ABSTRACT

The glacial chronology of the South Boulder Valley has been studied by the application of several relative age dating techniques with emphasis on soil development. The study area may have been glaciated in pre-Bull Lake time although no deposits of this age have been identified The oldest recognized till is the result of a single Bull Lake advance which moved 18 km from the cirques between 135,000 and 160,000 yr BP based on regional correlations. The Bull Lake moraine is farthest downvalley, lacks high relief, and has the greatest soil development, as is shown by reddish clay-rich B horizons. Expandable clays dominate the clay mineral assemblage in the upper 20 cm. Hornblende depletion of nearly 70 percent also characterize the post-Bull Lake soil in the upper 50 cm.

Nearly 95 percent of the till and outwash in the study area was deposited during two major Pinedale advances. The early Pinedale glaciation probably began 45,000 to 50,000 yr BP. Early Pinedale ice moved at least 14 km downvalley and deposited three moraines lacking high relief. Post-early Pinedale soils are weakly developed and lack strong profile expression. Expandable clays or illite dominate the clay mineral assemblage in the upper 50 cm. Hornblende depletion ranges from 23 to 60 percent The middle Pinedale glaciation is evidenced by one lateral and five recessional moraines. These moraines display a fresher topographic expression and greater surface boulder density than the older moraines downvalley. Post-middle Pinedale soils are comparable to post-early Pinedale soils in terms of dominant soil properties.

Cirque deposits are mostly young (Audubon and Gannett Peak) rock glaciers, protalus ramparts, and talus. Early Neoglacial moraines display a fresh topographic expression and are profusely covered with angular clasts. Post-Early Neoglacial soils also lack distinctive horizonation and have leached profiles to depths of about 50 cm There is no systematic depletion of hornblende in Early Neoglacial tills. Kaolinite and chlorite dominate the clay mineral assemblage throughout the soil profiles studied.