DEVELOPMENT OF GLACIAL-VALLEY CROSS-SECTIONS UNDER CONDITIONS OF SPATIALLY-VARIABLE RESISTANCE TO EROSION

A simulation model that examines the development of valley cross-profi les as a result of glacial erosion has been constructed by placing a f inite-element model for ice flow within an iterative program that modi fies channel cross-section form on the basis of an erosion equation. T he simulation model has previously been used to examine how different flow laws, erosion laws and ice discharge histories affect medium-scal e landform development. This allows us to better understand the develo pment of glacial landforms, and also provides a critical test for eros ion and flow laws. Within the erosion component of the simulation prog ram it is possible to specify spatial variations in bedrock erodibilit y in order to examine the effect of spatially variable lithologies on form development. As an initial test of form development under these c onditions a central area of enhanced erodibility is placed in the vall ey center. In this case active glacial channels develop that are narro wer and deeper than those which occur with homogenous bedrock. This ch ange in form occurs as long as the weaker zone is small enough that th e 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 l arge enough that it continues to have a significant impact on overall discharge through the section. Form variation occurs because the weake r central zone enhances the lateral gradient of the erosion rate, whic h is critical for form development. A central area of more erodible be drock changes not only the form of the active glacial channel, but als o the convexity of side slopes that are left above the glacier as it e rodes 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 combin ation of active glacial and hillslope processes in a landscape with sp atially and temporally variable ice discharge and rock characteristics can create a diverse set of cross-section forms.