136 



GEOLOGY 



parts of the Dakotas. The formations here are often beds of sand- 

 stone or shale, alternating with beds of clay. Climatic factors 

 are also concerned in the development of bad-land topography] 

 A semi-arid climate, where the precipitation is much concentrated, 

 seems to be most favorable for its development. The bad-land 

 topography is most striking in early maturity. 



Special forms of valleys; canyons. Various conditions influence 

 the size and shape of valleys, especially in the early stage of their 



Fig. 102. Grand Canyon of the Colorado. (Peabody.) 



development. High altitude of land favors swift flowing streams. 

 and the development of deep valleys. Such valleys will be narrow 

 if the conditions which determine widening are absent or unfavor- 

 able. 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. Narrow valleys with steep slopes will 

 also be favored if the valley is cut in rock which is capable of si an cl- 

 ing 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 



