Stevens Pass to Rainy Pass by Fred Beckey

Author:Fred Beckey
Language: eng
Format: epub
Publisher: The Mountaineers Books
Published: 2003-08-14T16:00:00+00:00

GLACIAL HISTORY

Glacial activity has had a continuous series of effects through the Pleistocene epoch and in Quaternary time. The thick Cordilleran Ice Sheet overrode much of the region and overflowed most present topographic divides to feed the deep outlet valleys. Steep slopes accelerated the headward and downward cutting, so that by the time Wisconsin stage ice began to disappear, the glaciers had cut themselves into deep valleys and away from accumulation sources. Thick ice and rapid runoff were highly effective in carving major valleys to base level, a process begun by the mature dissection of stream erosion.

The ice sheet also advanced into the North Cascades from Canada and entered the upper Chelan drainage basin over high passes near the range crest.6 Continental ice also flowed south through the Okanogan Valley, forming broad lobes on the Columbia Plateau, and gouging out the Columbia Valley. This ice flowed into the mouth of Chelan Valley until it met alpine glaciers emanating from the mountain region.7 It is believed the continental ice began its retreat before the Lake Missoula floodwaters last poured across the Channeled Scablands an estimated 14,000 years ago.

Distribution of Glacier Peak tephra indicates that the southern margin of the ice sheet had retreated at least 80 km from its terminal moraine on the Plateau by the time “G” layer was deposited, perhaps between 12,750 and 11,250 years ago.8

The late-glacial advance along the Cascade crest apparently culminated between about 12,000 and 11,000 years ago. At the last glacial maximum, valley glaciers in the North Cascades terminated as much as 50 km east of the drainage divide.9 The glacier that occupied Chelan Valley was the longest on the east slope of the Cascades, and it cut the bottom some 400 ft below sea level.

Various observations point to rapid and extensive ice wastage in Cascade valleys following the advance of about 14,000 years ago; apparently all but the headward portion of most valleys were deglaciated by about 12,750 years ago.10

Quaternary surficial deposits are products of (1) glaciation by multiple advances of the Puget Lobe of the Cordilleran Ice Sheet and the ice-marginal drainage in the lowlands along and west of the Cascade Range front and (2) cirque and valley glaciers and their outwash streams.

Though deeply scalloped by glacial cirques, much of the Cascade crest still bears remnants of its preglacial topography (showing that the range was never completely overridden by ice). After the withdrawal of the regional ice cap, large valley glaciers aligned and straightened trenches by the truncation of projecting spurs and cut off hanging valleys. The absence of Skagit and Chelan Valley moraines is evidence that the ice sheet postdated the most significant alpine phase. Where alpine glaciers have largely disappeared, small lakes often occupy cirque depressions, and some lakes are held behind massive morainal embankments.

The worldwide return to colder climatological conditions (Neoglaciation), after nearly complete retreat of glaciers in middle-latitude areas of North America, is also reflected in the North Cascades. Little Ice Age fluctuations, which began about five centuries ago, reflected worldwide expansion of glaciers.

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