96 



PHYSICAL GEOLOGY 



a high plateau, 6000 to 8000 feet above the sea, in which it has cut 

 a trench a mile deep in certain places. The climate is arid ; the 

 gradient of the valley is steep; the amount of sediment is sufficient 

 to furnish tools for cutting, but not so great as to overload the stream ; 

 the rocks are sandstones, limestones, and shales, overlying granite. 

 The Grand Canyon in Arizona is about 220 miles long and may be 

 described as a valley within a valley, since, in certain localities, the 

 upper portion, cut in the softer, sedimentary rocks, is eight to ten miles 



wide, while the lowest 

 part cut in the hard 

 granite is barely wide 

 enough to hold the 

 river. The total 

 depth of the canyon is 

 almost a mile. The 

 canyons of the tribu- 

 tary streams branch 

 again and again as 

 theyare followed back, 

 and are miniatures of 

 the Grand Canyon. 



The gorge of the 

 Niagara River, Au- 

 sable Chasm, and 

 Watkins Glen, in New 

 York, are examples of 

 canyons developed in a moist region. In these cases the valleys 

 are all postglacial and have been cut so rapidly that their sides 

 have been but little widened by the weather. In Ausable Chasm 

 (Fig. 74) the verticality of the walls has been maintained in places by 

 vertical joints. 



Instances of Rapid Erosion. — The Duna, a river of eastern Prussia, blocked by 

 an ice jam in 1901, was forced to take a new course. In thirty-four hours it was able 

 to cut a u<>ru<- one meter to three and a half meters deep and four meters to eight meters 

 wide, representing an excavation of 2250 cubic meters of material. The bottom of 

 the Sill tunnel in Austria was provided with a pavement of granite slabs more than a 

 yard thick. Great quantities "of debris were swept over this pavement at a high 

 velocity, and so rapid u as the abrasion that it was found necessary to renew the granite 

 slabs after a single year. 



Effect of Deforestation on Rivers. — When forests are cut down 

 or the vegetation on the- hills is killed, the latter being sometimes the 



„ 45 



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Fig. 74. — Ausable Chasm, Chazy, New York. 

 is a young valley. (U. S. Geol. Surv.) 



Thi 



