The formation of ice-marginal embankments into ice-dammed lakes in the eastern Puget Lowland, Washington, U.S.A., during the late Pleistocene

Large embankments, typically several kilometers in lateral extent and many tens of meters high, choke the mouths of each alpine valley of the central Cascade Range, Washington State, U.S.A., at or near their junctions with the Puget Lowland. They comprise till and bedded gravel, sand, and silt, aggraded into ice-dammed lakes. The embankments lie within the late-Pleistocene Cordilleran ice-sheet limit and so do not mark the location of the ice-maximum terminus. Reconstruction of the subglacial hydraulic potential field indicates that these ice-dammed lakes would have drained subglacially via spillways located near the junction of each alpine valley and the Lowland. Physical processes tended to stabilize the grounding line for each ice tongue close to its respective spillway location. Because sedimentation rates are highest adjacent to the grounding line, subaqueous sedimentation formed a growing embankment there. In some valleys, subsequent subaerial lake drainage or decay of the active-ice dam resulted in late-stage deposition of deltas or valley trains. This analysis of ice-water behavior is based on physical principles that should be generally applicable to any environment where glaciers terminate against ice-dammed bodies of water.