Modulation of erosion on steep granitic slopes by boulder armoring, as revealed by cosmogenic 26Al and 10Be

Cosmogenic 26Al and 10Be in quartz from boulders, bedrock and sandy sediment from 21 small watersheds in the Diamond Mountains batholith, CA, USA, and two small watersheds from the nearby Fort Sage Mountains confirm that exposed granitic bedrock and boulders erode more slowly than the catchments in which they are found. Exposed bedrock and boulders are more abundant on steep slopes and may play an important role in regulating mountain erosion rates. Rapid transport of fine sediment on steep slopes exhumes resistant corestones which accumulate on the surface. The resulting boulder lag apparently shields the underlying soil and bedrock from erosion, even when the bedrock is deeply weathered and friable. Where steep slopes have an abundant boulder lag, they erode as slowly as gentler slopes nearby. In contrast, steep slopes lacking a boulder lag erode much more quickly than gentle slopes. Boulder armoring can modulate hillslope erosion such that erosion rates of summits, steep mountain flanks, and gentle footslopes are indistinguishable, thus permitting local relief and steep mountain slopes to persist for long periods of time.