MAPPING THE EXTENT AND CHRONOLOGY OF THE LATE WISCONSIN ICE SHEET IN WISCONSIN

A preliminary series of 43 maps at approximately 500-year intervals that show the likely extent of the Laurentide Ice Sheet in Wisconsin and parts of surrounding states was presented as a poster at GSA in fall, 2011. We used crosscutting relationships, stratigraphic relationships, and existing age data to assemble a series of maps that illustrated our interpretation of where the ice margin was at various times. We encouraged comments on the ice margin positions, and in particular, how they agreed or disagreed with interpretations in adjacent states. We now present somewhat revised maps that incorporate comments from others as well as the incorporation of the GIS files into an online web map application. Radiocarbon dates in Wisconsin and Illinois still provide most temporal constraints. The former extents of paleo-lakes Michigan, Superior, Saginaw, Scuppernong, Oshkosh, Yahara, and Wisconsin are shown for each time slice. As in the 2011 versions, 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 still no actual age control, but these maps should highlight important areas for future ice-sheet-chronology research. We expect them to continue to be modified in the future as more data become available.