214 ILLINOIS STATE ACADEMY OF SCIENCE 



dam will be 72 feet wide including a concrete apron but 

 exclusive of a 30-foot clay apron on the upstream side. It 

 will extend to a depth of 3i/^ feet below the lowest part 

 of the bed of the river, and cut-off sheeting will be driven 

 to a maximum depth of 30 feet below the base of the con- 

 crete portion of the dam. The ends of the dam will be 

 earth instead of concrete and cut-off sheeting will reach to 

 somewhat less depth than in the central portion of the 

 valley. 



Before undertaking the construction, the engineering 

 company made a series of test-holes across the valley to as- 

 certain the character and thickness of materials. Samples 

 were collected, but due to the fact that these were from a 

 "churn-drill," which separated the coarse from the fine, 

 nothing but sand and gravel was reported, whereas till was 

 found later to be involved. Upon these data the specifi- 

 cations were drawn and the work was undertaken. In 

 driving the cut-off sheeting, a hard-pan was encountered at 

 varying depths which was practically impossible to pene- 

 trate. It, therefore, became desirable to know the nature 

 of this hard pan, its thickness, and relations to underly- 

 ing materials. A geological examination was conse- 

 quently requested. 



GEOLOGICAL PHENOMENA 



Decatur is situated on the eastern border of the Shelby- 

 ville Moraine, which has a width here of several miles. The 

 altitude of the moraine at the river bluff is said to be about 

 650 feet above sea-level. The valley here is about 50 feet 

 deep, but farther west in the moraine it is 75 feet or more. 

 West of the moraine it is scarcely over 50 feet, but is much 

 wider than in the moraine. This seems to indicate that 

 the portion beyond the moraine is pre-Wisconsin in age, and 

 this is supported by some of the tributaries which are be- 

 headed by the Wisconsin deposits. ^ The incision of this 

 part of the valley in Illinoian drift would therefore make 

 its age post-Illinoian and pre-Wisconsin. 



Leverett states that the average thickness of the Shelby- 

 ville drift is 100 feet, an estimate based on the relief of 



I. Leverett, Frank: The Illinois Glacial Lobe, U. S. Geol. Surv., Mono. 38, 1899, 

 p. 518. 



