94 



GEOLOGY. 



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 de- 



FiG. 76. — Toadstool Park, Sioux Co., Neb. The peculiar topograph}^ is the result 

 of erosion working on jointed rocks of unequal hardness in an arid region of con- 

 siderable elevation where rainfall is unequally distributed. (Darton, U. S. Geol. 

 Surv.) 



velopment. If the altitude of the land be great, the gradient of the 

 streams at this stage will be high. A high gradient means a swift 

 stream, and a swift stream erodes chiefly at its bottom. High alti- 

 tudes therefore favor the development of deep valleys. Such valleys 

 will be narrow if the conditions which determine widening are absent 

 or unfavorable. Since slope wash is one of the main factors in the 

 widening of valleys, an arid climate favors the development of narrow 

 valleys, if there be sufficient water to maintain a vigorous stream. 

 Narrowness and steepness of slopes will also be favored if the valley 

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





