MODELING 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 mar ginal deposits, which leaves several significant unresolved controvers ies. Those controversies all involve the maximum altitude and shape of that interconnected ice mass, and include (1) the source of the Two M edicine piedmont lobe (local or Cordilleran ice), (2) correlation of t erminal deposits of the Flathead lobe (late lastglacial, early lastgla cial, or pre-lastglacial), and (3) regional paleoclimate at last glaci al maximum (dominant westerlies or local easterlies). Theoretical reco nstructions 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 d ominantly composed of northeast-flowing local ice, not ice of northwes tern (Canadian cordillera) origin. Flathead lobe deposits include last glacial deposits both supported by and without a component from the Sw an Valley. There is evidence of only a regional westerly flow of moist ure across the region. In addition, a proglacial lake 60 km long occup ied 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 c an be used to predict the evolution of form.