GLACIAL GEOLOGY OF THE ANTHRACITE RANGE, WEST ELK MOUNTAINS, COLORADO

Geomorphologic features strongly indicate the presence of a complex alpine glacier in the West Elk Mountains, western Colorado, during the late Pleistocene. This project reconstructed a maximum model of a glacial system that originated from both Storm Ridge and the Anthracite Range, using the locations of glacial moraines observed in the field to delineate ice system boundaries. Moraines were identified as oriented ridges composed primarily of boulders, creating a landscape of hummocky topography. In addition to determining the ice margins, the Equilibrium Line Altitude (ELA) was determined based on the elevation where lateral moraines terminated. The ELA is the line between the accumulation and ablation zones of a glacier, which also has paleoclimatic significance. The ELA for this system was 10,400 ft (3171 m). Accumulation area ratio, mass balance calculations, and shear stress values were used as checks to determine if the ice margins and the ELA suggested by field observations were reasonable.

Since there are no modern glaciers in the study area, the climate must have been colder and/or wetter to support these extensive ice masses. Therefore, the established paleo-ELA of 10,400 ft (3171 m) was compared to the estimated 14,000 ft (4268 m) perennial snowline to obtain a Pleistocene temperature depression. The perennial snowline altitude is the modern equivalent of the Pleistocene ELA. When a lapse rate of 6°C for every 1000 m of ELA depression was applied, the resulting Pleistocene temperature depression is 6.6°C. This temperature depression was primarily responsible for the existence of glaciers in the study area, while the precipitation rates are thought to have remained largely unchanged since the Pinedale glaciation (Leonard, 1989).