PAPERS ON GEOLOGY AND GEOGRAPHY 213 



THE GLACIAL HISTORY OF THE SANGAMON RIVER 



VALLEY AT DECATUR AND ITS BEARING 



ON THE RESERVOIR PROJECT 



Morris M. Leighton 

 State Geological Survey, Urbana 



Glacial Geology seems so far removed from applied sci- 

 ence that one might pass the judgment that it has no utili- 

 tarian end; that its chief justification lies in its contribu- 

 tions to pure geology, in helping to reveal to mankind that 

 history of his surroundings. Without doubt one of its 

 chief functions is that of supporting pure science, but one 

 needs only to come in touch with the soil specialist, or the 

 tunnel engineering expert, or the hydrographic engineer, or 

 the materials-survey man to be reminded that in all states 

 north of the glacial drift boundary and in glaciated moun- 

 tains, Glacial Geology has a very definite bearing on cer- 

 tain industrial projects. 



Experience has taught the engineer that the construc- 

 tion of a dam across any valley in the glaciated area must 

 be undertaken with caution. The materials beneath the 

 dam must have sufl^icient bearing power to support the 

 weight of the dam, and must be tight enough to prevent 

 seepage from beneath. The materials at the ends of the 

 dam and in the natural walls of the reservoir must also be- 

 impei'vious to excessive leakage. In order to pass judg- 

 ment upon the character of glacial material for these pur- 

 poses the engineer desires to know the properties of these 

 materials, the possible relationship of the porous to the 

 nonporous, and the characteristics of the drift-sheets at his 

 particular locality. Obviously this involves the principles 

 of glacial and aqueo-glacial sedimentation and in some 

 cases the differentiation of drift-sheets. 



THE DECATUR DAM 



At Decatur, Illinois, a dam is being constructed across 

 the Sangamon River Valley, which will be 15 feet hieh 

 above the present river level and which will impound the 

 waters upstream for a distance of about 12 miles and create 

 a reservoir over 3,900 acres of territory. The base of the 



