DEPOSITS OF THE LAKE CHICAGO BASIN 59 



» 



Stratum VII (Just above clay) 

 Animals 

 Naiad, fragments Planorbis antrosus 



Anodonta species " exacuous (rare) 



Valvata tricarinata " parvus 



Amnicola limosa Ancylus parallelus 



" lustrica Galba palustris 



Pkysa warreniana " reflexa 



Planorbis campanulatus Lymncsa slagnalis appressa 



Donacia proxima Coleoptera, punctostriate elytron 



The till beneath these sedimentary deposits is a portion of the Valparaiso 

 moraine, exhibiting the charactertistic undulating topography of a terminal 

 moraine. A mile or so west of the 92nd Street section, the naiad deposit and 

 the boulder pavement come within five feet of the surface, the boulder clay 

 increasing in thickness and the peat deposit decreasing in thickness. A few 

 hundred feet farther west these deposits again fall away to the bottom of the 

 canal. This elevation may have been an island during one of the low water 

 stages. The sequence of strata is the same in all locations, the variation being 

 in the relative thickness of the deposits. The interpretation of these deposits 

 may be summarized as follows: 



Stratum II, boulder clay. This deposit is very hard and contains an abun- 

 dance of rock material. 



Stratum III, boulder pavement, sand gravel. This stratum evidently repre- 

 sents the Glenwood, Bowmanville, and Calumet stages. During the Glen- 

 wood stage there would be some erosion of the boulder clay, notably, when 

 the water was lowering, preceding the Bowmanville stage. It was at this 

 period, probably, that the boulder pavement was formed. Later, during the 

 Bowmanville stage, a little clay was deposited, and, as the water was rising 

 to form the Calumet stage, a little gravel resulted. The bed of Unio shells 

 hang on the surface of this boulder pavement was probably deposited during 

 the latter part of the Calumet stage and the early part of the Toleston stage. 



Stratum IV, sand and gravel. The heavy bed of fine gravel and sand was 

 evidently formed during the low water period succeeding the Toleston stage 

 as it contains much beach-worn wood, worn shells and the usual mixture found 

 on a gravel y shore or in shallow water where there is somewhat of a current, 

 a condition easily provided by an outlet from a shallow lake such as Lake 

 Chicago is supposed to have been at this time. As the moraine beneath these 

 deposits is notably rolling the deep gravel and sand deposit may rest against 

 one of these dome-shaped elevations, which may have been near the surface 

 during this time. 



Stratum V,fine gray sand. This deposit is believed to represent a time when 

 the water was rising to form the Hammond stage. 



