THE INIERGLACIAL GORGE PROBLEM 



137 



ing many rapids and waterfalls. The length of these postglacial 

 gorges is usually less than three miles ; and the amount of fall 

 varies from about lOO feet to over 500 feet per mile. The descent 

 is usually accomplished by means of a series of cascades, though 



Fig. I. — Taghanic Falls. 



in a few cases there are cataracts of considerable heisfht. Tag:- 

 hanic' Falls, the highest, measures 200 feet. Many of these 

 falls have developed on account of an alteration of hard sandy 

 layers with layers of soft, easily eroded shale. A few cataracts 



^ This spelling is in accordance with that on the Genoa atlas sheet of U. S. Geo- 

 logical Survey. 



