216 ILLINOIS STATE ACADEMY OF SCIENCE 



SUPERPOSED DRIFT-SHEETS IN THE NORTH VALLEY WALL. 



In the north valley wall at the dam site, the construction 

 company has opened a clay pit for obtaining clay filling. 

 An unusual section is there exposed, having the following 

 sequence : 



3. Wisconsin till, overlain by "wash" from the slopes and capped 

 with soil; upper part, including "wash", is leached 3 to 4^ 

 feet, the underlying to 2H feet being calcareous; total 



exposed 41^ to 7 feet 



2. Old soil 6-8 inches 



1. lUinoian till, gray below the soil, brownish below, leached 3 to 



4 feet, calcareous below, compact; total exposed 6 feet 



A short distance west, the ditch for the cut-off sheeting, 

 which transects the valley slope, shows the same se- 

 quence, but here the trend of the exposure is at right an- 

 gles to the valley and shows the relation of the pre-Wiscon- 

 sin surface to the valley. The contact dips steeply toward 

 the valley in conformity with the valley wall. Thus it ap- 

 pears that the present north valley wall is almost essen- 

 tially that of pre-Wisconsin times. This is in agreement 

 with the belief that the thickness of the Wisconsin drift 

 was not sufficient, at least along the line of the Sangamon 

 Valley, to fill the valley. The failure of the hard-pan to 

 rise as much on the south side of the valley as on the north 

 is taken to indicate that the valley was wider in pre-Wis- 

 consin time than at present. This is in harmony with the 

 characteristics of the valley outside of the moraine. 



The hard-pan beneath the dam is at just such a level as 

 one would expect if the slope of the surface of the Illinoian 

 drift were projected into the valley. The sand deposit of 

 the valley, at this point, is, therefore, believed to rest di- 

 rectly on Illinoian drift, with little or no Wisconsin drift 

 intervening. The "hard-pan" is characteristic of the Illi- 

 noian. Leverett has recorded the general observation of 

 well drillers that the passage from Wisconsin to Illinoian 

 drift is marked by a difference in hardness, "the earlier 

 drift being partially cemented and much more difficult to 

 penetrate than the overlying later drift sheet."^ 



1. Idem, p. 199. 



