Timing and processes of deglaciation along the southern margin of the Cordilleran ice sheet

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Abstract
The Cordilleran ice sheet covered the northwest part of the North American continent during the last glaciation (Fig. 1). It developed from an initial core of coalescing mountain glaciers on Vancouver Island and the British Columbia mainland, spreading outward over a period of 10,000 to 15,000 yr. South of the ice-sheet limit, isolated alpine glaciers fluctuated in size, leaving a similar but not identical record of glacial advance and retreat. From the behavior of these glaciers, three questions have been posed and, in part, addressed by previous studies (including Crandell, 1965; Porter, 1976; Hicock and others, 1982; Clague, 1981; Waitt and Thorson, 1983). (1) Why did the Cordilleran ice sheet attain its maximum 5,000 yr later than its smaller counterparts to the south? (2) What was the extent of the alpine glaciers during the ice-sheet maximum, and why were most apparently well back from their maximum position? (3) What physical factors determined the rate and character of final ice-sheet retreat? This chapter approaches these questions by applying current knowledge of glacial mechanics, both theoretical and empirical, to various aspects of the inferred or reconstructed Cordilleran glaciers. The record of advance and retreat should reflect changes in the external environment (regional climate, sea-level changes), the glaciers’ physical responses to those changes, and changes that in turn result from ice growth and decay (isostasy, local climate). Existing data on the late-glacial advance-retreat chronology and climate constrain this analysis and provide an independent check on the conclusions suggested by this approach. Although the primary focus here is on the mechanics of deglaciation, this chapter considers the record of Cordilleran ice advance as well for several reasons. Retreat of most of the other North American ice sheets (Prest, 1969) coincided with the Cordilleran ice’s achievement of maximal advance, about 15 ka (Clague and others, 1980). Retreat of Cordilleran glaciers was neither monotonic nor entirely synchronous, in that some glaciers had retreated from their maximum positions 5,000 yr before others had finished advancing. Finally, the full advance-retreat record characterizes the physical behavior of the Cordilleran glaciers more completely, because the same physical principles determine glacier behavior regardless of the direction of motion of the ice terminus.