Seismic explorations on the floor of Yosemite Valley, California

Abstract

The depth and configuration of the bedrock floor beneath Yosemite Valley were determined by seismic surveys in 1935 and 1937. Seismic velocities of roughly 1.7, 2.5, 3.0, and 5.2 km/sec, and good to excellent reflections delineate at least three distinct layers within the valley fill resting on granitic bedrock. The upper layer with a maximum thickness of about 150 m extends from Mirror Lake to the Wisconsin end moraines near Bridalveil Meadow. It is thought to be primarily deltaic lake deposits of Wisconsin age. The intermediate and basal layers have maximum thicknesses of 220 and 300 m respectively, and the intermediate layer lies in a U-shaped trough seemingly gouged out of the basal layer. Both layers are thought to be remnants of earlier lake fillings, and at least the basal layer is pre-Wisconsin. The greatest thickness of fill, about 600 m, is near the head of the valley between Ahwahnee Hotel and Camp Curry. The bedrock floor of Yosemite Valley is an undulating surface with three separate basins and a total bedrock closure of at least 400 m, possibly approaching 500 m. The bedrock floor slopes steeply from the head of the valley to its deepest point, 600 m above sea level, between Ahwahnee Hotel and Camp Curry. Down-valley it rises rapidly about 300 m across a broad sill opposite Rocky Point. The second basin, 800 m above sea level, is opposite Cathedral Spires. From here the floor rises gradually down-valley to at least 1000 m above sea level opposite Artist Creek. It may rise another 100 m before the drop into a small basin more than 100 m deep at the Cascades. The amount of glacial excavation on the bedrock floor, essentially double the 450 m previously estimated, is attributed wholly to pre-Wisconsin glaciation. The greatest depth of excavation is in massive granitic rocks, and it is suggested that thick ice, exfoliation sheeting developed by pressure relief, and compressive flow in the glacier combined at this point to produce exceptionally effective erosion.

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