Stepped Topography of the Southern Sierra Nevada, California
Irregular steps characterize the topography on granitic terrane on the west slope of the southern Sierra Nevada. These steps are a few hundred feet to a few thousand feet high, one-quarter to 5 miles wide, and up to 10 miles long. Most steps face the San Joaquin Valley, but others line the canyons of the major rivers, facing the streams. Part of the eastern edge of the San Joaquin Valley is a smooth plain bevelled across granite, and has an origin similar to the steps. Outcrops are common on the fronts of the steps, and near the outer edges of the step treads, but are rare on the back parts of the treads, which are underlain by disintegrated granitic rock as much as 100 feet thick. Treads tend to slope back toward the next higher front. The stepped topography is confined to granitic rocks, and is believed to result primarily from the much more rapid weathering of granitic rocks where buried than where exposed. Weathering is predominantly by partial alteration and expansion of biotite, which shatters the rock. The disintegrated rock can be moved readily by small streams. The unweathered outcrops exposed by accelerated erosion act as local baselevels, because their large joint blocks cannot be moved by even the largest streams. Alternative hypotheses include faulting, differential erosion due to variations in bedrock lithology or in spacing of joints, and parallel retreat of the fronts, with the treads as piedmonttreppen . Evidence is presented that renders each of these hypotheses doubtful. The proposed hypothesis raises questions about the validity of ancient erosion surfaces in the Sierra Nevada.