Morphogenesis of Granitic Boulder Slopes in the Mojave Desert, California

Examination of quartz monzonite residuals in the western Mojave Desert indicates that current weathering of exposed granitic rocks is not renewing the boulder mantles characteristic of this region. Boulder mantles are presently disintegrating to expose massive outcrops that change thereafter only by the detachment of exfoliation shells. The existing boulders were originally isolated as corestones by subsurface chemical weathering and have been exposed by the stripping of a thick weathered mantle formed during pre-Quaternary periods of greater moisture availability. North of the San Bernardino Mountains deep-weathering profiles characterized by brick red surface horizons, rubefied calcrete crusts, and the presence of core-stones in a grus matrix are widely encountered beneath basaltic remnants having radiometric ages in excess of 8 m.y. Continuity between these relict weathering profiles and present boulder mantles establishes the boulders as inherited features. The gradual onset of aridity during the later Tertiary terminated subsurface weathering and accelerated surface erosion, bringing about the massive exposure of Tertiary corestones and the joint-controlled Tertiary "weathering front." Slope retreat and pedimentation occurred here in the presence of a soil cover; thus the extensive granitic pediments of the region are also relict forms inherited from the Tertiary landscape.