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Pre-Illinoian
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 ~2.5–0.5 Ma (million years ago), a period of ~.
==North America==
As the oldest stage in the North American regional subdivision of the Quaternary, the Pre-Illinoian precedes the Illinoian Stage. Researchers have identified 11 distinct glacial stages during the Pre-Illinoian Stage.
The Pleistocene prior to the Illinoian stage had previously been subdivided into the Nebraskian, Aftonian, Kansan, and Yarmouthian stages (ages). However, detailed studies of these stages revealed that the assumptions and criteria on which they were defined proved to be wrong to such a point that these stages became meaningless in terms of the actual glacial–interglacial record.
For example, instead of two glaciations having occurred prior to the Illinoian Stage, researchers found that 11 distinct glaciations had occurred. 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. Similarly, paleosols used in the definition of the stages were found to have been greatly miscorrelated, as they consisted of paleosols of greatly differing ages. Because of these and other major problems, the concepts on which the Nebraskian, Aftonian, Kansan, and Yarmouthian (Yarmouth) stages are defined were discredited. North American geologists discarded these stages as unusable and merged them into the Pre-Illinoian Stage.〔
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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