QUATERNARY GEOLOGY AND GEOMORPHOLOGY DIVISION DISTINGUISHED CAREER AWARD: CHANGES IN GLACIAL LANDFORMS AND SUBGLACIAL CONDITIONS IN THE ICE-MARGINAL ZONE OF THE LAURENTIDE ICE SHEET FROM SOUTHERN WISCONSIN TO MINNESOTA

About 30,000 cal BP ice advanced into Wisconsin over permafrost that was as much as 100 m thick. It is likely that relatively little work was done by the frozen-bed glacier until subglacial permafrost beneath thick ice slowly melted and more rapid sliding began to take place behind a frozen toe. This frozen bed zone must have widened toward the north. Tunnel channels carried large volumes of water to the ice margin while there was still at least a 15 km-wide frozen bed around the margin in southern Wisconsin. Tunnel channels are absent in central Illinois, abundant along the Green Bay Lobe margin where water beneath the ice flowed up the regional slope, and more widely spaced, but longer along the margins of ice out of the Lake Superior basin. Behind that frozen zone the bed was going through a transition from frozen to wet bed, and drumlins were forming. No flat till plain was left behind as it was farther south in Illinois. Although permafrost remained in the south until about 15,000 cal BP and later in the north, perhaps by about 22,000 cal BP climate began to warm sufficiently to allow sliding and basal water accumulation out to the outermost margin. Striations and grooves, indicating sliding, as well as small eskers are present within a few km of the outermost advance, indicating that ice warmed to the base before significant retreat. Internal relief in the end moraine increases from about 10 m in southern Wisconsin, to nearly 100 m in the north, reflecting an increasing thickness of supraglacial sediment in the north. High relief ice-walled-lake plains are common in the north, but nearly absent in the end moraines of southern Wisconsin. In northern Wisconsin, ice advances interpreted to be surges cross-cut deposits of somewhat earlier advances. The Green Bay Lobe readvances, however, seem to have followed the same path until very late in the deglaciation process. The diverse landscapes resulting from distinctly different subglacial conditions are well illustrated along the 1200–mile Ice Age National Scenic Trail and in the Ice Age National Scientific Reserve.