GLACIALLY ERODED LANDFORMS IN THE NORTHWESTERN SIERRA NEVADA, CALIFORNIA

Geomorphic features can be reliable indicators of the extent, relative age, and processes of late Quaternary alpine glacial advances, particularly where no glacial maps are available. Unfortunately, glacial reconnaissance mapping based on erosive forms has fallen from favor, although in many alpine areas erosional landforms are the dominant expression of glaciation. This poster presents evidence of glacial erosion in the northwestern Sierra and interpretations of glacial processes inferred from them. Landforms identified in this region for the first time include large roches moutonnées, a 220-meter-high crag-and-tail, and bedrock benches interpreted as former ice-marginal channels. Along the northern divide to the Royal Gorge of the North Fork American River, erosional forms indicate substantial upland erosion. This is in contrast to more common evidence of severe erosion confined to valley bottoms which implies progressive Quaternary canyon deepening.

Geomorphic inquiry historically went through a transition from studies of form to studies of process and stratigraphy. In the late 19th and early 20th centuries, topographic form in the Sierra Nevada was often linked to glacial process by geomorphologists such as LeConte, Gilbert, and Matthes. This practice waned, however, as stratigraphic studies began to focus on questions of Quaternary climate change. Yet, in many alpine regions, stratigraphic evidence is sparse, incomplete, or absent, so maps are not available and interpretations of Quaternary glaciation must be extrapolated over large distances. Extrapolations are dangerous in alpine areas where glacial advances can vary greatly between valleys, no radiometric dates exist, and sedimentary deposits are rare, coarse-grained, and barren of carbon for dating. Cosmogenic radionuclide surface-exposure dating may provide stratigraphic controls in these environments but provide limited information about contemporary processes. Modern glacial-process studies have progressed considerably over the past 50 years and now often allow unambiguous inferences to be made from form. Glacially eroded landforms can provide important information in alpine regions that allows mapping and interpretations of glacial processes.