RETREAT OF THE LAST CORDILLERAN ICE SHEET FROM THE NORTH CASCADE RANGE, WASHINGTON AND BRITISH COLUMBIA

Retreat of the Cordilleran Ice Sheet (CIS) from the North Cascade Range was complicated by rugged topography and steep climate gradients. The CIS flowed south from near 52° north latitude in British Columbia into the mountains via the Similkameen and Fraser valleys to the north, and from Puget Lowland on the west. At its maximum the ice sheet had a surface elevation of 2200 m in upper Skagit valley. Deglaciation commenced about 16 ka by frontal retreat of ice flanking the mountains and by ice surface lowering within the mountains. Down-wasting of 200 m exposed regional hydrologic divides and stranded ice masses more than 1000 m thick throughout the range. Isolated fragments of the ice sheet disintegrated rapidly from 14.5 to 13.5 ka, with the pattern and timing of deglaciation in each valley controlled by valley orientation, topography, and climate. The return of marine waters to Puget Sound led to the reorientation of ice flow in the lower Skagit and Nooksack valleys. Frontal retreat up these valleys allowed marine water to flood the valley mouths from 13.8 to 13.3 ka. Retreat of CIS remnants in deep mountain valleys was punctuated by millennial scale climate fluctuations that produced multiple recessional moraines and kame terraces from elevations of 200–1400 m. Deglaciation produced short-lived glacial lakes with outlets marked by meltwater canyons cut across divides and valley spurs. Boulder and cobble gravel deposits in upper Skagit valley and bull trout genetics indicate that proglacial lakes in the lower Fraser and upper Okanagan valleys drained south through the North Cascades at the end of marine isotope stage 2. More research is needed to determine the age of landforms and events associated with the rapid decline of the continental glacier.