Abstract

Bridgeport Basin, east of the Sierra Nevada in central California, contains deposits of the following Pleistocene glaciations, youngest to oldest: Tioga, Tenaya, Tahoe, Mono Basin, and Sherwin (?). Some evidence also suggests an advance between the Mono Basin and Sherwin(?). With respect to the last major interglacial, the first three glaciations are younger, position of the Mono Basin is debatable, and the others are older.

The following conclusions have been drawn. (1) Within Bridgeport Basin, Sherwin(?) glaciers were the most extensive. (2) Since Sherwin(?) time, warping and erosion have caused significant topographic changes in the piedmont area at the east Sierra foot. (3) Bridgeport Valley has been warped down relative to the range front and northward tilting of the piedmont has reversed a stream formerly flowing south into Mono Basin. (4) A gravel filling of Bridgeport Valley occurred to a depth of at least 250 ft (75 m), and locally twice that amount, at about the same time as the Sherwin(?) glaciation. Most of this fill is believed to have been warped down and covered by modern alluvium. (5) Gravels, at least 500 ft (150 m) thick, were also deposited in nearby Huntoon Valley in Sherwin(?) time, at least in part, because of meltwater overflow from a large pool of glacier ice occupying West Walker River. (6) Eastward flow of ice out of Robinson Creek during Mono Basin time built a moraine that escaped burial by the subsequent more extensive Tahoe glacier that flowed north along the present course of Robinson Creek. This fortuitous circumstance facilitates recognition of the Mono Basin glaciation. (7) Ice from Robinson Creek also flowed eastward through the Summers Meadows trough in Sherwin(?) time and probably also within the interval between the Sherwin(?) and Mono Basin glaciations. (8) The Tahoe glaciers of Robinson and Buckeye Creeks coalesced to form a piedmont bulb. (9) Qualitative relations and semiquantitative data indicate that the Tenaya is a valid and distinct glaciation between Tahoe and Tioga, although the small moraines suggest that it was a relatively short-lived event. (10) The size and spacing of recessional moraines demonstrate a reasonably consistent but not identical behavior of Tioga ice streams in the four principal canyons of Bridgeport Basin.

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