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 h ad a complex geologic history. Changes in the locations of accretionar y and erosional zones accompanied multiple and wide-ranging fluctuatio ns 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 mod em lakeshore and rim the Chicago/Calumet lacustrine plain. Where morai nes 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 term inated in separate spit systems on opposite ends of former lake embaym ents. A convergence zone of the western and eastern lakeshore littoral transport systems formed about 2.5 ka, first near the Illinois-Indian a state line and then shifting eastward along the Indiana shore. In th e past 2,000 years, major coastal changes have included southward migr ation of a beach-ridge plain near the Illinois-Wisconsin state line, e rosion along the Chicago central lakeshore, and erosion in the vicinit y of the Illinois-Indiana state line. Coastal erosion along the centra l Chicago lakeshore apparently played a major role in developing the m odem drainage pattern of the Chicago River.