CHARACTERIZING BOULDER DEPOSITION TO ASSESS ROCK-FALL HAZARD IN UPPER PINES CAMPGROUND, YOSEMITE NATIONAL PARK, CALIFORNIA

We used field mapping, x-ray fluorescence (XRF), and cosmogenic ¹⁰Be exposure dating to determine potential sources and ages of boulder deposits in Upper Pines Campground in Yosemite Valley, Yosemite National Park, California. These deposits rest up to 212 m beyond the base of adjacent talus slopes. We consider two possible sources for boulders at this location: (1) glacier deposition during retreat approximately 15-17 ka, or (2) rock-fall deposition after approximately 15 ka. We mapped all boulders >1 m³ with GPS and noted their angularity (ranging from sub-rounded to sub-angular) and volume (ranging from ~1-14 m³). The data generally fall into two spatially distinct subsets in the northwest (NW) and southeast (SE) portions of the study area. Histograms of angularity versus volume show equal distribution of sub-rounded boulders ≥11 m³ between datasets, suggesting that sub-rounded boulders are glacial in origin. In contrast, sub-angular boulders ≥11 m³ occur with a 20:1 ratio SE to NW, showing a strong clustering that suggests a rock-fall origin. We collected five samples from both sub-angular and sub-rounded boulders for cosmogenic ¹⁰Be exposure dating. Preliminary data of sub-angular boulders indicate exposure ages of 10 ± 1 ka, which we interpret as boulder depositional ages. Because these ages considerably post-date the inferred timing of deglaciation, we consider them most likely to be rock-fall. Six in situ XRF samples of Glacier Point Granodiorite (Kgp) contain 56-58 wt% SiO₂, contrasting with previous XRF analysis of Half Dome Granodiorite (Khd) by Gray et al. (2008) that yield 65-70 wt% SiO₂. Boulders from the study area are intermediate at 59-70 wt% SiO₂, suggesting that the rock-fall originated along a marginal contact between Kgp and Khd, located on the east side of Glacier Point at approximately 2000 m elevation. We provisionally conclude that at least some boulders in Upper Pines Campground result from a rock fall from Glacier Point.