EROSION AT THE SUMMIT: UNLOCKING THE KEY TO CIRQUE FORMATION

Cirques are abundant erosional landforms in glaciated alpine landscapes. The presence of these overdeepened basins at valley headwalls may be key to understanding erosional processes and the formation of relief in alpine landscapes. Though cirque formation is a classic problem in geology, the controls on cirque development, including ice dynamics, subglacial erosion, and headwall backwearing, are not well understood. These features are particularly intriguing because they are often used as proxies for past climate, e.g., cirque elevations are often used to indicate the location of the zero degree isotherm during the Last Glacial Maximum.

We initiated a field study at Grinnell Glacier in Montana to explore the glacial and geomorphic dynamics involved in cirque formation. In the summer of 2004 we installed a GPS unit to track glacier motion, as well as temperature sensors and ablation stakes to measure snow and ice melt. The average ice surface velocity for July and August was between 4.5 and 5.5 cm/day at the center of the glacier, where ice thickness is ~44 meters. The melt rate during this time was 3 to 6 cm/day (water equivalent). Both the surface velocity and melt were greater than those measured during past studies of Grinnell (e.g., USGS, 1980; Anderson and others, 1982). Possible diurnal variability in ice surface velocity, in addition to calculations of expected velocity due to internal deformation only, suggests that sliding is a significant component of measured surface motion.