422 THE ILLINOIS GLACIAL LOBE. 



glacial Hrii ids mi the Des Plaines and other tributaries entering' the basin 

 farther east, thus permitting the water to issue at the western end of the 

 basin, unburdened with glacial material. The stream discharging westward 

 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 tirst a higher gradient. It seems not improbable, also, that 

 some excavation was accomplished by the glacial floods in their passage 

 over tlu- 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 course of the outlet has received since the lake waters were withdrawn.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 rilling, Prof. L. E. Cooler has made some investigation and 

 concludes that from Peru to Peoria it will average 30 feet, and is appreciable 

 To the mouth of the Illinois, though probably somewhat less toward the 

 mouth of the river. 1 



In the 1 >es 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. 



From the topographic maps it appears that the lied of the lake outlet 

 declines from about 590 feet at Lemont, in the midst of the Valparaiso 

 system, to scarcely 500 feet at the head of the Illinois, or 90 feet in a 

 distance of 25 miles. Of this fall, 76 feet is made in a little less than 10 

 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 bv the maps to lie 

 somewhat irregular, ranging from 40 feet to about 70 feet, but an average 

 erosion of 50 feet may lie assumed. This deepening embraces not only 

 the work at the time the upper beach was forming, but also that carried on 

 during the formation of the second and third beaches, or down to the time 

 of the final abandonment of the lake outlet. The channel above Joliet has 



'Communicated to the writer. 



