THE WORK OF RUNNING WATER. 



99 



small part of the work which the stream must do to bring its drainage 

 basin to base-level. 



AMiile aridity and altitude are conditioDS which favor the develop- 

 ment of canyonS; as shown by the fact that most canyons are in high 



il3 



.■.::;a 



Fig. 82. — Cross-section of the Colorado Canyon. (After Gilbert and Brigham.) 



and dry regions, they are not indispensable. Niagara River has a 

 canyon below its falls (PI. IX), and the surrounding region is neither 

 high nor arid. The narrow part of the valley has been developed 







r.^>>^^u- 



^^^<xt.e 





^-^^x-:- 



r. 17 vxy-\ 





B 



■^^m^nnij^ \^ 



Fig. 83. — Detail of erosion in the Grand Canyon. The inequahties of slope are the 

 result of unequal hardness. The vertical planes which give the architectural effect 

 are the result of joints. (Holmes.^ 



by the recession of the falls, and is so young that side erosion has not 

 yet T\idened the valley or lowered its angle of slope to such an extent 



