From this database, several observations have emerged. 1. Many more ice-margin positions are mapped than ever described before, especially in eastern Nebraska and southern Iowa. 2. Melt-water drainage valleys are well preserved in bedrock and represent the most important long-term geomorphic legacy of glaciation. 3. Melt-water drainage routes are situated in two positions--parallel and normal to ice margins. The parallel routes are arranged in concentric, nested patterns that mark positions of glacier advance and retreat. The normal routes are either parallel or slightly diverging in the downstream direction. These drainage routes, at right angles to ice margins, presumably formed as either subglacial tunnel valleys or proglacial spillways, in places following preglacial valleys. They probably served repeatedly in both roles during multiple glacier advances and retreats. The overall pattern of ice margins and drainage routes strongly suggests a lobate style for pre-Illinoian glaciations, in which ice-lobe surging may have taken place.
Pre-Wisconsin homepage