Distribution Mapping of Prehistoric Rockfall Deposits in Yosemite Valley, California (2010)

Undergraduate: Miquela Ingalls

Faculty Advisor: Allen Glazner
Department: Geology

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Rock fall poses an ever-present danger in Yosemite Valley, one of the few places in the world with large numbers of people and significant infrastructure at the bases of tall cliffs. Rock falls have killed two people in the past decade and forced closure of one-third of the popular Curry Village cabins. We conducted a study of prehistoric rock fall deposits below El Capitan and Glacier Point to determine the source area and mode of transport. A rock fall is the free falling of rocks and debris that results in a random distribution of talus. Rockslides occur as a result of slope failure with talus deposits that represent the spatial distribution of the rocks at their source. We used GPS units to map the boulders during transects through the talus deposits at the two sites, and categorized each boulder by size, rock type, and any key features that could help us determine its source on the cliff face. The five rock types found in the El Capitan talus deposits are El Capitan Granite, Taft Granite, Taft diorite, diorite, and aplite. The rock types off of Glacier Point are Half Dome Granodiorite and Glacier Point Granodiorite. The distribution of rock types in the El Capitan deposit was more random than expected which indicates that the mode of transport resembled a fall rather than a slide. From the proportion of each rock type, a map of the source of the fall on the North America wall of El Capitan was constructed and compared to the original map created by rock climbers.