A BURSTING ICE DAM, A BIG FLOOD, AND SUB-LAKE MORAINES—THE TRANSITION FROM GLACIAL LAKE WISCONSIN TO GLACIAL LAKE OSHKOSH

During and soon after the most recent glacial maximum, two large ice-marginal lakes, glacial Lake Wisconsin and glacial Lake Oshkosh, formed beyond the Green Bay Lobe of the Laurentide Ice Sheet in central and east-central Wisconsin. The existence of these lakes has been known for many decades, and our recent studies of these lakes have revealed many new aspects of their history.

Glacial Lake Wisconsin formed when the Green Bay Lobe flowed onto the eastern end of the Baraboo Hills, blocking the southward-flowing Wisconsin River. During the most recent glacial maximum, this lake drained through a northwestern outlet. As the ice margin began to recede eastward, a new basin—the Lewiston basin—formed north of the Baraboo Hills between the ice margin and the outermost moraine deposited by the Green Bay Lobe. The water level of glacial Lake Wisconsin in this basin stabilized at an elevation of about 970 ft. The lake continued to expand as the ice margin receded eastward. A series of distinct north–south trending flat-topped moraines formed in the eastern part of the Lewiston basin as the western margin of the Green Bay Lobe receded. These moraines have distinct fan deltas on their western flanks, indicating they were deposited in the lake.

The lakes in the Lewiston and the main basin of glacial Lake Wisconsin abruptly drained down the Wisconsin River through the Alloa outlet (at an elevation of about 825 ft) when the ice dam at the east end of the Baraboo Hills thinned and then burst. This outlet was not as low as the lowest part of the floor of the Lewiston basin, and as a result some water remained ponded north of the east end of the Baraboo Hills.

As the margin of the Green Bay Lobe continued to recede northeastward, the Dekorra outlet (at an elevation of about 775 ft) opened and drained the last remnants of glacial Lake Wisconsin. Glacial Lake Oshkosh then began to form beyond the margin of the Green Bay Lobe. The early phases of this lake (at an elevation of about 780 ft) drained southward through the Dekorra outlet to the Wisconsin River. With the continued recession of the Green Bay Lobe, progressively lower outlets across the Door Peninsula in the northeastern part of the basin were opened, allowing glacial Lake Oshkosh to drain eastward to Lake Michigan.