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In the Illinois Valley region, the Altonian Substage (between 50,000 and 70,000 to 28,000 B. P.), Farmdalian Substage (28,000 to 22,000 B. P.), and Woodfordian Substage (22,000 to 12, 500 B. P.) of the Wisconsinan Stage are represented by loess, water-laid elastics, and glacial till. Molluscan faunas of these sediments are here listed both geographically and stratigraphically, and 61 species are illustrated. Woodland faunas distinctive of the Roxana silt (Altonian) are found at 11 localities. Two of the Farmdalian localities yielded aquatic faunas and one woodland fauna. Faunas characteristic of the Woodfordian are abundant but differ in the various geographic areas. The faunas of the several Woodfordian rock-stratigraphic units are not distinguishable, but those of the three substages are distinctly so. During Wisconsinan time the glaciers did not produce rigorous climates in Illinois very far from the ice fronts. During Altonian and part of Farmdalian time the southern half of Illinois was heavily forested, but this was not so in Woodfordian time. WISCONSINAN MOLLUSCAN FAUNAS 3 George E. Ekblaw and H. B. Willman of the Illinois Geological Survey, and Paul R. Shaffer of the University of Illinois assisted in the field and in making stratigraphic interpretations. All radiocarbon dates referred to were determined in the Washington, D. C., laboratory of the United States Geological Survey. STRATIGRAPHY OF FOSSILIFEROUS UNITS The Wisconsinan Stage in the Lake Michigan glacial lobe recently has been discussed and reclassified by Frye and Willman (1960), and their rock-stratigraphic and time-stratigraphic terminology is used in this report. The molluscan faunas described herein were collected from deposits included within the Altonian, Farmdalian, and Woodfordian Substages of the Wisconsinan glacial stage. In radiocarbon years the time span of these substages started between 50,000 and 70,000 years before present (B. P.) and extended to 12,500 years before present. Altonian Substage All the loess of Altonian age is included in the Roxana silt (Frye and Willman, 1960). The Pleasant Grove School section, which occurs in the bluffs of the Mississippi River valley in Madison County, Illinois, was designated as the type section of the Roxana. Roxana silt is described in 7 of the 11 measured geologic sections included with this report. In the thick sections along the valley bluffs the Roxana silt contains at least five distinctive stratigraphic units. At some localities the base of the Roxana is colluvium, consisting of silt locally containing sand and pebbles like those in the underlying deposits. The basal colluvium, where observed, is noncalcareous and nonfossiliferous; it is in sharp contact with the beds below but is gradational with the overlying deposits. The second unit of the Roxana is commonly a gray, massive, noncalcareous and nonfossiliferous silt. Locally this zone may contain sand (Browns Mound section) or sandy silt, and in some places (Pleasant Grove section) it is marked at the top by a humic streak, or an ...

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During the closing years of the 19th century, Haworth [see 1 and 2 of “References” at end of paper] and Johnson [3] noted the abundant depressions resembling sink‐holes in the Southern High Plains, especially in Meade and Clark counties, Kansas, and Beaver County, Oklahoma. Johnson was of the opinion that all the depressions and basins in this region were due to solution, whereas Haworth [1] believed the Meade artesian basin to be structural, although he held to a solutional origin for the Ashland and Englewood basins, which are a few miles to the east.

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