WEST COAST GLACIATIONS, USA

The major controversies in correlation and timing of the glacial deposits on the west coast have been identified for 80 years or more. Yet, their resolution remains elusive for much of the record. Postglacial advances in the Sierra Nevada occurred beginning ~3 ka and at ~13.5 ka, before the Younger Dryas. The last glacial maximum culminated around 22-18 ka during MIS 2 but began earlier during MIS 3. MIS 6 advances appear to be widespread but poorly dated. The record for the North Cascades differs in that there does appear to be a YD advance, and MIS 5 advances. However, cosmic-ray exposure dating >50 ka has proven highly sensitive to erosion and burial, and correlation to better-dated lake cores is also difficult. Therefore, paleoclimatic characteristics calculated from the paleo equilibrium-line altitudes (ELA) derived from the glacial deposits cannot be fixed firmly in time, and have been hard to correlate from place to place.

Cirque elevations drop from southern California to the Canadian border. Where it has been possible to calculate the ELAs for a large number (70) of east-side Sierran valleys glaciated at different times in Pleistocene (Tioga and Tahoe glaciations), the trend lines drop linearly northward at 3.1 m/km for both advances, probably separated by 100 ka or more, and the ELA values are within 100 m of each other. The ELA and cirque-altitude trends are basically parallel. Even the ELAs for the most extensive (Sherwin, ~820 ka) glaciations are within ~200 m.

Glaciers on the American west coast respond mainly to summer temperatures and winter precipitation. Summertime cloudiness associated with the “Mexican monsoon” should depress southern ELAs, but probably not during glacial maxima. Two types of storm tracks bring winter precipitation to the coast: cold cyclonic disturbances from the North Pacific and subtropical warm fronts. Precipitation is commonly anticorrelated between California and the Pacific Northwest, and if persistent this effect should be preserved as flexure in ELA trends with latitude. However, no flexure is detected within the Sierra Nevada: ENSO effects must average out over time, and there is no evidence of preferential storm landfall latitudes. Although glacier extents have fluctuated widely over the Quaternary, the maximum glaciations have developed under similar climatic conditions.