PORTER’S CIRQUE FLOOR ALTITUDES AND THE EXTENSIVE MIS 5-3 GLACIATION OF THE OLYMPIC PENINSULA, WASHINGTON
Recently expanded radiocarbon and luminescence chronologies of Late Pleistocene glaciation on the western slope of the Olympic Mountains demonstrate that ice extent during several MIS 5-3 glacial expansions was substantially more extensive than MIS 2 ice extent, and that the large valley glaciers expanded rapidly to their terminal positions and quickly retreated. Well-developed chronologies in the Hoh and Quinault valleys indicate maximum Late Pleistocene glaciation during MIS 5/4, near-maximum MIS 3 glaciation, and restricted MIS 2 glaciation. Published fossil pollen and beetle analyses indicate that maximum temperature depression occurred during MIS 2, with more modest cooling during MIS 4(?) and 3.
We infer that Porter’s steep westward cirque floor elevation gradient reflects effective rain-to-snow conversion with modest temperature depression, coupled with consistently high precipitation. In particular, the dramatic westward depression of the snowline implies that mid-elevation cirques on numerous western Olympic ridges became ice sources during MIS 5-3. As cirques reflect cumulative glacial erosion through multiple glaciations, this pattern has likely reflected the dominant mode of ice growth. During MIS 2, widespread precipitation decreases, inferred from pollen data, suggest that the maximum temperature depression was less effective for snowline depression, and thus was less effective for incorporation of the western cirques into the valley glacier systems.