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Study of past lake-level change and isostasy in the upper Great Lakes demonstrates 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, we collected sand samples from soil pits and vibracores for optically stimulated luminescence age determinations. This preliminary paper addresses the sedimentological data used to determine the elevation of the conjoined upper Great Lakes when each beach ridge formed. The age data will be presented in future papers. INTRODUCTION The Nipissing phase of ancestral Lake Michigan (6,100 to 4,500 years ago) was the last pre-modern high phase of the lake, peaking at an elevation of approximately 6 m above current lake level 4,500 years ago (peak Nipissing phase elevation) (Thompson and others, 2011). At that time, Lakes Michigan, Huron, and Superior were confluent as a single ancestral lake, and shorelines formed contemporaneously along the coastlines of all three of the upper Great Lakes. Outflow during the Nipissing phase was originally northeastward through the North Bay outlet at the east end of present-day Lake Nipissing in Ontario and then shifted southward to discharge through the Port Huron outlet at the southern end of Lake Huron and also through the Chicago outlet at the southwestern margin of Lake Michigan. Subsequent lake-level fall and warping of the basin by ongoing glacial isostatic adjustments (GIA) have abandoned the Nipissing shoreline and altered associated strandplain elevations throughout the upper Great Lakes. Because the Nipissing phase sequence of shorelines formed during a relatively narrow length of

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