PRE-ILLINOIAN (?) TILL AND ASSOCIATED DEPOSITS. 109 



In this connection attention is called again to the section of a shaft at 

 Coatsburg, in Adams County, in which a laminated clay of considerable 

 depth underlies the till, but which is not a strictly glacial deposit. The 

 Bloomington section may be of similar character in its lower portion. 

 The sections above discussed comprise the most puzzling ones reported in 

 the geology of Illinois. The buried soils there reported are usually found, 

 when in the district outside the Wisconsin drift, either at the base of the 

 loess, which is a post-Illinoian interval (Sangamon), or at the bottom of 

 the glacial deposits, where only silt or sand occurs between the soil and the 

 underlying rock. 



Prof. R. D. Salisbury has reported the occurrence of two sheets of 

 drift in southeastern Illinois and southwestern Indiana. He considers them 

 representatives of two episodes of a single glacial epoch. The upper sheet 

 is thought to extend fully as far as the lower, if not beyond it. 1 As the 

 features referred to by Salisbury have never been investigated by the 

 writer, some hesitancy is felt in offering an interpretation. It is, however, 

 suggested that the invasion limited on the southwest by the ridged drift of 

 the Kaskaskia Basin may have formed the upper sheet, while the invasion, 

 which in southwestern Illinois was the more extensive one, may have formed 

 the lower sheet. Possibly the interval will prove to be too long to support 

 this interpretation. In that case the lower sheet would be referable to a 

 pre-Illinoian invasion. 



In northern Illinois there are certain deposits which need consideration 

 in this connection. The studies of Mr. Ira M. Buell in eastern Winnebago 

 County led to the discovery of several exposures of glacial conglomerate 

 which appear along- the east bluff of Rock River. Bowlder-like masses of 

 this conglomerate have been incorporated in the till of that region, a good 

 illustration being found in the cuttings of the Illinois Central Railroad, 

 immediately southeast of Rockford. The firm cementation, and also a 

 deep orange-colored stain presented by the conglomerate from which these 

 bowlders are derived, seems to have been produced prior to the deposition 

 of the sheet of till in which they are embedded. This till is probably of 

 Iowan age. It therefore remains to be determined whether the conglom- 

 erate is of Illinoian age or of still earlier date. There are other deposits 

 in this region, noted by Buell, which- favor the view that there were not 



1 See Arkansas Geol. Survey, Crowleys Ridge, Report for 1889, Vol. II, p. 229. 



