Modelling of icecap glaciation of the northern Rocky Mountains of Montana

The extent of the lastglacial icecap over the northern Rocky Mountains of Montana has been inferred only from cursory correlation of its marginal deposits, which leaves several significant unresolved controversies. Those controversies all involve the maximum altitude and shape of that interconnected ice mass, and include (1) the source of the Two Medicine piedmont lobe (local or Cordilleran ice), (2) correlation of terminal deposits of the Flathead lobe (late lastglacial, early lastglacial, or prelastglacial), and (3) regional paleoclimate at last glacial maximum (dominant westerlies or local easterlies). Theoretical reconstructions of the glacial surface along major flowlines, constrained by ice-marginal features, nunataks and breached divides throughout the region, tentatively resolve those issues. The Two Medicine lobe was dominantly composed of northeast-flowing local ice, not ice of northwestern (Canadian cordillera) origin. Flathead lobe deposits include lastglacial deposits both supported by and without a component from the Swan Valley. There is evidence of only a regional westerly flow of moisture across the region. In addition, a proglacial lake 60 km long occupied the Swan Valley during the early stages of deglaciation. Although subject to revision from fieldwork, glacial process modelling remains one of the few geological arenas in which rigorous process modelling can be used to predict the evolution of form.