Abstract

Three major Pleistocene drift sheets preserved along a transect across the southern North Cascade Range are distinguished on the basis of stratigraphic relationships and differences in morphology, weathering characteristics, and soil-profile development. The two youngest drifts have been further subdivided into members representing second-order fluctuations of glacier termini. Deposits of former southeast-flowing valley glaciers in the upper Yakima River drainage basin can be traced across a low divide at Snoqualmie Pass to the west-draining valley of the South Fork of the Snoqualmie River where alpine drift is interstratified with deposits of the Puget Lobe of the Cordilleran Ice Sheet. Preliminary paleomagnetic measurements indicate that the deeply weathered and extensively eroded oldest drift (Thorp) antedates the Brunhes-Matuyama reversal (700,000 yr). Relative-age criteria suggest that the time elapsed between deposition of Thorp drift and the intermediate drift (Kittitas) was substantially longer than that between the intermediate drift and the youngest drift (Lakedale). Soil developed on Kittitas Drift shows pronounced clay enrichment in the B horizon, in marked contrast to the weakly developed post-Lakedale soil, suggesting that the Kittitas ice advances antedate the last interglaciation of the global marine record and therefore are more than 120,000 yr old. On the basis of reconstructed ice gradients, the next-to-youngest member (Domerie) of the Lakedale Drift is believed to correlate broadly with Vashon Drift that was deposited during the last major expansion of the Puget Lobe between 15,000 and 13,500 yr ago. Two more-extensive, pre-Domerie advances of Lakedale glaciers (Bullfrog and Ronald) preceded the maximum stand of the Puget Lobe, as indicated by stratigraphic relationships and reconstructed glacier profiles. A late Lakedale readvance led to deposition of the Hyak Member prior to 11,050 yr ago in valley heads draining high-altitude source areas.

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