STAGNATION OF THE PUGET LOBE OF THE CORDILLERAN ICE SHEET, NORTHWEST WASHINGTON

Multiple arguments suggest that at the end of the last (Vashon) glaciation the Puget lobe of the Cordilleran ice sheet stopped moving and melted in place for much of its retreat. 1) There are no terminal moraines between Olympia and Coupeville (northern Whidbey Island). 2) The ice flow direction on northern Whidbey Island switched from south to west-southwest, in concert with establishment of a stable ice margin that extended from Coupeville to Victoria, BC. 3) Ice-stagnation features (eskers, cross-flute channels, kame-kettle surface, lumpy ice-ablation surface) are widespread in the Puget Lowland. 4) At the NW corner of the Puget lobe, the outfall of glacial Lake Bretz did not use the lowest-elevation pathway. This requires that the ice surface in the area of outfall sloped down to the east when drainage initiated. This slope is incompatible with an active Puget lobe. 5) As noted by Clague and James, isostatic rebound was exceptionally rapid, suggesting rapid unloading. 6) Coherent elevations of deltas built into glacial lakes Russell and Bretz, during a time of rapid rebound, indicate that the lakes were short-lived. Arguments 1 – 4 are based on geomorphic mapping from high-resolution lidar topography.

Catastrophic retreat of the Juan de Fuca lobe provides a mechanism for Puget Lobe stagnation. Retreat was likely facilitated by positive feedback: floating of the ice sheet margin near Cape Flattery would have reduced basal shear stress, which aided thinning of the ice sheet margin, which caused more of the ice sheet to float. With late-glacial isostatic compensation the now-level floor of the Strait of Juan de Fuca was deeper to the east, towards the ice source, and thus grounding would not have been a brake on this feedback loop.

These inferences improve our stratigraphic framework for hazards and resources investigations in the region. This history may be an instructive analog for possible future events at margins of the Greenland and Antarctic ice sheets.