6 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 304 



The Farmdale silt is the most extensively dated stratigraphic unit in the 

 Pleistocene of Illinois, but at only two localities has an adequate molluscan fauna 

 been obtained. At the Wedron Quarry section a date of 26, 800 ± 700 (W-871) was 

 determined on flecks of wood associated with the shells, and earlier a date of 

 24, 000 ± 700 (W-79) had been determined on a sample of wood from the immediately 

 overlying bed. At "The Rocks" section, southeast of the Shawneetown bridge across 

 the Ohio River, a date of 22, 200 ± 450 (W-867) was obtained on the shells. The 

 time span of the Farmdalian is judged to extend from 28,000 to 22,000 B. P. 



Dates related to the age of the fauna from the Morton loess have been re- 

 ported from the vicinity of the Farm Creek section where wood dates of 20, 340 ± 

 750 (W-349) and 20,700 ± 650 (W-399) have been obtained. Although in general 

 not directly associated with faunal localities, there are a large number of dates 

 from the Farmdale silts below the Morton loess and a few dates from the Shelbyville 

 till above it. The available dates indicate a range in radiocarbon years of 20, 000 

 to 22,000 for this faunal zone. 



Beyond the limits of the Shelbyville moraine, the Peoria loess includes de- 

 posits and faunas that are equivalent in age to the Morton loess, the overlying 

 Woodfordian till, and the Richland loess. Two radiocarbon dates have been deter- 

 mined from the snail shells of this fauna; they are 20,300 ± 400 (W-870) from the 

 Enion area (Fulton County), and 17, 100 ± 300 (W-730) from the Burdick Branch ex- 

 posures north of the Pleasant Grove section. In addition, several dates determined 

 on wood have direct relation to the age of the fauna of the Peoria loess. Wood from 

 immediately below the fossiliferous loess in the Enion area has been dated 23, 500 ± 

 400 (W-745), and this agrees well with the date of 20, 300 determined on shells 

 from the upper part of the overlying loess. No radiocarbon dates have been ob- 

 tained directly from the Richland loess, but wood from fossiliferous terrace deposits 

 in Fulton County has been dated 15,600 ± 600 (W-381). From these data it appears 

 that the age of the molluscan fauna of the Peoria loess, and its stratigraphic equiv- 

 alents, ranges from approximately 22,000 to approximately 15,000 radiocarbon years 

 before the present. 



Several anomalous dates do not properly fit the patterns developed by the 

 majority of the dates. It will be noted that in the Lake Bloomington Spillway section 

 an upper date is listed as greater than 34, 000 (W-67), whereas a date from wood 

 lower in the section is reported as 31,000 or older (W-186). This anomaly is as yet 

 unexplained. A date determined on shells from the Peters exposures, north of the 

 Pleasant Grove section, is reported at 27,500 ± 900 (W-868), which is 5,000 to 

 10,000 years younger than would be anticipated for Roxana faunas in the area. The 

 sample of shell material that was dated from this exposure was collected by several 

 persons on several visits, and it is judged that a small quantity of shell material 

 from the overlying Peoria loess (dated 17, 100 ± 300 [W-730] at the Burdick Branch 

 exposures a third of a mile to the south) was intermixed in the sample, which could 

 easily account for the discrepancy. 



FOSSIL MOLLUSCAN FAUNAS 



Recent revision of the classification of the late Pleistocene deposits in Il- 

 linois has produced the framework necessary for a systematic study of fossil mol- 

 luscan faunas associated with the stratigraphic units currently assigned to the Wis- 

 consinan Stage. Knowledge of the relative ages of many units, derived from radio- 

 carbon dates, stratigraphic position, and relationship of moraines to the fossil- 



