AN ALGONQUIN SHORELINE IN SOUTHERN LAKE MICHIGAN

Large-scale and long-term fluctuations of lake level were common in the Great Lakes over the past 14,500 years. Relict shorelines related to these changes formed when lake levels were above current elevations. Three such shorelines--the Glenwood, Calumet, and Toleston--exist along the southern shore of Lake Michigan, arcing across northwest Indiana. The middle shoreline, known as the Calumet Beach, formed during the Calumet phase of ancestral Lake Michigan from 11.8 to 11.2 ka. It is generally believed that lake-level elevation was approximately 189 m above sea level during the development of the Calumet Beach. The main Algonquin phase (11.0 to 10.1 ka), prominent in depositional features along northern Lake Michigan, has not been recognized in the south. This has prompted many researchers to believe it is submerged below modern lake level in response to isostatic tilting of the Lake Michigan Basin. Recent data, however, suggest that a possible Algonquin phase shoreline does exist above modern lake level in southern Lake Michigan.

The Calumet Beach was a mainland-attached beach that terminated in a spit just west of the Illinois-Indiana state line. Along the lakeward margin of the Calumet Beach there is a pronounced scarp, showing evidence of erosion along the western end of the beach west of Deep River in Indiana. The recently documented scarp is 2.0-3.0 m high and is cut directly into till. The 183 m elevation at the base of the scarp is similar to that of Calumet foreshore deposits of 184 m east of Deep River. These elevations are 5 to 6 m below the reference elevation for the Calumet Beach (189 m). Moreover, radiocarbon dates of these deposits range from 10.4 to 10.6 ka making them more consistent with deposits Algonquin in age.