SUPPORTING EVIDENCE FOR A LARGE ROCK FALL 9.6 ± 1 KA FROM GLACIER POINT IN YOSEMITE VALLEY, CALIFORNIA

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 high discharge flood events, (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 postdates glacial retreat from Yosemite Valley, effectively ruling out glacial deposition. Discharge and bed stress calculations indicate that flooding was capable of entraining boulders at confined upstream locations, but it is unlikely to have transported boulders >500 m farther to Upper Pines. 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, and subsequently partially buried by alluvial fan aggradation, modifying the original geomorphic expression.