The Pre-Illinoian Stage is used by Quaternary geologists for the early and middle Pleistocene glacial and interglacial periods of geologic time in North America from ~2.5–0.2 Ma (million years ago).
In addition, what was presumed to have been a single volcanic ash bed, which was used to correlate and differentiate between Kansan and Nebraskan glacial deposits, was found to be three volcanic ash beds of greatly differing ages.
Because of these and other major problems, the concepts on which the Nebraskan, Aftonian, Kansan, and Yarmouthian (Yarmouth) stages are defined were discredited.
[2][8] The Pre-Illinoian stage is contemporary with the Bramertonian, Pre-Pastonian, Pastonian, Beestonian, Cromerian, Anglian, Hoxnian, and lowermost Wolstonian stages of the British Isles combined.
[2][3][10][11] More recent geologic mapping, coring, and optically stimulated luminescence (OSL) dating of Illinoian glacial tills (Glasford Formation) and outwash (Pearl Formation) of the Illinoian Glacial Lobe in North-central Illinois demonstrates that the start of the Illinoian stage and end of the Pre-Illinoian stage correlates with the beginning of Marine Isotope Stage 6 at 191,000 BP.