SOUTHERN LAURENTIDE ICE LOBES WERE CREATED BY ICE STREAMS - DES-MOINES LOBE IN MINNESOTA, USA

Regional mapping in southern Minnesota has illuminated a suite of land forms developed by the Des Moines Lobe that delimit the position of th e lobe at its maximum and at lesser readvances. The ice lobe repeatedl y advanced, discharged its subglacial water, and subsequently stagnate d. Recent glaciological research on Antarctic ice streams has led some glacial geologists to postulate that ice streams drained parts of the marine-based areas of the Laurentide Ice Sheet. I postulate that such ice streams may develop in land-based areas of an ice sheet as well, and that the Des Moines Lobe, 200 km wide and 900 km long, was an outl et glacier of an ice stream. It appears to have been able to advance b eyond the Laurentide Ice Sheet as long as adequate water pressure was maintained. However, the outer part of the lobe stagnated because subg lacial water that facilitated the flow was able to drain away through tunnel valleys. Stagnation of the lobe is not equivalent to stoppage o f the ice stream, because ice repeatedly advanced into and onto the st agnant margins, stacking ice and debris. Similar landforms are also se en in other lobes of the upper midwestern United States.