The glacial geology of southwestern Minnesota with emphasis on the deposits and dynamics of the Des Moines lobe

This thesis is based on mapping that was conducted in a 40,000 sq km region in southwestern Minnesota. The map, published by the Minnesota Geological Survey at a scale of 1:200,000, is Part One of the thesis. In Part Two, I describe the mapping methods and the geomorphology and glacial history of the study area.;Part Four further develops the proposed dynamics of the Des Moines lobe. The ice lobe repeatedly advanced, discharged its subglacial water, and subsequently stagnated. Recent glaciological research on Antarctic ice streams has led some glacial geologists to postulate that ice streams drained marine-based portions of the Laurentide and Scandinavian ice sheets. I postulate that such ice streams may develop in land-based areas of an ice sheet as well and that the Des Moines lobe was an outlet glacier of an ice stream. It appears to have been able to advance beyond the Laurentide Ice Sheet as long as adequate water pressure was maintained. However, the outer part of the lobe stagnated because subglacial water that facilitated the flow was able to drain away through tunnel valleys. Stagnation of the lobe is not equivalent to stoppage of the ice stream, for ice repeatedly advanced into and onto the stagnant margins, stacking ice and debris. Similar landforms are also seen in other lobes of the upper midwestern United States.;Part Three focuses on the subglacial drainage that developed beneath the Des Moines lobe at its maximum and also at the limits of its less extensive readvances. Subglacial water may have facilitated the advances of the lobe. Upon release of the water through the subglacial conduits, the lobe stagnated.