INTRODUCTION Glaciation of Mission Valley The Cordilleran Ice Sheet invaded northwestern Montana multiple times during the Pleistocene, overtopping mountain ranges, widening pre-glacial river valleys, and depositing moraines composed of clay-gravel diamicts that often lack a full complement of boulders. Mission Valley is a structural trough that extends south of Flathead Lake between Polson, MT and the Ravalli Hill divide. Two prominent, horseshoe-shaped moraines indicate the