424 THE ILLINOIS GLACIAL LOBE. 



The width of the outlet between Morris and Hennepin averages about 

 1 .\ miles. The excavation is largely in soft St. Peter sandstone, there being 

 nearly continuous rock bluffs to a height of 60 to 75 feet above the level of 

 the bed of the outlet. This sandstone and the 0oal Measures sandstone 

 which in places overlies it present much less resistance to stream action than 

 the firm Lockport (Niagara) limestone. The resistance 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 below the great bend of the Illinois is less than 1 00 feet above 

 the present stream, since the glacial terraces in which the lake outlet was 

 excavated seldom reach a level 100 feet above the bed of the outlet, while 

 below the mouth of the Sangamon they rise scarcely 50 feet above that 

 level. If the 30 feet of filling estimated by Professor Cooley be added, it 

 seems a liberal estimate 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 2 up to 

 about 5 miles, with an average of perhaps 3 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 1 mile up to about 5 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 probably not less than 3 cubic miles. With the 

 exception of about 15 miles between Lemont and Joliet and 40 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 resist- 

 ance to erosion, being least in the section where the resistant limestone was 

 eroded and greatest where there were only drift beds to remove, while in 

 the sandstone the channel is of intermediate breadth. The breadth is also 

 to some degree dependent upon the slope of the bed, being narrower in the 

 portions with rapid fall than 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 vol- 

 ume, probably filling the channel from bluff to bluff, and a current sum- 





