THE JULES SOIL AND ASSOCIATED MOLLUSCAN FAUNAS 9 



small field sample. Each of these species was also encountered in the lower 

 sample, taken from the loess above the Jules Soil complex (fig. 2). The ecolog- 

 ical requirements of the total faunal assemblage suggest a moist, wooded slope. 

 In contrast to the settings at Cottonwood South, Bald Bluff, and the previously 

 mentioned locality in Tazewell County where intermittent ponding is suggested, 

 the molluscan fauna at Bunkum South does not include Lymnaea parva and L. dalli. 

 This fact, coupled with the topographic position on the bluff line and the coarse 

 texture of the loess, suggests that even intermittent ponding did not occur at 

 this locality. There seems to be no stratigraphic significance to the differences 

 between the upper and the lower faunal assemblage; rather, a comparison of the 

 two suggests that the more delicate shells may have been removed from the upper 

 horizon by downward movement of carbonates. 



Bald Bluff Faunal Assemblage . The faunal assemblage at this locality is 

 similar to those at the other three localities, although removed from them by a 

 considerable distance. The habitat seems to have been a poorly drained upland 

 prairie to woodland border environment, as attested to by the composition of the 

 fauna, but under a climatic regimen approximately equivalent to that prevailing 

 at the same time at the other localities. 



DISCUSSION AND CONCLUSIONS 



The Jules Soil is of stratigraphic significance because its development 

 coincides with the maximum glacial retreat of the Lake Michigan Lobe during the 

 Woodfordian Substage (Frye and Willman, 1973) and may coincide with the inter- 

 val designated as the Erie Interstade (Dreimanis and Goldthwait, 1973; Morner 

 and Dreimanis, 1973). In the Peoria Loess of the middle Illinois Valley, the soil 

 has been stratigraphically placed between clay mineral Zones II and III previously 

 described by Frye, Glass, and Willman (1968). Its clay mineral assemblage 

 sharply contrasts with that of the loess above and below it because the soil has 

 a much higher percentage of expandable clay minerals. In fact, the clay mineral 

 composition of the Jules Soil resembles that of clay mineral Zones I and IV 

 (derived from a Mississippi Valley source having a relatively high percentage of 

 montmorillonite) more closely than it does Zones II and III, which reflect a source 

 higher in illite in the Lake Michigan Lobe outwash of the Illinois Valley. 



The morphology of the Jules Soil at the Jules and Cottonwood School South 

 Sections indicates that it did not form by in situ development downward from a 

 stable land surface, but rather that it is an A-horizon that continued to develop 

 as it was slowly extended upward by continuing but small additions of eolian 

 sediment on the soil surface. This history is indicated by the thickness and 

 character of the A-horizon, which retains primary carbonates throughout, although 

 it is generally lacking in aragonite snail shells; by the lack of a textural, or 

 even a cambic, B-horizon below an A-horizon 1 foot or more thick; and by the 

 lack of discernible weathering effect on the expandable clay minerals. The fact 

 that the percentage of less-than-2-micron clay in the soil is much higher than in 

 the loess above and below it suggests that at least part of the eolian sediment 

 was derived from a more remote source than the nearby Illinois Valley, which was 

 the source for the bulk of the Peoria Loess along the valley bluffs. Also, the pro- 

 gressive upward increase in percentage of clay and of expandable clay minerals 

 suggests that the nearby Illinois Valley loess source was contributing only a 

 small percent of the deposited sediment by the end of Jules Soil development, 



