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 strat- 

 igraphic 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 Will- 

 man, 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 sec- 

 tion) or sandy silt, and in some places (Pleasant Grove section) it is marked at the 

 top by a humic streak, or an A-C soil profile. Both of the two lower units of the 

 Roxana may or may not be present in a particular exposure, they are not generally 

 loess, and they are clearly separable from the three higher units. 



The third unit of the Roxana is light to dark pink, massive, coarse loess, 

 and contains fossil snails in the upper part. The carbonate content is largely dol- 

 omite, and the zone is gradational with the overlying deposits. In some exposures 

 the pink color diminishes upward and the upper part of the zone consists of inter- 

 stratified pale pink and yellow-tan loess. 



The fourth unit generally is gray-tan to yellow-tan, calcareous, fossiliferous 

 loess. At many places it contains lenticular streaks of tan sand and coarse silt 

 and locally may be quite sandy. The top of this zone is commonly gradational with 

 the fifth, or uppermost, unit of the Roxana. 



The fifth unit of the Roxana is pink-tan, massive loess. Commonly it is non- 

 calcareous in the upper part and dolomitic and fossiliferous in the lower part, but 

 at some places it is weakly calcareous throughout. Although the contact at the top 

 of the Roxana generally is sharp, it is not strikingly apparent in many exposures be- 

 cause of the absence of a B-zone in the leached material in the upper few feet. 



