Abstract

Large boulders exceeding 10 m3 in exposed volume are widely scattered throughout Upper Pines Campground in eastern Yosemite Valley, Yosemite National Park, California. These enigmatic boulders rest up to 330 m from the base of adjacent talus slopes but lack geomorphic expressions typical of other large rock fall, debris flow, or glacial deposits in Yosemite. We evaluated four hypotheses for boulder deposition: (1) glacial deposition during ice retreat 15–17 ka, (2) fluvial deposition during a high-discharge flood event, (3) debris flow deposition, and (4) rock fall deposition. We utilized field mapping, spatial analysis, cosmogenic 10Be exposure dating, and X-ray fluorescence analysis to investigate possible modes of deposition. A mean boulder exposure age of 9.6 ± 1 ka considerably post-dates glacial retreat from Yosemite Valley, effectively ruling out glacial deposition. Discharge and bed stress calculations indicate that although flooding could have been capable of entraining boulders at confined upstream locations, it is unlikely to have transported boulders as far as the Upper Pines area. Slope comparisons and evaluation of surface morphology of debris flow fans in Yosemite Valley suggest that the boulders did not result from debris flows. Geochemical results identify a majority of boulders in Upper Pines as granodiorite of Glacier Point, corresponding to bedrock samples located at the summit of Glacier Point. We interpret boulders in Upper Pines Campground to result from a single large rock fall event originating from the east face of Glacier Point circa 9.6 ± 1 ka; they were subsequently partially buried by alluvial fan aggradation, modifying the original geomorphic expression.

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