Abstract

Soils and subsurface rock-weathering features developed in Sherwin and pre-Sherwin tills were studied in the Sherwin Till type locality and in Bridgeport Basin to determine those characteristics useful (1) in distinguishing these deposits from those of Tahoe age, and (2) in subdividing and correlating pre-Tahoe deposits along the eastern side of the Sierra Nevada. To aid in these two goals, laboratory data were obtained on pH, particle-size distribution, two fractions each of free Fe and free Al, and two P fractions, but only data on particle size and one Fe fraction proved to be useful. If stable surface sites are compared, soils formed in Sherwin Till have much better developed Bt horizons, as indicated by clay content, clay films, and redness, than those in Tahoe Till. Grusification of granitic clasts is about the same in both Tahoe and pre-Tahoe tills, but metamorphic and volcanic rocks are much more weathered in pre-Tahoe deposits. The best developed soils thought to be of Sherwin age are those in Bridgeport Basin, but the correlation with the type locality of the Sherwin Till is uncertain. Soils formed in type Sherwin Till are less well developed than those formed in Bridgeport Basin deposits, probably either because the former soils are younger, having formed on an exhumed surface following the removal of the overlying Bishop Tuff, or because of climatic and lithologic differences. Data on a weakly developed soil in the uppermost part of the type Sherwin Till, buried by the Bishop Tuff, help to confirm Sharp's (1968) estimate of the date of the Sherwin Glaciation at about 0.75 m.y. B.P. Type Sherwin Till buries a soil formed in still older till in Rock Creek gorge, and a minimum time for the formation of that soil is about 100,000 yr.

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