Late Wisconsinan and Holocene Geologic History of the Illinois-Indiana Coast of Lake Michigan

During the past 14,500 radiocarbon years, southern Lake Michigan has had a complex geologic history. Changes in the locations of accretionary and erosional zones accompanied multiple and wide-ranging fluctuations in lake level. End moraines bordering southern Lake Michigan dammed the ancestral lake to levels above those that exist now. Along 120 km of the Illinois-Indiana coast, end moraines are landward from the modern lakeshore and rim the Chicago/Calumet lacustrine plain. Where moraines intersect the lakeshore, coastal erosion has supplied the littoral sediment stream. During a series of peak lake levels, the lacustrine plain was the sink for littoral sediment supplied from the western and eastern lakeshores. Before about 2.5 ka, this littoral transport terminated in separate spit systems on opposite ends of former lake embayments. A convergence zone of the western and eastern lakeshore littoral transport systems formed about 2.5 ka, first near the Illinois-Indiana state line and then shifting eastward along the Indiana shore. In the past 2,000 years, major coastal changes have included southward migration of a beach-ridge plain near the Illinois-Wisconsin state line, erosion along the Chicago central lakeshore, and erosion in the vicinity of the Illinois-Indiana state line. Coastal erosion along the central Chicago lakeshore apparently played a major role in developing the modern drainage pattern of the Chicago River.