Geophysical Investigation of the Royal Arches Meadow Rock Avalanche in Yosemite Valley – CA

Since the retreat of the Last Glacial Maximum (~15,000 years ago), rockfalls have been the major force shaping Yosemite Valley, California. Rock avalanches are an especially large rockfall/rock slide that extends far beyond the cliff where they originate. These events are infrequent, but can reach hundreds of meters into the valley, and deposit an extremely large volume of debris when compared with regular rockfall events. Yosemite Valley is home of at least ten rock avalanche deposits, with the Royal Arches Meadow rock avalanche (RAMRA), situated in eastern Yosemite Valley, being the oldest event (~14,000 yr BP). Because this event occurred shortly after the Last Glacial Maximum (LGM), mapping the interface between this rock avalanche and the underlying valley sediments can give us insights about the valley elevation and overall geomorphic state of Yosemite Valley shortly after the LGM. Holocene aggradation covers parts of the deposit, reducing its surface expression. This represents a challenge for estimating the dimensions of the deposit. To overcome this obstacle, we used a combination of geophysical methods (Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR)) to image the interface between the RAMRA and underlying valley sediments. The strong dielectric permittivity and electrical resistivity contrast between the rock avalanche and the underlying sediments make both electrical resistivity tomography and ground penetrating radar ideal methods for our purpose. This then allowed us to infer that the surface of the valley underneath the Royal Arches Meadow Rock Avalanche is in average 1209m with a variation of +/- 3.2m.