Semiquantitative Differentiation of Glacial Moraines near Convict Lake, Sierra Nevada, California

Geometry, setting, and lithological constitution make Pleistocene moraines near Convict Lake well suited for development and testing of semiquantitative methods of temporal differentiation. The methods applied consist of surface-boulder frequency, weathering of granitic surface boulders, grain-size distribution in the upper 12 inches of these deposits, and color and pH variations in the upper 24r-36 inches. Weathering of granitic boulders is one of the more sensitive and consistent temporal indicators. It is evaluated on two bases: the preservation of remnants of original abrasion surfaces and the development of fretted forms by granular disintegration. On Tioga deposits in this area, an average approaching 25 per cent of all granitic surface boulders is judged unweathered on the abrasion basis and nearly 90 per cent on the fretted basis. On Tahoe deposits, the average values are about 3 and 50 per cent, respectively. Application of these techniques to moraines near Convict Lake is more than a calisthenic exercise. A huge, 1,000-foot-high lateral moraine extending east-northeast from Convict Lake has been attributed by earlier workers largely to the early Pleistocene Sherwin glaciation, and a massive lobate complex of loop and lateral moraines, due north of Convict Lake, has been regarded as a composite, late Pleistocene, Tahoe-Tioga feature. Semiquantitative data indicate that the lobate complex is the product of a single glaciation, the Tioga, and that the large lateral is primarily a Tahoe feature. Both are thus late Pleistocene and presumably younger than a nearby 285,000 ± 30,000 year old rhyolitic mass. No Tenaya or Sherwin deposits are recognized near Convict Lake, and the semiquantitative methods used are not effective in denning the temporal position of the possibly still older McGee till.