MAP DETAIL from
"Map Showing the Thickness and Character of Quaternary Sediments in the Glaciated United States East of the Rocky Mountains"
U.S. Geological Survey Miscellaneous Investigations Series Map I-1970
1993-1998
by David R. Soller
In general, weaker, more erodable rocks lie in topographically low areas, whereas higher areas are underlain by more resistant rock. Glaciers tend to flow in topographically low areas, further eroding the weaker rocks, and they tend to flow around topographic highs. The glaciers tend to flow in lobes, depositing sediment most thickly along the lobe margins. Therefore, bedrock topographic highs tend to be buried by the thickest glacial sediment, and the lows are more commonly covered by thinner glacial deposits. These images show this pattern, near Duluth and the western part of Lake Superior. The ice flowed out of Lake Superior and across the topographic low to the southwest, depositing a thin layer of glacial sediment; however, the glaciers were diverted somewhat around a small bedrock topographic high in the center of the valley, leaving thick sediment.
