Surface Macrofabric of Boulder Dominated Desert Mountain Slopes, California, USA

Rhyolite domes formed over a million year continuum in eastern California are used to study boulder dominated slopes. Slopes in this study are steep (～25° to ～35°) and are made of coarse boulder sized blocks. These slopes include well varnished vertically oriented eolluvial deposits that have been likened to relict periglacial stone stripes, or as indicated in this study, are the result of ongoing desert slope processes. The deposits are common throughout the arid southwestern US, but their morphometric character, fabric, and rates of formation have not been assessed systematically.Results indicate that boulder deposits examined here are remnant from the original surface formed during volcanic eruption and that these boulder slope deposits evolve slowly. Grain size, grain shape and grain angularity do not change significantly from genesis to ～0.6 Ma; trends in the data change markedly after that time. Mean eigenvectors indicate a fabric oriented downhill, parallel to the slope,consistent with the visual impression that long thin to plate-like rocks orient themselves similarly; however,fabric is actually randomly dispersed, similar to that at slope genesis, as indicated by the eigenvalue analysis resultants of C and K. Interestingly, grains remain or become more angular over the million-year time scale of the study as they decrease in size,indicating active in situ weathering processes on individual grains; this result is counter to the common assumption that as grains weather they become more rounded over time.

Rhyolite domes formed over a million year continuum in eastern California are used to study boulder dominated slopes. Slopes in this study are steep （-25°to-35°） and are made of coarse boulder sized blocks. These slopes include well varnished vertically oriented colluvial deposits that have been likened to relict periglacial stone stripes, or as indicated in this study, are the result of ongoing desert slope processes. The deposits are common throughout the arid southwestern US, but their morphometric character, fabric, and rates of formation have not been assessed systematically. Results indicate that boulder deposits examined here are remnant from the original surface formed during volcanic eruption and that these boulder slope deposits evolve slowly. Grain size, grain shape and grain angularity do not change significantly from genesis to -0.6 Ma; trends in the data change markedly after that time. Mean eigenvectors indicate a fabric oriented downhill, parallel to the slope, consistent with the visual impression that long thin to plate-like rocks orient themselves similarly; however, fabric is actually randomly dispersed, similar to that at slope genesis, as indicated by the eigenvalue analysis resultants of C and K. Interestingly, grainsremain or become more angular over the million-year time scale of the study as they decrease in size, indicating active in situ weathering processes on individual grains; this result is counter to the common assumption that as grains weather they become more rounded over time.