GEOLOGICAL AND NATURAL HISTORY SURVEY. 61 



Niagara limestone. Its resistance, therefore, may not be markedly 

 greater than that of the beds of glacial drift. 



As noted above, the level at which excavation by lake waters 

 began in the section belov^^ the great bend of the Illinois, is less 

 than lOO feet above the present stream, since the glacial terraces in 

 which the lake outlet was excavated seldom reach a level loo feet 

 above the bed of the outlet, while below the mouth of the Sanga- 

 mon they rise scarcely 50 feet above that level. If the 30 feet of 

 filling estimated by Prof. Cooley be added it seems a liberal esti- 

 mate to allow 75 feet of average excavation in this lower section of 

 200 miles. It may not have been more than two-thirds that 

 amount. The width of the outlet in this lower section ranges from 

 two up to about five miles, with an average of perhaps three miles. 

 This excavation is in a loose, easily eroded bed of sand and fine 

 gravel, which had been deposited largely by glacial streams. 



Summing up the above estimates, it appears that the outlet 

 has a width ranging from one mile up to about five miles, and a 

 depth ranging from 20 feet up to 70 feet. Its length from Summit 

 to the mouth of the Illinois is 300 miles. The excavation is prob- 

 ably not less than three cubic miles. With the exception of about 

 fifteen miles between Lemont and Joliet and forty miles between 

 Morris and Peru, where rock strata have been eroded, the excava- 

 tion is almost entirely in beds of drift. The width varies with the 

 resistance to erosion, being least in the section where the Niagara 

 limestone was eroded and greatest where there were only drift 

 beds to remove, while in the sandstone the channel is of inter- 

 mediate breadth. The breadth is also to some degree dependent 

 upon the slope of the bed, being narrower in the portions, with 

 rapid fall, th^m in portions having a low rate of descent. 



Throughout the entire length of the outlet the bluffs are steep, 

 like a river bank, and deposits made by side streams on the edge 

 of the valley are very meager, a feature which indicates that the 

 stream had great volume, probably filling the channel from blufif 

 to bluff, and a current sufficiently strong to carry away nearly all 

 the detritus brought into it by the side streams. 



The rapids between Romeo and Joliet occur in a section where 

 the limestone is friable, and it is thought by Professor Cooley that 

 the friability is such that falls could not have been maintained, or 

 even established. The removal of this, the main barrier in the 

 course of the outlet, it is estimated, would require the excavation 

 of a channel in rock only about 20 miles in length and 25 to 75 feet 

 in depth. Of this excavation scarcely one-tenth part was accom- 



