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Sediments, soils, and fossils are used to interpret paleoenvironmental conditions during the last interglacial-glacial transition in Illinois. The sediments include the classic Sangamonian-Wisconsinan and Wisconsinan/Farmdalian-Woodfordian successions; both consist of loess overlying pedogenically modified or generally colluviated sediment. Although the contacts between the loess and soil are defined as isochronous, they are diachronous. The age of basal Wisconsinan deposits spans from at least 50 to 22 ka, and the age of basal Woodfordian sediments spans from 25 to 20 ka.

Recognition of the last interglacial episode, the Sangamonian Age, is based on Mollisols, Alfisols, and Ultisols that formed in Illinoian glacigenic deposits. Sangamonian flora and fauna, although sparsely preserved, include pollen well represented by deciduous trees, grasses, and Ambrosia. Sangamonian vertebrates included giant tortoise, mastodon, giant beaver, snapping turtle, and short-nosed gar. The Wisconsinan Age (Altonian Subage) began as Roxana Silt (loess) was deposited under periglacial conditions. The vegetation was characterized by coniferous trees that grew in weakly developed, often organic-rich, cryoturbated soils. Periglacial conditions persisted during the Altonian and Farmdalian Subages, from before 50 ka to about 25 ka, after which loess, outwash, and till were deposited under glacial conditions during the Woodfordian Subage and much of the remainder of the Wisconsinan Age. The vegetation that grew during the last glacial episode includes plants that are now found at and north of the treeline in Canada.

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