496 THE ILLINOIS GLACIAL LOBE. 



have opened new courses since the close of the Iowan stage of glaciation 

 it is the rule to find rock rapids, and even low falls. 



Hershey devotes a considerable part of his paper to the discussion of the 

 C( imparative amount of erosion in these valleys and those opened at the Iowan 

 and Wisconsin stages of glaciation, and concludes that the time required in 

 their excavation is several times that of the Iowan as well as of the Wisconsin. 



Rock gorges occur to some extent along the valleys of western Illinois 

 south of Rock River, but they are not of a class similar to those of north- 

 western Illinois just discussed. They occur usually where streams are 

 opening a course entirely independent of preglacial A^alleys, and where 

 after cutting through the entire depth of drift they have begun the excava- 

 tion of the rock. These valleys often have rocky rapids and low falls. It 

 is difficult to compare the amount of work accomplished by these streams 

 with that accomplished by the streams of northwestern Illinois. The 

 streams of western Illinois have had usually a large amount of drift to 

 remove, and consequently have been prevented from beginning the excava- 

 tion of the rock at so early a date as was possible for streams in northwest- 

 ern Illinois. The valleys which they have excavated in the drift, show 

 much greater cubic contents than the rock gorges of similar sized streams 

 of northwestern Illinois. It remains to be determined whether the work 

 accomplished in removing the drift deposits is sufficient to offset the imma- 

 ture state of development on the rock gorges, or whether the excavation of 

 the rock gorges of the two districts began at different dates. In western 

 Illinois the excavatiou may confidently be placed after the lllinoian ice 

 invasion, but in northwestern Illinois the possibility of deflection by a pre- 

 Illinoian invasion must be considered. 



ILLINOIS RIVER DRAINAGE BASIN. 



The watershed of the Illinois River extends in a broad band, averaging 

 100 miles in width, in a northeast-southwest direction directly across the 

 center of Illinois. From the northeastern extremity of this band there are 

 two projections — one north into Wisconsin, including- the Fox and Des 

 Plaines basin; the other east into Indiana, including the Kankakee and its 

 main tributary, the Iroquois. The name Illinois is applied to the river 

 from the junction of the Kankakee and Des Plaines. The western side of 

 the watershed is 20 to 40 miles in width, while the eastern is 60 to 80 miles. 

 The entire area is estimated b}' Greenleaf, in his report for the Tenth Census, 



