Absence of Altonian glaciation in Illinois

Illinois was free of glacier ice from about 130,000 to 25,000 yr B.P. Deposits of this time interval in northeastern Illinois, represented by core samples from test-hole ISGS S-30, are composed of leached, nonglacigenic, stratified, or pedoturbated silty sediments that typically are organic-rich and pedogenically modified. These deposits overlie the Sangamon Soil, which is identified by its stratigraphic position, soil morphology, and distinct alteration products in the clay-mineral fraction, including a variably swelling vermiculite-like phase and a randomly interstratified kaolinite/10 Å phase. The regional extent of these deposits suggests that if Altonian ice existed in the Lake Michigan basin, it did not extend westward beyond the cuesta of Silurian dolomite that rims the southwestern shore of Lake Michigan.