POSTGLACIAL BIOTA OF THE GREAT LAKES REGION 123 



Undetermined proboscidians. 



Near Madison, teeth and small bones; Fond du Lac, small leg bones in ditch; Lake 

 Monona, Madison, vertebra. 



II. ILLINOIS 



An account of the lake stages, with their attendant life, has been given in 

 Chapters LT and III. Outside this lake basin there are many records of life 

 which are referable to post-Wisconsin time, but which cannot be definitely 

 placed in any one of the lake stages. These may be considered under two 

 heads; 1, fluviatile, and 2, terrestrial. 



1. Fluviatile Deposits 



Shimek 16 has described a deposit of shells near Rock Island, in a terrace 

 north of Turkey Island. The terrace is said by Leverett 17 to be of Wisconsin 

 age. The diagrams in figure 2 indicate the position and sequence of the strata 

 and the location of the section. Shimek 18 believes that C was evidently an 

 old sand bar upon which were heaped the shells while it lay in the path of a 

 strong current, this supposition being indicated by the coarseness of the mater- 

 ial. The middle stratum was deposited while strong currents of flood times 

 alternated with the more sluggish currents of low water and we find a conse- 

 quent commingling of coarse and fine material. The upper deposit containing 

 carbonaceous material was deposited during floods when silt-laden water backed 

 up and overflowed the land after the river had receded from the old sand-bar. 

 The upper deposits are somewhat loess-like in character. As shown by Lever- 

 ett, this terrace is of wide extent. Shimek made borings 40, 50, and 150 feet 

 from the edge of the bank and found the sections substantially the same as the 

 river bank section. This deposit was probably contemporaneous with an early 

 stage of Great Lake history, perhaps correlative with the Calumet stage, 

 and certainly as early as the Toleston stage. As indicated in the diagram 

 the lower deposits contain fluviatile life only, while the upper deposits are made 

 up mainly of terrestrial life. The absence of Unios, other than fragments, is 

 noteworthy. The upper stratum represents a period of time somewhat later 

 than the lowest stratum, indicating that the volume of water was much less. 



Fluviatile species, mostly from stratum C 19 

 Unio, fragments Pyrgulopsis scalariformis 



Sphaerium striatinum Somalgyrns subglobosus 



" simile{= sulcatum) " integer 



Pisidium abditum Pomaliopsis lapidaria 



» Bull. Lab. Nat. Hist. State Univ. Iowa, II, pp. 170-171. 



17 Illinois Glacial Lobe, plate VI. 



18 Op. tit., pp. 173-174. 



" The newer nomenclature is used in these lists. The species here listed as coarctatum 

 was possibly a form of exilis; coarctatum is a southern species. 



