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. If the Illinoian Stage is limited in duration to Marine Isotope Stage 6, the Yarmouth Soil (paleosol) spans a period of geologic time equivalent to Marine Isotope stages 7, 8, 9, 10, and 11.