Development of glacial-valley cross sections under conditions of spatially variable resistance to erosion

Abstract

A simulation model that examines the development of valley cross-profiles as a result of glacial erosion has been constructed by placing a finite-element model for ice flow within an iterative program that modifies channel cross-section form on the basis of an erosion equation. The simulation model has previously been used to examine how different flow laws, erosion laws and ice discharge histories affect medium-scale landform development. This allows us to better understand the development of glacial landforms, and also provides a critical test for erosion and flow laws. Within the erosion component of the simulation program it is possible to specify spatial variations in bedrock erodibility in order to examine the effect of spatially variable lithologies on form development. As an initial test of form development under these conditions a central area of enhanced erodibility is placed in the valley center. In this case active glacial channels develop that are narrower and deeper than those which occur with homogenous bedrock. This change in form occurs as long as the weaker zone is small enough that the active channel does not eventually narrow to a stage such that it is entirely contained within the weaker material, and as long as it is large enough that it continues to have a significant impact on overall discharge through the section. Form variation occurs because the weaker central zone enhances the lateral gradient of the erosion rate, which is critical for form development. A central area of more erodible bedrock changes not only the form of the active glacial channel, but also the convexity of side slopes that are left above the glacier as it erodes downwards into the landscape. The absence of a classic U-shaped cross section should not be taken, a priori, to suggest either absence of glacial erosion or ineffective glacial erosion. Rather, the combination of active glacial and hillslope processes in a landscape with spatially and temporally variable ice discharge and rock characteristics can create a diverse set of cross-section forms.