THE STRATIGRAPHIC RECORD AND CHRONOLOGY OF LATE GLACIAL ADVANCE AND RETREAT IN EASTERN WISCONSIN

The Kewaunee Formation, deposited in the area covered by the Green Bay and Lake Michigan lobes, consists of till, lake sediment, and fluvial sediment deposited during glacial advances between about 19,500 and 13,200 BP (cal.). Maximum advance was to the south end of Lake Winnebago in the Fox River valley and to just south of Milwaukee in the Lake Michigan basin. Radiocarbon dates from eastern Wisconsin indicate that late glacial ice-free times separated by periods of ice cover can be recognized. AMS dates between about 17,000 and 18,000 BP (cal.) and 15,500 and 16,500 BP (cal.) indicate ice free times with tundra conditions and permafrost. These ice free times correspond to times of minor warming in the GRIP core. No radiocarbon dates between about 14,000 and 15,400 BP (cal.) are known at this time in the area covered by Kewaunee Formation deposits, suggesting ice cover. Ice of this event deposited Valders, Chilton, and Kirby Lake tills and was followed by retreat of ice at least to the northern end of Lake Michigan. This retreat of ice allowed growth of the Two Creeks forest (many dates from about 13,200 to 13,900 cal.). By this time, spruce trees grew, there was apparently no permafrost, Glacial Lake Oshkosh had drained, and lake level in the Lake Michigan basin had fallen from the Glenwood level to below its present level. The subsequent advance, driven by pre-younger Dryas cooling, blocked the northern end of Lake Michigan and, as lake level rose to the Calumet level, the Two Creeks forest at the type locality was drowned about 13,200 cal. years ago. Ice continued to advance, depositing the Middle Inlet, Glenmore, and Two Rivers tills as far south as the north end of Lake Winnebago and the present city of Two Rivers. Ice appears to have retreated out of the northern end of Lake Michigan, allowing water level to fall from the Calumet level, by about 12,600 BP (cal.).