Cosmogenic Al-26 and Be-10 in quartz from boulders, bedrock and sandy sedim
ent from 21 small watersheds in the Diamond Mountains batholith, CA, USA, a
nd two small watersheds from the nearby Fort Sage Mountains confirm that ex
posed granitic bedrock and boulders erode more slowly than the catchments i
n which they are found. Exposed bedrock and boulders are more abundant on s
teep slopes and may play an important role in regulating mountain erosion r
ates. Rapid transport of fine sediment on steep slopes exhumes resistant co
restones which accumulate on the surface. The resulting boulder lag apparen
tly shields the underlying soil and bedrock from erosion, even when the bed
rock is deeply weathered and friable. Where steep slopes have an abundant b
oulder lag, they erode as slowly as gentler slopes nearby. In contrast, ste
ep 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 indistinguishabl
e, thus permitting local relief and steep mountain slopes to persist for lo
ng periods of time. (C) 2001 Elsevier Science B.V. All rights reserved.