PORTER’S CIRQUE FLOOR ALTITUDES AND THE EXTENSIVE MIS 5-3 GLACIATION OF THE OLYMPIC PENINSULA, WASHINGTON

Findings from Stephen Porter’s early Pacific Northwest studies aid interpretation of the unique mass balance patterns and geochronology of major valley glaciers on the Olympic Peninsula. Two years after arriving at the University of Washington, Porter (1964) mapped and interpreted regional Pleistocene snowline patterns as inferred from 409 measured cirque floor altitudes across the Olympic and Cascade ranges. The most relevant finding for our purposes is a clear southwestward steepening of the Pleistocene cirque floor altitude trend across the Olympics, relative to modern glacier snowlines, implying much greater snowline depression on the western slopes of the range.

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.