Theories that propose feedbacks among climate, tectonics, andsurface processes commonly assume that erosion is enhanced byglacial activity. Indeed, studies have shown that glaciers appearto limit the elevation of mountain ranges; however, comparisonsbetween rates of glacial and nonglacial erosion are difficultto make. Ideally, such comparisons must hold precipitation andlithology constant, while only varying the erosional regime.Located in a climatic transition zone during the Pleistocene,the east-west–trending valleys of the Bitterroot Rangepresent an opportunity for a quantitative analysis of glacialand nonglacial erosion because the north-facing sides of thevalleys were glaciated, whereas the south-facing slopes werenot. The different erosional regimes operating on either sideof the valleys created strongly asymmetric ridges. Ridgelinesseparating the east-west–trending valleys have been pushedsouthward by glacial headwall retreat such that ridge-to-valleydistances are 50% greater on the north-facing slopes than onthe south-facing slopes. In addition, mean hillslope anglesare 6° lower on the glaciated slopes than on the unglaciatedslopes, and calculations of geophysical relief suggest that,on average, glaciers have removed nearly twice as much rockas nonglacial processes. Finally, we conclude that, althoughrates of vertical incision by glacial processes in the BitterrootRange were more rapid than nonglacial processes, the dominantgeomorphological impact of glaciers was lateral erosion by headwallretreat.