2 ILLINOIS STATE GEOLOGICAL SURVEY CIRCULAR 461 



STRATIGRAPHY OF FOSSILIFEROUS UNITS 



The stratigraphy of the Pleistocene deposits of Illinois has recently been 

 described and extensively reclassified by Willman and Frye (1970), and the strat- 

 igraphic terminology used in this report is in accord with their reclassification. 

 Stratigraphic names used here are shown in figure 2. Because of the recency of 

 this more detailed stratigraphic report, the stratigraphy of the deposits is not re- 

 described in detail here. Rather, the molluscan faunas are placed briefly in their 

 proper stratigraphic setting, and the reader is referred to the previously published 

 reports for more detailed descriptions of units. The discussion of the stratigraphy 

 and faunas is arranged by time-stratigraphic unit, but the faunas are related to 

 the rock- stratigraphic unit in which they occur. Seven described stratigraphic 

 sections (including eight faunal localities) are included with this report, and de- 

 scribed stratigraphic sections for 14 additional faunal localities have been pub- 

 lished previously and are cited in the locality list accompanying figure 3. Fos- 

 sil shells were collected from silt, silt and sand, or clayey silt and occurred in 

 eight rock-stratigraphic units. 



Kansan Stage 



The fossiliferous deposits of the Kansan Stage in Illinois are all included 

 within the Banner Formation (Willman and Frye, 1970). The Banner Formation con- 

 sists of glacial tills and intercalated outwash of sand and gravel and silt. At the 

 base there occurs a proglacial silt unit classed as the Harkness Silt Member, and 

 at the top occurs the silty clay and accretion deposits of the Lierle Clay Member. 

 Further subdivision of the Banner has been undertaken only in part of eastern Illi- 

 nois, where four till members and one fossiliferous silt, the Belgium Member, 

 have been defined (fig. 2) (Johnson, Gross, and Moran, in press; Johnson, 1971). 

 The Banner Formation is described in two of the stratigraphic sections in this re- 

 port, and deposits now assigned to the Banner are described in five of the pre- 

 viously published stratigraphic sections cited in the list of localities following 

 figure 3. 



The Banner Formation is predominantly glacial till and coarse outwash, but 

 the molluscan faunas occur in silt and sandy silt units. The Harkness Silt Member 

 occurs below the lowest of the tills and rests directly on the top of the Afton Soil 

 developed in deposits of Nebraskan age. Fossil snails have been collected from the 

 Harkness Silt Member only at the Zion Church Section (Frye, Willman, and Glass, 

 1964; Willman and Frye, 1970) near the bluff of the Mississippi River Valley in 

 extreme western Illinois. The silt appears to have been deposited in water short- 

 ly before the advance of the Kansan glacier into Illinois from the northwest. The 

 shells are sparsely distributed throughout the deposit, and a meaningful fauna was 

 obtained only by bulk sampling and concentration of the shells by washing. 



The Banner Formation, above the Harkness Silt Member, consists of inter- 

 stratified till, outwash, and silts. At some localities the silts contain fossil 

 mollusks. Several of the collections were made from silts that occur below one or 

 more tills of Kansan age, but because the deposit did not rest on the top of the 

 Afton Soil it was not classified as Harkness Silt Member. Notable among such 

 localities are the Long Lake Section (this report) and the Groveland Southwest 

 Section (fig. 3). Here, silts occur immediately above bedrock and are either 

 overlain by till of the Banner Formation (Groveland SW Section) or present the 



