Late Pleistocene glaciations in the northwestern Sierra Nevada, California

Pleistocene fluvial landforms and riparian ecosystems in central California responded to climate changes in the Sierra Nevada, yet the glacial history of the western Sierra remains largely unknown. Three glacial stages in the northwestern Sierra Nevada are documented by field mapping and cosmogenic radionuclide surface-exposure (CRSE) ages. Two CRSE ages of erratic boulders on an isolated till above Bear Valley provide a limiting minimum age of 76,400 +/- 3800 Be-10 yr. Another boulder age provides a limiting minimum age of 48,800 +/- 3200 Be-10 yr for a broad-crested moraine ridge within Bear Valley. Three CRSE ages producing an average age of 18,600 +/- 1180 yr were drawn from two boulders near a sharp-crested bouldery lateral moraine that represents an extensive Tioga glaciation in Bear Valley. Nine CRSE ages from striated bedrock along a steep valley transect average 14,100 1500 yr and suggest rapid late-glacial ice retreat from lower Fordyce Canyon with no subsequent extensive glaciations. These ages are generally consistent with glacial and pluvial records in east-central California and Nevada. (C) 2002 University of Washington.