Illinoian (stage)

At its type exposure in Peoria County, Illinois, the Illinoian deposits consist of three till members of the Glasford Formation. They overlay Pre-Illinoian tills of the Banner Formation, in which the Yarmouth Soil (paleosol) has developed. 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. A paleosol, called the Pike Soil, separates two of the till members within the Glasford Formation. ==Correlation==

Since 1986, the Illinoian Stage has been interpreted as consisting of two glaciations, the early Illinoian (Marine Isotope Stage 8) and late Illinoian glaciations (Marine Isotope Stage 6) and the intervening interglacial period (Marine Isotope stage 7). In this interpretation, the Pike Soil is proposed to an interglacial (Marine Isotope Stage 7) paleosol. According to this interpretation, the Illinoian Stage started about 300,000 years ago and ended about 130,000 years ago. 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. The oldest fluvial sediments, which overlay bedrock in the deepest part of the valley, were dated by OSL dating to around 190,000 BP. These OSL dates demonstrate that the Illinoian Stage is temporarily equivalent only to Marine Isotope Stage 6, which ended at 130,000 BP and started at 191,000 BP. and, thus, started about 352,000 years ago and ended 130,000 years ago. to designate glacial deposits and paleosols lying between the Sangamon and Yarmouth soils (paleosols). ==Extent==

During the Illinoian Stage, the Laurentide Ice Sheet covered about 85 percent of Illinois. At its maximum extent during this stage, this ice sheet reached its southernmost extent in North America near Carbondale, Illinois. At their maximum extent, the edge of Illinoian ice sheet(s) lay further south than the southernmost extent, i.e. Douglas County, Kansas, of any of the Pre-Illinoian ice sheets. ==See also==