4 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 461 



physical appearance of having been incorporated into till at the overriding margin 

 of the glacier (Long Lake Section). Such faunas may be of approximately the same 

 age as the fauna from the Harkness Silt at the Zion Church Section, but the strat- 

 igraphic evidence is not clearly indicative of such a conclusion. 



Most of the other faunas from the Banner Formation were collected from 

 silt beds that either are interstratified with tills or that occur at such a position 

 that they are presumed to be interstratified with tills of Kansan age. At the Tin- 

 dall School Section (Willman and Frye, 1970), which is the type section of the 

 Banner Formation, the fauna occurs in fine sand and silt above calcareous till but 

 below a thick sequence of tills and interstratified sand and silt. At the Big Sister 

 Creek Section (this report) the fauna was collected from a silt bed above calcar- 

 eous till that is classed as part of the Banner Formation. A similar stratigraphic 

 relation occurs at the Taylorville Dam Section (Johnson, 1964), at the Harmattan 

 Strip Mine Section No. 2 (Johnson, 1971), and at the Petersburg Dam Section No. 

 2 (Johnson, 1964). It is evident that all faunas listed as Kansan in this report 

 occur stratigraphically well below the top of the Banner Formation and in a posi- 

 tion that calls for an age assignment older than late Kansan. 



Illinoian Stage 



The Illinoian Stage has been subdivided into three substages by Willman 

 and Frye (1970): the Liman (oldest), Monican, and Jubileean (youngest). In the 

 following discussions of molluscan faunas, those from the Liman Substage are 

 considered as a unit, but those of the Monican and Jubileean Substages are 

 grouped together. 



Liman Substage 



The faunas of Liman age described here were all collected from the Peters- 

 burg Silt (Willman, Glass, and Frye, 1963; Frye, Willman, and Glass, 1964), 

 which is the lowest formation of Illinoian age over much of western and central 

 Illinois and which also occurs sporadically in the eastern and southern parts of 

 the state. It consists of water-laid silt and in places contains a significant per- 

 centage of loess. The Petersburg Silt rests on the Yarmouth Soil or older deposits, 

 is generally a proglacial deposit that was overridden by the advancing earliest 

 Illinoian glacier, and is overlain by the basal tills or coarse outwash of the Glas- 

 ford Formation. The composition of the silts suggests that initial deposition was 

 the result of local erosion of the pre-existing deposits caused by the climatic 

 change accompanying the advancing Illinoian glacier, but the upper part of the 

 unit reflects the composition of the overlying till and is the result of deposition 

 of proglacial outwash in lakes and the deposition of loess from adjacent outwash- 

 carrying valleys. At some places the overriding glacier incorporated fossiliferous 

 silt into the lowermost few feet of the till (e.g., Petersburg Section, Willman and 

 Frye, 1970). 



The Petersburg Silt is terminated by vertical cutoff at the limit of glacial 

 till of Illinoian age. Beyond the glacial limit silts that may be stratigraphically 

 equivalent to the Petersburg are combined with silts stratigraphically equivalent 

 to silts in the Glasford Formation and to the overlying Teneriffe Silt to form the 

 Loveland Silt. Only one molluscan fauna from the Loveland Silt is listed here. 

 Because it was collected from the upper part of the formation, it is placed in the 

 Monican and Jubileean Substages. 



