Preliminary Analysis of Middle Holocene Lake-Level Change of Ancestral Lake Michigan, Wentworth Woods, Cook County, Illinois

Study of past lake-level change and isostasy in the upper Great Lakes has demonstrated the need to reconstruct relative lake-level history at each outlet during the Nipissing phase of ancestral Lakes Huron, Michigan, and Superior. Although elevation and age data exist for the Port Huron/Sarnia and Sault outlets of Lake Huron and Lake Superior, respectively, no paleohydrograph has been created for southern Lake Michigan near the Chicago outlet. The Wentworth Woods area of the Cook County Forest Preserve, Illinois, contains more than 30 beach ridges that formed during the rise and fall from the peak elevation of the Nipissing phase. These relict shorelines were vibracored to recover basal foreshore sediments that can be used as a proxy for lake-level elevation at the time of individual shoreline formation. In addition, sand samples from soil pits and vibracores were collected for optically stimulated luminescence age determinations. This report addresses the sedimentological data used to determine the elevation of the conjoined upper Great Lakes (Lake Nipissing) when each beach ridge formed. The age data will be presented in future reports.