GEOLOGICAL AND NATURAL HISTORY SURVEY. 69 



Plaines and other tributaries entering the basin farther east, thus 

 permitting the water to issue at the western end of the basin, un- 

 burdened with glacial material. The stream discharging west- 

 ward from this basin would therefore have a tendency to deepen 

 the new valley opened across the Marseilles moraine, and in all 

 probability would have extended its excavation at least through 

 the new portion of the valley to Hennepin, there being in that 

 section a gradient of several inches per mile and possibly at first 

 a gradient of several feet. It seems not improbable, also, that 

 some excavation was accomplished by the glacial floods in their 

 passage over the terraces in the lower portion of the Illinois Valley, 

 the advantages for erosion being as good for these floods as for the 

 later ones fed by Lake Chicago. 



It is also necessary to estimate the amount of filling which the 

 lower course of the outlet has received since the lake waters were 

 withdrawn. Concerning this filling. Prof. L. E. Cooley has made 

 some investigation and concludes that it will average about 30 

 feet from Peru to the mouth of the Illinois, (i.) 



In the Des Plaines Valley the erosion of the Valparaiso 

 moraine and of the terraces outside of it was probably very largely 

 effected by the lake waters. An examination of this portion of the 

 outlet will therefore be likely to afford a fair understanding of the 

 size of the channel which it formed. 



Turning to the topographic maps, it appears that the bed of 

 the lake outlet declines from about 590 feet A. T. at Lemont, in 

 the midst of the Valparaiso moraine, to scarcely 500 feet at the 

 head of the Illinois, or 90 feet in a distance of 25 miles. Of this 

 fall yd feet is made in a little less than ten miles from Romeo to 

 Joliet pool. The glacial terraces which border the outlet decline 

 from about 630 feet to 570 feet between Lemont and the head of 

 the Illinois. This deepening of the channel is shown by the maps 

 to be somewhat irregular, ranging from 40 feet to about 70 feet, 

 but an average erosion of 50 feet may be assumed. This deepen- 

 ing embraces not only the work at the time the upper beach was 

 forming, but also that carried on during the formation of the 

 middle and third beaches, or down to the time of the final abandon- 

 ment of the lake outlet. The channel above Joliet has a breadth of 

 one to one and a half miles, averaging perhaps one and a quarter 

 miles. Between Joliet and the head of the Illinois several island- 

 like remnants of the glacial terraces remain in the midst of the 

 channel, making it more difficult to estimate the breadth, but it is 



[1] Communicated to the writer, 



