WISCONSINAN MOLLUSCAN FAUNAS 5 



Away from its major valley source, the Peoria loess thins more gradually 

 than does the Roxana (Smith, 1942), but generally within a few miles of the valley 

 bluff it becomes leached and nonfossiliferous . Although the internal subdivisions 

 of the Roxana become indistinguishable within a relatively short distance of the 

 major valleys, differentiation of the Peoria loess from that of the Farmdale and 

 Roxana is possible far from the source valleys, even where the total loess section 

 has become thin and is completely leached. 



Within the limits of the Shelbyville moraine, the Woodfordian loess that 

 occurs stratigraphically below the Shelbyville and younger tills and above the 

 Farmdale silts is called the Morton loess. The Morton is a calcareous to weakly 

 calcareous, massive, sparsely fossiliferous, gray to gray-tan, thin loess. It is 

 clearly distinguishable from the noncalcareous, dark, humic to peaty Farmdale silts 

 below and the glacial till above. No evidence of weathering has been observed at 

 its top, and the stratigraphic relations, as well as carbon-14 dates, demonstrate 

 its conformability with the overlying Woodfordian till. It is equivalent in age to, 

 and stratigraphically continuous with, the lowermost part of the Peoria loess. 



Loess overlies the Woodfordian tills along the Illinois River valley from the 

 Peoria area where the Shelbyville moraine crosses the valley northeast to Grundy 

 County. The loess, which is equivalent in age to, and stratigraphically continuous 

 with, the upper part of the Peoria loess, has been named the Richland loess (Frye 

 and Willman, 1960). It is tan to gray-tan, massive silt, calcareous below the sur- 

 face soil profile, and commonly fossiliferous where it is calcareous. It rests on 

 calcareous glacial till that shows no evidence of weathering at the top, and in gen- 

 eral becomes progressively thinner up the valley as the underlying Woodfordian till 

 sheets become progressively younger. The Richland loess thins away from the valley 

 bluffs, and fossiliferous deposits are limited to relatively narrow belts adjacent to 

 the valley. 



Several molluscan faunas are listed from silts interbedded with glacial till 

 (Lake Bloomington Spillway section), from included masses of sands and silts with- 

 in glacial till, and from terrace deposits. However, it is not our purpose here to 

 redescribe the stratigraphy of the many Wisconsinan glacial pulses. In such cases 

 stratigraphic terminology is that in accepted use by the Illinois State Geological 

 Survey and documented in many Survey publications. 



RADIOCARBON DATES OF FAUNAS 



All of the Wisconsinan faunas described here are correlated with a time 

 scale based on carbon-14 dates determined in the Washington, D. C, laboratory 

 of the United States Geological Survey (Frye and Willman, 1960) . Thus the bio- 

 stratigraphic zones defined by the faunal assemblages also may be referred to in 

 terms of isotopically determined years. Dates ranging from 37,000 ± 1500 (W-869) 

 to 17,100 ± 300 (W-730) were determined directly on shells collected from these 

 faunal zones, but a much larger number of dates have been determined on wood from 

 the beds that contain the shells and from beds above and below the fossil zones. 



Snail shells from the Roxana silt have been dated at 37,000 ± 1500 (W-869) 

 at the Gale section and at 35,200 ± 1000 (W-729) at the Pleasant Grove section. 

 As the fossil zone within the Roxana is restricted approximately to its midportion, 

 these two dates are judged to give a satisfactory indication of its age. On the 

 basis of these dates, it seems reasonable that a range of radiocarbon years of 

 32,000 to 40,000 B. P. would include all of the elements of this fauna in Illinois. 



