494 



THE ILLINOIS GLACIAL LOBE. 



cases, however, the stream is thrown across the divide into another pregla- 

 cial valley. The streams in cutting new courses through the rock ledges 

 have found the material so resistant that very narrow channels have been 

 formed which, because of their narrowness and the precipitous rock cliffs 

 on their borders, are known as rock gorges. Several of these rock gorges 

 in Stephenson County have been examined with considerable care and dis- 

 cussed by Hershey. 1 His paper contains the following table of measure- 

 ments and estimates of a few of these gorges. There is added a more 

 recent measurement of a gorge on Carroll Creek just west of Mount Carroll: 



Measurements of rock gorges in northwestern Illinois, by Oscar R. Hershey. 



From the above table it appears that the gorges show remarkable 

 variations in size, the causes for which are not made clear. The small ratio 

 of width of bottom to width of stream displayed by the two streams hav- 

 ing the largest drainage area raises the suspicion that the smaller streams 

 may have had softer material to work upon and thus have been able to 

 accomplish a greater amount of excavation than that displayed by the two 

 larger streams. The writer has had opportunity to examine only two of 

 the gorges, No. 1 and No. 8 of the above table. These gorges are of some- 

 what different type. The gorge No. 1, on the small stream north of Free- 

 port, has bluffs so broken down that an ascent may easily be made at almost 

 any point. As shown in the table, the top of the gorge has a breadth nearly 

 three times as great as at the bottom, though the gorge is scarcely 30 feet 



1 Pleistocene rock gorges of northwestern Illinois, by Oscar H. Hershey : Am. Geologist, Vol. XII, 

 1893, pp. 314-323. 



