REFINING THE LATE GLACIAL CHRONOLOGY OF THE GREEN BAY LOBE OF THE LAURENTIDE ICE SHEET USING ICE-MARGINAL LACUSTRINE SEDIMENTS
Both of those datasets address the timing of onset of retreat of ice from the last maximum position. Sediments collected from within the Devils Lake Gorge in the Baraboo Hills are providing insight into the timing of ice advancing to its maximum position. Late Wisconsin ice blocked both ends of the gorge, creating a lake during the glacial maximum and the lower, modern, Devils Lake. We have collected a 9.1-m core into lacustrine sediments immediately south of Devils Lake; the base of the core is 9.2 m higher than modern lake level, suggesting that the sediments could only have been deposited when ice was completely blocking both ends of the gorge. Three radiocarbon dates from the bottom 2.4 m of the core range between 20,480 +/- 100 14C yr BP (24,890 – 24,050 cal yr BP) and 19,100 +/- 80 14C yr BP (23,290 – 23,060 cal yr BP), indicating that the Green Bay Lobe had advanced to its maximum position by that time. Combined with the previous studies, these data further constrain the timing of late Wisconsin advance and retreat of the Green Bay Lobe.