THE JULES SOIL AND ASSOCIATED MOLLUSCAN FAUNAS 3 



The Peoria Loess includes all of the eolian silts and fine sands above 

 the Robein Silt of Farmdalian age and beyond the limit of Woodfordian glacial 

 tills. It is typically developed south of Peoria along the Illinois Valley and in 

 the Mississippi Valley south of the southern terminus of the Illinois River. The 

 Peoria Loess in the Frederick South Section and the Cottonwood School Section 

 were suggested as reference sections for the type Woodfordian (Frye, Willman, 

 Rubin, and Black, 1968). 



More than 30 radiocarbon dates determined in central and northern Illi- 

 nois place the Robein Silt, of Farmdalian age, within the time span of 28,000 to 

 22,000 radiocarbon years B.P. (Willman and Frye, 1970). Several additional 

 dates from the Morton Loess (stratigraphically equivalent to the lowermost part 

 of the Peoria Loess) and from the basal part of the Wedron Formation tills fall 

 within the range of 19,000 to 22,000 radiocarbon years B.P. Within the Peoria 

 Loess are two radiocarbon dates, 17,950 ±550 (W-1055) from the Collinsville 

 Section and 17, 100 ±300 (W-73 0) from the Burdick Branch Section (Frye, Glass, 

 and Willman, 1962), that were determined on materials that are stratigraphically 

 below the position of the Jules Soil. 



Radiocarbon dates determined from the Cottonwood School South and 

 Jules Sections are placed stratigraphically in the described section in this report. 

 The date of 10,410 ±650 (ISGS-138), determined on snail shells from 1 1/2 to 

 2 1/2 feet above the top of the Jules Soil, is the youngest date so far determined 

 from the Peoria Loess in Illinois. The youngest date previously determined is 

 13,700 ±230 (1-1720) on snail shells from the uppermost part of the Peoria at 

 the Bald Bluff Section (Frye, Glass, and Willman, 1968) in the bluffs of the 

 Mississippi Valley in Henderson County. Immediately below the youngest date 

 at Cottonwood School South, however, a date of 15,640 ±580 (ISGS-137) was 

 determined on snail shells from the one-foot interval immediately above the top 

 of the Jules Soil. In an effort to date the soil directly, dates were run on organic 

 carbon extracted from the sediments. At Cottonwood School South the organic 

 carbon content of the one foot of Jules Soil from which humus was extracted 

 ranges from 0.20 percent to 0.37 percent. The dates from organic carbon from 

 the Jules Soil were 15,020 ±300 (ISGS-179) at the Jules Section, and 12,740 ±210 

 (ISGS-195) at the Cottonwood School South Section. As the probability of some 

 contamination by modern hair-roots in such shallowly buried soils is high, these 

 soil dates are probably somewhat too young. A reasonable evaluation of the 

 available dates indicates that the Jules Soil developed during part of the interval 

 between 15,500 and 16,500 B.P. rather than at about 16,500 B.P. (Frye, Glass, 

 and Willman, 1968) or 17,000 B.P. (Frye and Willman, 1973) as has been 

 suggested. 



In an attempt to relate the nodules of CaCO} that occur in the one-foot 

 zone below the base of the soil to the soil-forming process, they were radio- 

 carbon dated. One-third, by weight, of a group of nodules was dissolved and 

 dated as 5,030 ±100 (ISGS-139A), another one-third was dissolved and dated as 

 5,867 ±80 (ISGS-139B), and the remaining one-third was dissolved and dated as 

 7,370 ±140 (ISGS-139C). From the dates obtained, it is concluded that most of 

 the concretionary growth of the nodules took place after the development of the 

 Jules Soil and, as some concretions occur within the Jules Soil itself, they may 

 be largely related to the development of the modern surface solum. 



The Jules Soil is an immature A-C profile. Carbonate minerals persist 

 through the solum; but with the exception of a rare Succinea, snail shells are 

 not present in bed 3, the A- horizon of the soil. The A- horizon is darker colored 

 and less friable than the loess above and below; it is more clayey and is streaked 

 with some secondary carbonate, but it lacks distinctive soil structure. The 



