LOESS THICKNESS IN ILLINOIS1

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Abstract
This paper presents thickness maps of the Peoria loess, Roxana loess, and total loess (Peoria and Roxana combined) for Illinois. The Peoria and Roxana loesses, both deposited in the Wisconsinan glacial stage, are the two loess beds of pedological significance in Illinois. Of the two, the surficial Peoria is the more important, because it is the parent material of soils covering about 63 percent of Illinois. About 6600 thickness measurements, made on nearly level, stable landscapes throughout the state, were used in constructing the isopach loess maps. Peoria loess thickness ranges from more than 7.5 m to less than 0.5 m. The thickest areas are adjacent to the major Pleistocene rivers, which carried Wisconsinan glacial melt waters. The thinnest loess is in northeastern Illinois, where much of the loess is beneath more recent till deposits or has been incorporated in the younger till. Roxana loess is only 15 to 50 percent as thick as the Peoria, and it thins more rapidly with distance from a source. Mixed sediment zones, commonly present at the loess-till boundary, are a problem in areas where the loess is thinner than about 1.5m and the soil is formed in more than one material. However, these zones can be recognized and accommodated pedologically as a second soil parent material. Suggestions are made for the strati-graphic placement of the most extensive of these zones in south-central and southern Illinois. Evidence in various areas in northwestern Illinois, both north and south of the Green-Rock River Lowland, suggests that a period of extremely severe erosion probably occurred contemporaneous with a portion of the cutting of the Iowan erosion surface in Iowa some 18 000 yr B.P. The erosion removed various portions of the loess, and in some places portions of the underlying Sangamon paleosol and its substratum till were also removed. In a number of instances this differential erosion resulted in thin loess areas lying rather abruptly next to thick loess areas. © Williams & Wilkins 1986. All Rights Reserved.

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