Chemical Weathering at Low Temperatures

IN 1941 the author had an opportunity to study the small nivation depressions and associated snowdrifts on the north side of the San Gabriel Mountains, just north of Los Angeles. Analysis of ground water collected under the drifts showed that it contained calcium bicarbonate in solution. As a result of this observation, field work was undertaken, in 1947 and I948, in the Snoqualmie Pass region in the Cascade Mountains of Washington, to investigate the problem of chemical weathering at low temperatures. Snoqualmie Pass, elevation 3008 feet, forms the divide between the South Fork of the Snoqualmie River and tributaries of the Yakima River. Northeast of the pass the Cascades are 5500 to 7000 feet high; to the southwest they are 4500 to 6000 feet. The ridges trend in a general northwestsoutheast direction and are markedly asymmetrical. On the longer and more gentle northeast slopes are numerous cirque and nivation depressions, some of them filled with lakes. Talus slopes have been built up around the bases of the rock walls of the cirques. The southwest slopes are short and steep, and the absence of cirques, nivation depressions, and deeply eroded stream valleys leaves them relatively smooth and unbroken (Figs. 2 and 3). In Snoqualmie Pass the total precipitation is 87 inches. Fifty per cent is in the form of snow, the yearly fall averaging 398 inches. On the ridges 2000 feet above, it is probably much greater. Although the ridges are more or less transverse to the prevailing westerly, southwesterly, or southerly winds, they are not high enough to be barriers to drifting snow. Instead, the heavy snowfall of the windward slopes and the crests is drifted north-northeastward and accumulates on the leeward slopes to great depths, perhaps 40 or 50 feet on the average and greater in depressions (Figs. 4 and 5). The first spring thaw removes the thinner parts of the snow cover and exposes the bedrock to the mechanical weathering caused by alternating freezing and thawing. As a result the surface becomes covered with coarse, angular rock fragments. This process continues longest around the margins of snow patches, where meltwater is abundant, and it has generally been assumed that such mechanical weathering, together with the removal of the

Astrobiology · Earth science · Geochemistry · Physics · Weathering · Environmental Science · Geology · Geology and Paleoclimatology Research · Groundwater and Isotope Geochemistry · Water Quality and Resources Studies

• Cascade alp slopes and gipfelfluren as clima-geomorphic phenomena

  Open Access•Will F Thompson•Erdkunde•1962

  Add to list