AVERAGE THICKNESS OF THE DRIFT IN ILLINOIS. 545 



the average thickness for each township is 77.9 feet. In making the esti- 

 mates care has been taken to avoid duplications; thus where several borings 

 have been made on a single square mile which differ very little in distance 

 to rock they are considered as one boring. If, however, borings in close 

 proximity show a wide difference, they are each included in the computation. 



After averaging the drift by townships the question was considered 

 whether the townships in which rock has been struck fairly represent the 

 thickness of the drift. This involved the consideration of their distribution 

 in reference to moraines and in reference to preglacial valleys, both being 

 lines where the drift has exceptional thickness. It was found that the 

 moraines have been about as thoroughly tested as the tracts of thinner drift 

 separating them, but an examination into the distribution along preglacial 

 valleys has led to the conclusion that they are not adequately represented. 

 Of the 1,179 borings which reach rock, 1,065, or fully 90 per cent, enter it 

 near the level of the preglacial uplands, while only 114, or scarcely 10 per 

 cent, pass markedly below the general level of those uplands before striking 

 rock. It is difficult to decide upon the portion of the surface to be included 

 in the valleys, especially if their intricate network of tributaries is included, 

 but it may confidently be placed at not less than 30 per cent; possibly it 

 may reach 50 per cent. Upon computation it is found that the average 

 depth to rock of the borings along preglacial valleys is 172.44 feet, while 

 the depth of those on the uplands is 67.87 feet. If the uplands comprise 

 70 per cent of the glaciated portion of Illinois and the preglacial valleys 

 30 per cent, the average thickness of drift will be 99.23 feet. If the uplands 

 comprise 60 per cent and the valleys 40 per cent, the average will be 109.7 

 feet. If the uplands comprise only 50 per cent and the valleys 50 per cent, 

 the average will be 120.15 feet. By this method of computation, therefore, 

 the thickness appears to be not less than 99 feet, and it may be 120 feet or 

 even more. 



The thickness is on the whole greater within the area covered by the 

 Wisconsin drift than in other portions of the State. Of the 487 borings 

 within that area which reach rock, the average drift thickness is 121 feet. 

 Of these borings only 52, or about 10 per cent, have struck preglacial val- 

 leys. The borings on the preglacial uplands enter rock at an average depth 

 of 109.7 feet, while those along preglacial valleys average 221 feet to the 



MON XXXV11I 35 



