PAPERS ON GEOLOGY AND GEOGRAPHY 215 



the moraine above. If this were the case, it would seem 

 that the valley through the moraine must have been com- 

 pletely filled, but this figure seems excessive, and there are 

 reasons for believing that the valley was not buried. 



After the hard-pan was discovered, a new series of well- 

 borings was made, this time in such a way that samples 

 of the material as it actually occurs were taken at desired 

 intervals. Three well-borings and four test-pits were put 

 down on the north side and eleven well-borings and four 

 test-pits on the south side. These revealed the following 

 strata, as shown in Figure 1. 



GENERALIZED SECTION OF THE STRATA IN THE SANGAMON 



RIVER VALLEY NEAR DECATUR, AS DETERMINED FROM 



BORINGS AND TEST PITS. 



Thickness 

 Feet 



3. Soil and flood-plain alluvium, a brownish sandy silt toward the 



V2dley walls, becoming darker near the stream 5-10 



2. Sand and gravel, distinctly sandy, thins out toward the valley 



walls, thickens toward the stream, maximum thickness 35 



1. Till, called "hardpan" by the drUlers, dense, compact clay with 

 scattered pebbles and bowlders, difficult to penetrate, contains 

 a few thin layers of sand, of which the thickest, 12 feet, is on the 

 south side, maximum thickness penetrated 54 



An examination was made of the glacial till exposed in 

 the new sewage ditch along the valley farther west in the 

 moraine. The till here contained considerable masses and 

 thick lenses of gravel and sand, some of which show marked 

 contortion by ice movement. One of these lenses was about 

 200 yards long. If such a lens were to occur in the till be- 

 neath the dam, it might permit excess leakage. But the till 

 in the sewage ditch seems quite clearly to belong to the 

 Shelby\ille moraine, while the till beneath the dam appears 

 to be Illinoian, of the ground moraine type. If so, their dif- 

 ference in origin would appear to have a direct bearing on 

 the question of dam construction. The reasons for this 

 discrimination will now be considered. 



