2 DRIFT DEPOSITS OF ILLINOIS. 



Second The drift deposits are also the main source of our 

 water supply, and of those homely but indispensable products, 

 sand, clay and gravel, which enter so largely into the industrial 

 pursuits of the laboring classes. Every man who sinks a well, 

 digs a cellar or a ditch, or grades a roadway, penetrates this 

 formation, and hence it becomes the one with which the people 

 are most frequently brought in contact, and therefore the one 

 in which they are more directly interested than in any other of 

 the geological formations. 



In order to arrive at a satisfactory solution of the phenomena 

 presented by the drift deposits, it will be necessary to consider, 

 briefly, the condition that prevailed at the commencement of 

 the period which they represent. 



At the close of the carboniferous era, nearly the whole area 

 of the State of Illinois, as well as that of the adjacent States 

 on the north and east, was elevated above the ocean's level, 

 beneath which it has not since been submerged. During all thai 

 vast period of time, during which the Jurassic, Cretaceous and 

 Tertiary formations were deposited over such portions of the 

 continent as were still beneath the ocean, nearlv the entire area 

 of Illinois was above the ocean's level, and subject to the erosive 

 agencies that are always prevalent upon the land. 



During the earlier portion of this period, the river valleys 

 were excavated, as well as others that are now filled with drift 

 material, the existence of which is entirely unknown and unsus- 

 pected until revealed by artificial excavations. The eroding- 

 forces of this period were not confined to the excavation of the 

 river valleys, but they carried away a vast amount of solid 

 rock strata, not only from the northern portion of the state, 

 where the evidence of erosion is most apparent, but probably 

 over nearly the whole area of the state as well. 



Prof. D. J. Whitney 7 , in his report on the lead region, in the 

 first volume of the geology of AVisconsin, estimates the amount 

 of solid rock strata removed by erosive agencies over the region 

 south of the AVisconsin river, at three hundred and fifty to four 

 hundred feet, and it is highly probable that even this estimate, 

 large as it may seem, does not fully represent the extent of the 

 erosion that has taken place not only in northern Illinois, but 

 over the greater portion of its entire area. 



