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Alpine Land Forms of Western United States 1 BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 44. PP. 927-950, 8 FIGS. OCTOBER 31. 1933 ALPINE LAND FORMS OF WESTERN UNITED STATES 1 BY RICHARD JOEL RUSSELL (Presented before the Geological Society, December 28, 1932) CONTENTS Page Introduction ............................................................................................................... 927 Climatic factors......................................................................................................... 928 Geomorphic processes............................................................................................... 930 Groups of land forms............................................................................................... 931 Nivation depressions.......................................................................................... 931 Solifluction slopes............................................................................................... 939 Prevalence of alpine processes................................... ........................................... 944 Overemphasis on glaciation..................................................................................... 946 Possible misinterpretations..................................................................................... 947 Conclusions ................................................................................................................. 948 I ntroduction Land forms of the alpine portions of western United States are ordi­ narily described as though they were chiefly the product of erosional and depositional processes familiar at lower elevations, their characteristic variant being Quaternary glaciation. In reality, this conception is far from representing the actual facts. With all glacial forms subtracted, alpine landscapes would be strikingly different from landscapes of lesser elevation in similar latitudes. In many respects they resemble the desert, but their closest relatives exist in Arctic borderlands. It is the purpose of this paper to describe some of the more common alpine land forms and to emphasize the importance of certain significant processes. It is written in the light of field experience extending through the past 12 years and covering many of the highest regions in western United States, chiefly in California, Nevada, Colorado, and Wyoming. Field work in northeastern California in the summers between 1920 and 1925 served to arouse interest in alpine problems and led directly to 1 Manuscript received by the Secretary of the Society, February 9, 1933. (927) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/44/5/927/3430438/BUL44_5-0927.pdf by guest on 28 September 2021 928 R. J. RUSSELL----ALPINE LAND FORMS practically every conclusion here presented. Intensive field work was carried on during August and September, 1927, in Colorado, primarily for the purpose of checking observations and conclusions. In the sum­ mer of 1928 the writer had the good fortune to conduct Professor Albrecht Penck, of Berlin, across Tioga Pass of the High Sierra of California, in order to discuss with him, on the ground, the major theses here ad­ vanced. These discussions were renewed in Upper Bavaria in the summer of 1931. As the region under discussion owes its individuality principally to its climatic setting, it can best be defined in terms of climate. Using the classification of Koppen,2 a mean temperature for the warmest month between 0°C. and 10°C. may be used to define “tundra climate.” If the mean temperature of the coldest month departs less than 5°C. from that of the warmest month, the climate may be called ‘isothermal tundra,” a type more characteristic of low than of middle latitudes. At Moraine Lake, Colorado, elevation 10,265 feet, the warm month temperature is 13.5°C. and the cold month -6.1°C.; thus, the climate misses tundra classification by a small margin. Pikes Peak summit has a true tundra climate, extending downward to an elevation of about 12,000 feet. The optimum condition for the landscapes to be described occurs in climates of the tundra type. These climates occur only toward the high­ est summits in western United States, starting somewhat above the cold timber line and possibly nowhere extending through a vertical range of more than 3000 feet. Alpine land forms, however, are not restricted to the region of optimum development. In decreasing perfection they ex­ tend downward to “microthermal” 3 climates, where they are gradually replaced by forms characteristic of the ordinary boreal scene. C l im a t ic F actors Most significant among climatic factors controlling denudation and erosion in alpine regions appear to be: (1) Coldness. Winter temperatures remain below freezing for long periods, and summer temperatures, as expressed in monthly averages, are not high. Over extensive regions, temperatures in the shade seldom rise much above freezing. During the season of thaw, freezing temperatures are experienced on most nights. The number of alternations between effective freeze and thaw in the course of a year is large. 2 W. Iitippen : Die Klimate der Erde. Berlin and Leipzig, 1923, pp. 120, 161-167. 3 Coldest month mean temperature below O6 C. but warmest month above 10° C. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/44/5/927/3430438/BUL44_5-0927.pdf by guest on 28 September 2021 CLIMATIC FACTOKS 929 If effective freeze is considered to be a drop in temperature from above 32°F. to below 28°F., and thaw, a rise above 32°F., then Durango, Colo­ rado, from 1921 to 1930 experienced an alternation between effective freeze and thaw on an average of 124 times per year, based on official maximum and minimum thermometer records. On the ground the num­ ber would be higher. Durango lies at an elevation of 6589 feet, well below tundra climate. During 1930, Durango experienced 168 alterna­ tions, and Leadville, elevation 10,248 feet, 185. The writer is now undertaking an investigation of the frequency of freeze and thaw for the whole of the United States, and although the statistical part of the study is far from complete, it now appears that most tundra climate areas experience alternations between freeze and thaw on one-third to one-half of the year. (2) Low evaporation rate. Mainly as the result of low temperatures, evaporation rates are so low in alpine regions that even places experiencing a mean precipitation of only eight or nine inches in the course of the year must be considered as having a humid climate. Even the wind-swept summits lie in a region of rather continuous high relative humidity, as is evidenced by the ease with which banner and other types of clouds are formed. Evaporation rates for the year, 1930, for a station within the Sierra Nevada (on its more arid side) and a station at its western base, are as follows: Monthly Evaporation (inches) Station Elevation May June July August September Tahoe (within Sierra Nevada).......................... 6230 feet 4.12 7.31 7.50 6.12 4.13 Oakdale (at its western base)................................ 215 feet 7.75 14.53 15.18 13.48 8.05 (3) Snowy ness. Most of the precipitation falls as snow. In the extreme "West this condition is accentuated, because winter is the season of maximum precipitation. Thus, in spite of comparatively mild mean annual temperatures, the heaviest snowfall in the United States occurs in the three Pacific Coast States. Even in Colorado, where the precipi­ tation maximum occurs in summer, less than one-quarter of that above Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/44/5/927/3430438/BUL44_5-0927.pdf by guest on 28 September 2021 930 B. J. BUSSELL----ALPINE LAND FOEMS the timber line falls as rain. The proportion of snow and hail increases toward the summits. Mean monthly precipitation (all types, including snow) and snowfall, in inches, for two Colorado stations is as follows: Long’s Peak Lake Moraine (Elevation, 8956 feet) (Elevation, 10,265 feet) Precipitation* Snowfall Precipitation* Snowfall (including snow) (including snow) January....................... 0.82 11.2 0.74 12.3 February.................... 1.20 16.0 0.97 16.8 March......................... 1.92 25.4 1.76 27.3 April............................ 3.07 32.6 3.23 36.1 May............................ 2.33 14.8 2.46 15.8 June............................ 1.62 1.0 2.61 2.9 July............................. 3.23 0.2 4.44 Trace August........................ 2.16 0.0 3.66 0.1 September.................. 1.75 4.9 1.56 2.6 October....................... 1.54 15.4 1.63 16.4 November.................. 0.81 11.0 0.80 12.9 December................... 0.93 13.5 0.89 14.9 * Hail is included in total precipitation but not in snowfall. Ge o m o r p h ic P eo c esses Though the majority of constructional land forms encountered at higher elevations, such as forms of volcanic or tectonic origin, are, in their pristine state, identical with those of lower elevations, their de­ struction involves sequential changes through a series of forms charac­ teristically alpine in nature. Under the climatic conditions outlined above, the denudational processes familiar at lower elevations are, to a large degree, replaced by others, notable among which are nivation, solifluction, and frost action. Nivation refers particularly to the type of rock disintegration that takes place marginally around patches of snow. It is a form of weather­ ing, consummated rapidly, approaching perfection under the favorable combination of sufficient water
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