GLACIAL LAKES WISCONSIN AND OSHKOSH—TWO VERY DIFFERENT LATE-GLACIAL ICE-MARGINAL LAKES IN WISCONSIN

Two large ice-marginal lakes, glacial Lakes Wisconsin and Oshkosh, were dominant features in the late-glacial landscape of Wisconsin. These lakes formed along the edge of the Green Bay Lobe, and nearly overlapped in time, but their histories are very different.

Glacial Lake Wisconsin formed when the western edge of the Green Bay Lobe advanced onto the eastern end of the Baraboo Hills, damming the Wisconsin River. The resulting lake in central Wisconsin drained northwestward to the East Fork of the Black River. When the Green Bay Lobe receded, the lake expanded eastward and then drained southward to the Wisconsin River around the east end of the Baraboo Hills. The rapid drainage of this, and possibly earlier versions, of glacial Lake Wisconsin carved the network of sandstone gorges known as the Wisconsin Dells. Throughout most of its existence glacial Lake Wisconsin was separated from the Green Bay Lobe by broad outwash plains and there were no major advances of ice into the lake. Shore features of glacial Lake Wisconsin are poorly preserved and sediment deposited in the lake is dominantly sand. No radiocarbon dates closely constrain the timing of events in the history of glacial Lake Wisconsin.

Lake Oshkosh was a much more complex ice-marginal lake. It formed as the Green Bay Lobe receded from the Green Bay lowland. This lake first drained westward through an outlet at Portage to the Wisconsin River. As the Green Bay Lobe continued to recede, a series of progressively lower outlets eastward to the Lake Michigan basin became available, the Portage outlet was abandoned, and lake level dropped. The geomorphology of the area and the sedimentary sequence in the lake basin indicate that the Green Bay Lobe readvanced into Lake Oshkosh at least twice, at about 13,200 and 11,500 RCYBP, blocking most or all of the eastern outlets and raising lake level until drainage shifted back to the Portage outlet. Sediment deposited in glacial Lake Oshkosh is characterized by thick sequences of laminated silt and clay-rich lake sediment interbedded with till and sandy lake sediment and outwash. Locally, shore features of Lake Oshkosh are fairly well preserved. Organic material interbedded with lake sediment has yielded a number of radiocarbon dates that constrain the timing of events in the history of glacial Lake Oshkosh.