LAURENTIDE ICE SHEET EXTENT MAPS COVERING WISCONSIN AND PARTS OF ADJACENT STATES—THE NUCLEUS OF A LONG TERM PROJECT

A new series of 45 maps at approximately 500-year intervals that show the likely extent of the Laurentide Ice Sheet in Wisconsin and parts of surrounding states will help form the basis for improving the chronology of ice advance and retreat in this area. Radiocarbon dates in Wisconsin and Illinois provide some temporal constraints on the initial advance of the Lake Michigan Lobe. In Wisconsin, however, there are no radiocarbon dates between about 30,000 to 16,000 cal BP that constrain ice margin positions, while there are no data for any time interval that date the ice advance out of the Lake Superior basin. New cosmogenic and OSL dates are providing more control on ice margin fluctuations, but they remain limited in number and extent. We have used crosscutting relationships, stratigraphic relationships, and existing age data to assemble a series of maps that illustrate our "best guess" as to where the ice margin was at various times. The former extents of paleo-lakes Michigan, Superior, Saginaw, Scuppernong, Oshkosh, Yahara, and Wisconsin are shown for each time slice. For our maps, paleo-lake elevations were calculated by incorporating an isostatic rebound model that calculates land surface elevations at various times in the past. For each time slice we subtracted those elevations from the present day DEM. The extent of these former lakes is at present indicated by the distribution of lake sediments and, in a few places, beaches. Those elevations were extrapolated onto the paleo-DEMs. For many of the maps there is no actual age control, but these maps should highlight important areas for future ice-sheet-chronology research. We expect them to be modified in the future based on viewer comments, and as more data become available.