Abstract

Glaciers shape alpine landscapes. They broaden valley bottoms, enhance local valley relief, generate multiple steps, overdeepen valley floors, and cause tributary valleys to hang. These distinctive glacial signatures result from 104–105 yr of erosion, during which swings in climate drive advances and retreats of alpine glaciers. We use a numerical model of glacial erosion to explore the development of the longitudinal profiles of glaciated valleys. The model is driven by the past 400 k.y. of variable climate. Because both sliding speed, which dictates abrasion rate, and water-pressure fluctuations, which strongly modulate quarrying rate, should peak at the equilibrium-line altitude (ELA), we expect the locus of most rapid erosion to follow the transient ELA. Simulations of a single glacial valley show rapid flattening of the longitudinal profile. Inclusion of a tributary glacier creates a step immediately downvalley of the tributary junction that persists over multiple glaciations and commonly leaves the tributary valley hanging. Steps and overdeepenings result from an increase in ice discharge immediately below the tributary junction, which is accommodated primarily by increased ice thickness and hence sliding rate. The size of the step increases with the ratio of tributary to trunk ice discharge, while the height of a hanging valley reflects the difference in the time-integrated ice discharge in tributary and trunk valleys and therefore increases as the discharge ratio decreases.

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