492 THE ILLINOIS GLACIAL LOBE. 



channeling, therefore, seems referable mainly to the interval preceding the 

 Iowan glaciatiou, and only in small part to that succeeding it. 



An instance of a rock gorge formed subsequent to the Iowan stage of 

 glaciatiou is found in the west part of the city of Rockford, where Kent 

 Creek enters Rock River over a series of cascades. The stage of excavation 

 here is far less mature than in the rock gorges which were opened between 

 the Illinoian and Iowan stages or in the new course of the Rock River 

 under discussion. 



Concerning the lower portion of the new course of Rock River, between 

 the city of Sterling and the moiith of the stream, a few words seem necessary. 

 The river enters the sandy plain known as the Green River Basin near the 

 city of Sterling, a plain which stands but 25 to 40 feet above the stream. In 

 the 80 miles from Sterling to its mouth the river makes a descent of 84 feet, 

 of which 15 feet occur in passing the rapids at that city, and about 25 feet 

 more in the 18 miles between Sterling and Lyndon. The average descent, 

 including the rapids, is about 1 foot to the mile, or nearly the same rate as in 

 the narrow portion of its course in the 50 miles above Sterling. This lower 

 portion of the stream is evidently independent of preglacial lines, for it fre- 

 quently encounters rock ledges, and its bed is entirely floored with rock for 

 several miles in the vicinity of .its mouth. The Green River Basin has appar- 

 ently been deeply filled with drift, so that the stream encounters rock only 

 in its passage across preglacial interfluvial tracts. It seems probable that 

 the preglacial channels in this basin reach a level 100 feet or more below 

 the interfluvial tracts, or sufficiently low to correspond with the rock bottom 

 of the preglacial Rock and Mississippi rivers in neighboring districts. 



Much of the erosion accomplished by Rock River in the Green River 

 Basin has occurred since the Wisconsin stage of glaciation, for a large part 

 of the surface gravel and sand in which the stream has its channel appears 

 to be an outwash from a moraine of Wisconsin age at the head of the basin. 

 The size of the valley, moreover, corresponds to that of the post- Wisconsin 

 Valley in the upper portion of the stream. The rapids at Sterling also 

 ally it with gorges opened since the Iowan, and are consistent with a post- 

 Wisconsin age. 



GREEN RIVER. 



The headwaters of this eastern tributary of Rock River are found in 

 the elevated moraine forming the border of the Wisconsin drift in south- 

 eastern Lee County, Illinois, and standing 950 to 1,000 feet above tide. 



