Near-surface geophysical imaging of a talus deposit in Yosemite Valley, California

Talus at the base of cliffs in Yosemite Valley, CA, represents rock fall and debris avalanche accumulation occurring since the glacial retreat after the last glacial maximum. This ongoing mass wasting subjects humans and infrastructure to hazards and risk. In order to quantify post-glacial rock-fall rates, talus volumes are needed for the deposits of interest. We used three near-surface geophysical methods (ground penetrating radar, electrical resistivity, and seismic refraction) to locate the basal contact of talus below Glacier Point, near Curry Village in the eastern Yosemite Valley. The coarseness of the talus deposit limited our ability to use these methods in some areas, and the geometry at the base of the cliff restricted our ability to conduct seismic refraction and electrical resistivity across the talus-bedrock boundary there. Nonetheless, we were able to detect the basal boundary of talus on top of both bedrock and glacio-fluvial sediment fill. Geophysical imaging revealed an apparent onlapping relationship of talus over aggrading post-glacial sediment fill, and our data support the proposition of approximately 5 m of valley floor aggradation since deglaciation. The bedrock-talus contact is characterized by a dip of 52-64 degrees , consistent with the dip of the cliff surface above the talus apex. Ground penetrating radar and resistivity were the most diagnostic methods, in addition to being the most rapid and easiest to implement on this type of deposit.