110 ILLINOIS ACADEMY OF SCIENCE. 



Anodonta grandis. Ancylus sp. 



Pisidium (several species). Physa ancillaria warreniana. 



Sphaerhim simile. Planorhis trivolvis. 



Goniobasis livescens. Planorbis campanulatus. 



Ainnicola lustrica. Planorbis hicarinatus. 



Amnicola limosa. Planorbis deflectus. 



Valvata tricarinata. Lymnaea stagnalis appressa. 



Campeloma integrum. Galba reilexa. 



The presence of this life in a silt deposit, overlying a sand 

 deposit, is conclusive evidence that the early statement of Dr. 

 Andrews^ concerning a post-Glenwood low-water stage was cor- 

 rect, although this is questioned by Dr. Goldthwait in a recent 

 paper in which this deposit is referred to the Calumet period.* 



That this stage was one of very low water is apparently proven 

 by the fact that these shell deposits do not extend far beyond 

 the 585 foot contour or about a mile and a half north of Foster 

 Avenue. As the leaves of an oak and the cones and wood of a 

 spruce are also found in these strata, it would seem that this 

 deposit represents mainly shallow ponds formed, possibly, in large 

 kettle holes in the ground moraine. The ground moraine at this 

 point, as seen in cross sections, is strikingly undulating, forming 

 depressions from six to ten feet in depth, and of a sufficient size 

 to form a pond of good area. It is interesting to note that at 

 Lemont a bed of silt with shells is encountered overlying the 

 Niagara limestone, which may represent this stage. It is over- 

 laid by six feet of carbonaceous soil and peat containing mollusks. 

 As both deposits contain the same genera and nearly the same 

 species, it seems evident that the fauna near Foster Avenue 

 migrated thence by way of the Des Plaines outlet. 



It must be borne in mind that during the several advances of the 

 ice all life was either exterminated within the englaciated area, 

 or was driven south of the ice border. Consequently, a return 

 of life to the country left by the receding ice sheet could only be 

 from the south. The aquatic life could return only by way of the 

 natural waterways provided by the glacial streams issuing from 

 the ice-bound lakes. The oak {Qucrcus marceyana) and the 

 spruce (Picea evanstoni=^ canadensis?) probably grew on the 

 higher ground. The latter does not at the present time grow 

 within about one hundred miles of this locality. The spruce cones 

 are notably abundant in deposits near Devon Avenue. 



"Trans. Chi. Acad. Sci., II, pp. 1-24, 1877; Leverett, Bull. Geol. Surv. Chi. 

 Acad. Sci., II, p. 71, 1897; Alden, Chicago Folio (81) p. 9, 1902. 

 *Bull. 7, 111. Geol. Surv., p. 61, 1908. 



