CANYONS 87 



parts of the Pakotas. Many of the formations here an- sandstone 

 or shah', alternating with beds of unindurated clay. Climatic factors 

 also are concerned in the development of this topography. A semi- 

 arid climate, where the precipitation is much concentrated, seems to 

 be most favorable for its development. 



Canyons. Various conditions influence the size and shape of 

 valleys, especially in the early stage of their development. High 

 altitude favors swiftness of flow, and the development of deep 

 \ alleys. Such valleys will be narrow if the conditions which deter- 

 mine widening are absent or unfavorable. An arid climate favors 

 the development of narrow valleys if there is sufficient water to 

 maintain a vigorous stream, because there is little slope wash. 



I 



Fig. 71. Grand Canyon of the Colorado. (Peabody.) 



Narrow valleys with steep slopes will also be favored if the valley 

 is cut in rock which is capable of standing with steep faces. Thus 

 a stream may develop a narrow valley in firm rock, where it would 

 not do so in loose gravel. Aridity, high altitude, and the proper 

 sort of rock structure therefore favor the development of deep narrow 

 valleys. Such valleys are canyons, and many of the young valleys 

 in the western part of the United States, where these conditions pre- 

 vail, belong to this class. 



While all canyons are valleys, most, valleys are not canyons. 



