The effect of valley confluence and bedrock geology upon the location and depth of glacial overdeepenings

Abstract

ABSTRACT Overdeepenings are erosional landforms, cut by glaciers into bedrock in basins and valleys. Overdeepening is the glaciological and geomorphological process that produces these landforms. The overdeepening process is important because it has the potential to influence the response of ice masses to climatic changes. In this paper, we analyze topographic and bathymetric digital elevation models to examine several hundred glacial overdeepenings in Labrador, Canada. We investigate controls upon the location and depth of overdeepenings. Our analyses show that the location of overdeepenings correlate strongly with confluences of glacial valleys and, importantly, that the correlation is strongest where confluence geometry requires the speed-up of ice-flow due to change in valley cross-sectional area. Further, we find that the magnitude of ice-flow speed-up correlates with depth of overdeepenings only for confluences situated in or near major geological fault-zones. Our findings therefore support the hypothesis that overdeepening can be initiated by an increase in ice velocity. Further, we conclude that overdeepening is most efficacious where fractured bedrock enables efficient quarrying. In summary, we find that the primary control upon the location of overdeepenings arises from confluences of glacial valleys due to ice speeding up at these locations, and that the depth of overdeepenings are controlled by rock mass strength. These findings are relevant for landscape evolution modelling and may be used in model testing.