10 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 486 



while the sharp decrease in total clay and in percent of expandable clay minerals 

 at the top of the soil indicates a sudden influx of outwash into the Illinois 

 Valley (figs. 1 and 2). 



The maximum thickness of the Jules Soil zone as shown by the Bunkum 

 South Section (fig. 2) in the bluff of the Mississippi River Valley, below the 

 mouths of both the Illinois River and the Missouri River, confirms these general 

 conclusions. In addition, the relations in the thickest sequence at the bluff 

 line indicate that the very slow loess deposition during the episode of Jules Soil 

 development was interrupted by minor pulses of more rapid deposition. At the 

 position of the Bunkum South Section* the sediment source for loess has its maxi- 

 mum complexity, as sediment from the Lake Michigan Lobe was contributed by 

 way of the Illinois Valley, from the north by way of the upper Mississippi Valley, 

 and from the west and northwest by way of the Missouri Valley. The complex 

 interrelations of these three contrasting source regions are shown by the clay 

 mineral assemblages (fig. 2), since the Illinois Valley source is characterized 

 by a high percentage of illite, the upper Mississippi source by a moderately high 

 percentage of expandable clay minerals, and the Missouri Valley source by an 

 exceptionally high percentage of expandable clay minerals. As at the Jules and 

 Cottonwood School South Sections, the episode of Jules Soil development is 

 characterized by a marked decrease in sediment from the Lake Michigan Lobe by 

 way of the Illinois Valley. 



These data on the Jules Soil are all consistent with the previous interpre- 

 tation (Frye, Glass, and Willman, 1968; Frye and Willman, 1973) that the soil 

 marks the stratigraphic position of the most extensive glacial withdrawal of the 

 Lake Michigan Lobe during Woodfordian time. The glacier front retreated into the 

 basin of Lake Michigan and much of the outwash that was discharged from the 

 glacier was trapped in the slack waters of the pro-glacial lake. Thus, a signif- 

 icant part of the eolian sediment that was deposited at the Cottonwood School 

 South locality and other localities of the middle Illinois Valley was derived from 

 the high-montmorillonite source of the Mississippi Valley and possibly from 

 other sources farther west. These more remote sources furnished finer textured 

 materials, and the clay fraction contained a much higher percentage of expand- 

 able clay minerals, which is characteristic of the northwestern source region. 

 The very slow rate of deposition of eolian sediment allowed A-horizon develop- 

 ment to keep pace with the slow rate of deposition, and therefore the Jules Soil 

 is recognizable. 



Of all the data presented here in regard to the Jules Soil, the only item 

 that is not compatible with the bulk of the data is the date of 10,410 ±650 (ISGS- 

 138) determined on shells only 1 to 2 feet above the date of 15,640 ±580 (ISGS- 

 137) determined on shells within 1 foot above the top of the soil. As there is no 

 evidence of soil formation between the dates, as the texture and the clay mineral 

 assemblage of the two zones is similar, as the molluscan faunas of the two zones 

 do not indicate a marked stratigraphic separation, and as there was no glacial 

 outwash being discharged down the Illinois Valley at that time, it seems reason- 

 able to conclude that the uppermost date is too young. If this conclusion is 

 conceded, then the uppermost part of the Peoria Loess at the Cottonwood School 

 South and Jules Sections is latest Woodfordian in age and approximately equiv- 

 alent in age to the uppermost Peoria Loess of the Mississippi Valley (Bald Bluff 

 Section), which contains the youngest dated fauna at 13,700 ±230 (1-1720). 



