AN ALGONQUIN SHORELINE IN SOUTHERN LAKE MICHIGAN, PART II

Three former shorelines are present in northwest Indiana arcing along the southern boundary of modern-day Lake Michigan. These shorelines are the Glenwood (reference elevation of 195m [AMSL]), Calumet (189m), and the Toleston (183m) Beaches. They are differentiated by their geomorphic position, topography, stratigraphy, and radiocarbon ages. Algonquin shorelines, well-documented relict shorelines observed elsewhere throughout the Great Lakes, have not been recognized along southern Lake Michigan. Recent data based on a series of 19 vibracores taken along transects at four locations in northwest Indiana indicate a previously unidentified shoreline position. This shoreline is located lakeward of the Calumet Beach. It is represented by basal foreshore deposits ranging in elevation from 183 to 187 m east of Deep River and by a pronounced erosional scarp cut directly into till at an elevation of 183 to 186 m west of Deep River. Samples collected from a peat found below foreshore deposits at Dune Acres, Indiana, yield 14C dates from the base, within, and top of 11,130 (±140), 10,570 (±50), and 10,740 (±130) years, respectively. These Algonquin dates, along with basal foreshore elevations and geomorphic position provide evidence for the existence of a high-level Algonquin shoreline along southern Lake Michigan. The above data suggest that current isostatic rebound models, which place the Algonquin shoreline deposits below modern lake-level, are not adequately representing basin warping in southern Lake Michigan. The hinge model of Goldthwait (1908), which corresponds with modern lake-level gauge records, may better represent Lake Michigan basin rheology. This research establishes the Algonquin phase of ancestral Lake Michigan as a high-level water plane along southern Lake Michigan. The lake-level curve for southern Lake Michigan will need to be redrawn in order to accommodate the Algonquin shoreline features.