and meltwater areas of Møre, during which the ice divide was ftow patterns, which show drainage nearly lowered rapidly. This late glaei al retreat may have radially from a culmination zone in the SW led to a backward lowering of the ice-sheet (Jotunheimen), contemporaneously with ice surface, causing a migration of the culmination drainage froma culmination area in NW (Skjåk). zone(s) c. 100 km toward S/SE over the northern In the NE parts of southern Norway the surface Gudbrandsdalen region. The ice marginal retreat of the continental ice sheet sloped toward NW, as may have been caused by a slight initial with­ shown by the sequence of deglaciation in northern drawal from the grounding line at the edge of the Østerdalen and in Rondane (G. Holmsen 1915; continental shelf. This may have resulted in a Mannerfelt 1940; Strøm 1956; Gjessing 1960). break-up accompanied by a rapid terminal retreat When the ice surface bad lowered to 1100 m a.s.l. to the next anchor points at shoals, headlands or at Dovrefjell and Drivdalen, a great amount of constrictions along the coast of western Norway ice and water drained toward Dovrefjell and (Fig. 14). The same process also prevailed along Drivdalen (Fig. l) (P. Holmsen 1964; Sollid the southern margin of the ice sheet (Thiede 1964). This shows that the main culmination zone NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Block fields in southern Norway 165 of the continental ice sheet had migrated south of gradient ice-sheet surface during the Late Weich­ the present water divide at that period (see Fig. selian glacial maximum too. 14). On the Hardangervidda plateau, an eastward According to the presented model, the con­ migration of the ice divide zone of c. 50 km re­ tinental ice sheet during the Late Weichselian corded between phases Il and Ill (Fig. 15), was glacial maximum sloped with average gradients explained by Vorren (1977) as a result of a poss­ of c. 9 m/km toward W /NW in the Møre area and ible glacial surge along Hardangerfjorden. How­ approximately 3 m/km fromthe culmination zone ever, according to the presented ice-sheet model, at Tafjordfjella-Lesja-Dovre toward the SE (Fig. the shift of the culmination zone can be explained 14). The highest-lying lateral drainage systems in by an ice-marginal retreat from the maximum the upper Gudbrandsdalen region (the Nunatak position in the central North Sea to the coast- and phase of Garnes & Bergersen 1980) are parallel fjord areas of western Norway (Fig. 15). The to the reconstructed surface of the ice sheet during marginal retreat of the ice sheet accelerated the Late Weichselian glacial maximum and are 14 000-13000 B.P. (Mangerud et al. 1979; Jansen mapped to approximately two hundred metres & Bjørklund 1985), while the coast of western below the regional weathering boundary. As a Norway was deglaciated during the Bølling result, the NW meltwater drainage could have Chronozone (Mangerud 1977). The climatic started when the margin of the ice sheet had deterioration during the Younger Dryas led to a retreated to the fjord areas of Møre. Conse­ build-up of an accumulation zone along the main quently, the highest-lying meltwater channels watershed, causing a migration of the ice divide may have been formed during the initial degla­ to the west (e.g. Vorren 1977), and an extensive ciation in the later part of the Late Weichselian, glacier readvance took place along the western and not in the Preboreal Chronozone, as pre­ part of Southern Norway (Mangerud et al. 1979). viously proposed by Garnes & Bergersen (1980). In the high-altitude areas of the Gudbrandsdalen region, lateral meltwater channels dose to the pass-points show that the gradients of the inland Summary and conclusions ice sheet during the early or middle phase(s) of (1) Autochthonous block fields in southern the deglaciation were lower than 0. 5% proxi­ Norway are located to the Jotunheimen-Dovre­ mally, and even less distally. This flat surface Rondane-upper Hemsedal/Hallingdal areas, and of the inland ice sheet strongly supports a low- to the inner Nordfjord-Møre regions. The lower

PHASE: I/IV Ill � -lee divides-- �

Hardanger- jll kulen m a.sJ Hallingskarvet _ .. _ .. +· . . . 2000 Folgefonna .. _. ·- • 4- A "'t> � -- " " = -"""' "" � --�;.: '.=.:t>·_·::::. ;:_:_:_;���------. 1600 A f\-" A ,.k'" A /\,...... � 1\ A A 1:'--A ;(> �-- " .K' A 1'.... �" " 1\ A /\A M A A • 1200 • ,o��e , \C!J'O\'\ ,..,.,t- /.· Hardangervidda �� • M �o •'" �"!�!/' - · e<_,v G-•"''�- -· • s��-- · " �"'"'"'�-..<-q, - 800 " '$.\ee�.... a·i�ø.:'..�!:...... AA A \vtt/9J \&'-e ".., "d' .,..;,;..., e·"· • ..,8·1cns•l��� � • • • • 400 " <;>' e� fi • Eidfjord d La\O ·� •• • • • .J.. �-<.ef'' • • SuQ,Q.•.S.\! .. • ' • • BERGEN ·-=:-=------• ---'T he • • • ' ------Nor-gian--J______:______• - O The continental shelf Trench Hardangerfjorden -400

-800

150km -1200

Fig. 15. Longitudinal profile from the marginal position of the ice sheet during the Late Weichselian glacial maximum in the central North Sea (Sejrup et al. 1987) to Hardangervidda. Possible correlations between ice-sheet profiles and ice divides during phases II-IV on Hardangervidda (Vorren 1977) are tentatively suggested. (For location of the profile, see Fig. 13.) 166 A. Nesje et al. NORSK GEOLOGISK TIDSSKRIFT 68 (1988) boundaries of block fields and trimlines show in the Gudbrandsdalen and Hardangervidda a regionally and altitudinally consistent pattern, regions, respectively. This induced glacier and and describe an erosive weathering boundary meltwater transport toward the main watershed. indicative of the upper limit of one or more ice In a four-phase model for southern Norway, the sheets. The autochthonous block fieldswere prob­ ice drainage phases 11/B and Ill/C of Vorren ably not overridden by at least the Late Weich­ (1977) and Garnes & Bergersen (1980) are most selian ice sheet. It is, however, possible that some probably from the Late Weichselian glacial maxi­ allochthonous and para-autochthonous block­ mum, and from the early deglaciation period, field areas above the regional weathering bound­ respectively. ary were covered by local snow fields or minor By considering the areas which are included in ice caps. These may have been either too thin to the model, regions with E and SE migrating ice erode the block fields already formed, or have divides are undoubtedly closely related to low been periodically /permanently frozen to the sub­ pass-points between eastern and western parts of stratum, and therefore not able to erode the southern Norway. already ex1stmg block fields completely. However, this can explain the existence of locally The result of the presented model for the degla­ derived blocks within para-autochthonous block ciation postdating the Late Weichselian glacial fields, which are transported either by local maximum is in agreement with slightly modified plateau/cirque glaciers or by nivation processes. ideas as presented by Hansen as early as 1886 and 1890. Since the presented model only gives a (2) The presented ice-sheet model suggests a low­ regional overview, there might be local deviations gradient, poly-centred ice sheet with the main ice from the proposal. More detailed field work is divide zone located dose to the main watershed, therefore required to date the weathering bound­ and with local domes above pla tea u areas between ary more accurately, and to verify the model in valleys and fjords. The highest ice-sheet surface different parts of southern Norway. was located to central Jotunheimen and along the central mountain range. However, the regions of Acknowledgements. - We thank all those who provided infor­ maximum relative ice thickness were located to mation through oral and written communication about the areal the central lowland area of eastern Norway, to distribution of block fields in southern Norway. E. Irgens and J. Ellingsen are acknowledged for drawing the Figures. the Trøndelag region, and along the deeper fjords of western Norway. Undoubtedly, this must have Manuscript received December 1987 had a significant effect on the pattern of glacio­ isostatic depression in southern Norway.

(3) The model explains the distribution of alpine References landscapes and cirque topography in southern Norway. Aa, A. R. 1982: lee movements and deglaciation in the area between Sogndal and Jostedalsbreen, western Norway. Norsk Geologisk Tidsskrift 62, 179-190. (4) The geographical location of refuge plants Aa, A. R. & Mangerud, J. 1981: Glasialgeologi og veg­ shows a remarkable overlap with the distribution etasjonsinnvandring i Indre Nordhordland, Vest-Norge. Nor­ of the supposed ice-free areas covered by auto­ ges geologiske undersøkelse369, 33--75. chthonous block fields. Therefore, the distri­ Andersen, B. G. 1979: The deglaciation of Norway 15,000-- 10,000 B.P. Boreas 8, 79-87. bution of these species does not need to imply an Andersen, B. G. 1981: Late Weichselian lee Sheets in Eurasia, immigration to the central mountain regions of Greenland and Norway. In: Denton, G. H. & Hughes, T. J. these plants neither from coastal fore land refuges (eds.): The Last Great lee Sheets, 20-27. John Wiley & Sons, nor from areas outside the marginal ice limit. This New York. Andersen, B. G., Nydal, R., Wangen, O. P. & Østmo, S. R. strongly favours the 'refuge theory' as presented 1981: Weicheslian before 15000 years B.P. at Jæren-Karmøy by botanists at the end of the nineteenth century. in southwestern Norway. Boreas 10, 297-314. Barth, E. K. 1971: Rondane. Norges nasjonalparker l. 2nd (5) As a result of a backward lowering of the ice ed., 127 pp. sheet during the ice marginal retreat to the coast Bergersen, O. F. & Garnes, K. 1971: Evidence of sub-till sediments from a Weichselian Interstadial in the Gud­ and fjord areas of western Norway, the main brandsdal valley, Central East Norway. Norsk geografisk culmination zones migrated toward SE and E Tidsskrift 245, 99-108. NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Block fields in southern Norway 167

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