548 



THE ILLINOIS GLACIAL LOBE. 



feet of drift, and so many portions of the Illinoian drift area in Western 

 Illinois, for they are built up about to the level of the summits of the rock 

 ridges. It is estimated that such areas embrace only about 3,550 square 

 miles. More than half the Illinoian drift area, embracing about 19,275 

 square miles, appears to have been filled to about two-thirds the capacity 

 of the preglacial valleys and to have perhaps 100 feet of drift. There is 

 an area of nearly 7,000 square miles lying mainly in southern Illinois but 

 embracing also portions of Stevenson, "Winnebago, Ogle, Carroll, and Jo 

 Daviess counties, in northwestern ' Illinois, where the average thickness can 

 scarcely reach 50 feet, the valleys being filled only to one-third or one- 

 fourth their full capacity. There remain about 8,000 square miles in 

 south-central and southwestern Illinois in which it is somewhat greater than 

 in the districts just mentioned and may average 75 feet. 



Summing up the above estimates in a tabular statement and averaging 

 the results for the State, we have the following table: 



Distribution, by depths, of glacial drift in Illinois. 



The above computations are necessarily less exact than those based 

 upon borings, but the method may prove helpful in supplementing one 

 based entirely on borings. It is some satisfaction to find that the results 

 obtained by the two methods are not so widely discordant as to leave 

 doubtful the general average thickness. As the computation just made 

 represents the maximum estimate of thickness, it should be compared with 

 the highest of the corrected estimates from borings. With this it is in 

 essential agreement, being but 9 feet higher. The average thickness for 



