Late Wisconsinan drift stratigraphy of the Lake Michigan Lobe in southwestern Michigan
Three Lake Michigan Lobe till sheets are exposed in a 20- to 25-m-high section (Glenn Shores section) along the Lake Michigan shoreline in southwestern Michigan. The Glenn Shores till (lowest) is directly overlain by stratified silt, sand, and gravel containing organic material dated at 37,150 to >48,000 yr B.P. The Ganges till (middle) and Saugatuck till (uppermost) are overlain by lacustrine sediments. The three till units can be differentiated by the 7A/10A X-ray diffraction peak-height ratio of their clay size fraction. Clay-mineral analyses of more than 200 samples collected from moraines, till plains, and multi-till exposures 2 to 60 km inland suggest that Saugatuck till at the lakeshore is equivalent to till in the Lake Border Valparaiso, and Sturgis-Kalamazoo morainic systems, whereas Ganges till can be traced in the subsurface from the lakeshore to where it crops out within the Tekonsha Moraine. The presence of lacustrine sediments between Ganges and Saugatuck tills implies that a significant retreat (>60 km) of the Lake Michigan Lobe occurred between the formation of the Tekonsha Moraine and that of the Sturgis-Kalamazoo Morainic System. On the basis of morainal correlations, we propose that Saugatuck till correlates with Wadsworth Till in Illinois, whereas Ganges till probably correlates with the upper part of the Yorkville Till in Illinois.