The Illinoian Stage is the name used by Quaternary geologists in North America to designate the Penultimate Glacial Period c.191,000 to c.130,000 years ago, during the late Middle Pleistocene (Chibanian), when sediments comprising the Illinoian Glacial Lobe were deposited.
[1] At its type exposure in Peoria County, Illinois, the Illinoian deposits consist of three till members of the Glasford Formation.
In this exposure, the Illinoian Glasford Formation, in which the interglacial Sangamon Soil (palesosol) has developed, is overlain by early Wisconsinan stage loess, called the Roxana Silt.
[6][7] However, later studies of the fluvial deposits of the Pearl Formation and Illinoian glacial tills of the Glasford Formation, which fill an ancient and buried Mississippi River valley in north-central Illinois, demonstrated that the Illinoian Stage in its type area consists of glaciations that occurred only during Marine Isotope Stage 6.
The age of proglacial fluvial sediments underlying the oldest known glacial till (Kellerville Member) of the Glasford Formation yield optically stimulated luminescence (OSL) dates that averaged 160,000 BP.