AN OVERVIEW OF GLACIAL LANDFORMS AND POSTULATED SUBGLACIAL CONDITIONS OF THE LAURENTIDE ICE SHEET IN THE WESTERN GREAT LAKES AREA

The last glaciation left behind a great variety of landscapes. Different landforms occur in different areas, suggesting contrasting glacial environments. One of the most conspicuous differences is the shape of till surfaces behind end moraines. In Minnesota and Wisconsin there are thousands of drumlins. In Illinois south of Chicago, Indiana, Ohio, and Iowa, drumlins are absent, and the till surface behind end moraines is very low-relief till plain. In some places, particularly in the area covered by the Des Moines Lobe, ridges that appear to have squeezed into basal crevasses are present. Internal relief in end moraines is generally low in much of Illinois (commonly only a few meters), to about 10 m in central Wisconsin, to nearly 100 m in parts of northern Wisconsin. Along the west side of the Green Bay Lobe tunnel channels intersect the outermost end moraines and lead to large boulder fans. These landforms are absent south of the latitude of Chicago. Low-relief ice-walled lake plains are present in northern Illinois, and these are more abundant northward along the west side of the Green Bay Lobe, and become even more common in the moraines left behind by lobes coming out of the Lake Superior basin. All of these observations seem consistent with modeling results indicating that the Green Bay Lobe and ice lobes from the Lake Superior basin first advanced over thick permafrost about 30,000 cal. years ago. There is no evidence of the ice advancing into forest vegetation as there is farther south. I suggest that for the first approximately 10,000 years the glacier was frozen to its bed throughout the Green Bay Lobe and northern lobes. By 20,000 years ago water was present beneath the central part of the Green Bay Lobe and warming at the bed was taking place throughout. As the area of wet bed produced by basal melting expanded margin-ward, water broke through the permafrost seal in places in the outer 20 km of the glacier, producing Nye channels that now remain as tunnel channels. I suggest that a patchy frozen and unfrozen bed led to the formation of drumlins. Eventually water was present at the bed throughout and sliding produced striations and substantial streamlining of drumlins. By this time, formation of tunnel channels had ceased and eskers were deposited in Rothlisberger-type tunnels.