294 LIFE OF THE PLEISTOCENE 



In northeastern Illinois three drift sheets are apparently penetrated in 

 well borings. These vary in thickness as noted below. 57 



Upper or Wisconsin drift sheet 50-100 feet. 



Middle drift sheet 15-50 " 



Lower drift sheet 45-175 " 



The upper or Wisconsin till is soft, while the two lower drifts are very hard — 

 hence called by tihe drillers "hard-pan. " Just what the two lower drift sheets 

 are has not been definitely indicated. One is certainly Illinoian. The present 

 aspect of the Iowan drift would seem to exclude that till sheet. It might be 

 the equivalent of the Kansan; or these two lower sheets might be variations 

 of one sheet, the Illinoian. Until more data are available it seems unsafe to 

 claim more than one soil horizon — the Sangamon. 



The Sangamon interval is represented in the Upper Illinois Valley, where 

 the river has cut thru the drift deposits. On the west bank of Spring Creek, 

 about a mile below Dalzell, 57a a section shows the following strata. The life 

 has not been identified as far as known to the writer. 



Bloomington till (Wisconsin) 40 feet 



Loess 15-20 feet 



Silt at base of section crowded with shells and roots of plants (probably Sanga- 

 mon) x 



c. Florencia Formation 



Some years ago, Mr. Hershey 58 described a fossil-bearing formation from 

 near Freeport, Stephenson County, to which he gave the name "Florencia 

 Formation " and assigned it to a post-Kansan interval, below the Iowan loess. 

 The formation may be thus briefly described. The basal member (which 

 rests on drift) is composed of coarse, subangular gravel, containing a few mol- 

 lusks and some drift wood. The thickness is unknown, but is believed to be 

 as much as 20 feet. Resting upon this gravel are three deposits, a dark blue- 

 green silt, light brownish gray sand, and dark brown carbonaceous clay or muck. 

 The thickness of these deposits is variable. The muck is overlaid by laminated 

 and variegated clays. The Florencia deposits are of fluviatile origin and were 

 formed in the bed of an interglacial stream, probably an ancient Pecatonica 

 River. 



Hershey says: 59 "It rests upon the Kansan drift sheet everywhere except 

 where post-Kansan erosion has completely removed the till and other glacial 

 deposits. It is, therefore, separated from the latter by an erosion interval 

 of the length of which the interglacial rock gorges of this region are the gauge. 



" Leverett, op. cit., 142-143. 



" a Sauer, Bull. 111. State Geol. Surv., No. 27, p. 73, 1916. 



•• Amer. Journ. Sci., (iv), IV, pp. 90-98. 



"Op. cit., pp. 93-94. 



