166 



PHYSICAL GEOLOGY 



the topography of the region midway between the present 

 and the final stage?) The conditions which brought about 



the formation of the 

 Colorado Canon are 

 those which most fa- 

 vor canon develop- 

 ment. They are (1) 

 a considerable alti- 

 tude, (2) a dry cli- 

 mate, (3) rocks that 

 will stand in cliffs, 

 and (4) a vigorous 

 stream. As implied 



FIG. 166. A canon in a humid region. Val- , , 



ley of the New River in the Allegheny Pla- above, however, not 



teau. all canons and canon- 



like valleys are in 

 arid regions. Figure 166 shows a canon in a moist region. 



Rock terraces. Rock terraces (Fig. 167) occur on the 

 sides of many valleys cut in horizontal beds of unequal 

 strength. The terraces are formed by the strong beds, 

 which are worn back less rapidly than the weak beds above 

 and below them. 



Elevations due to unequal erosion. The relatively rapid 

 erosion of soft rocks has left associated harder rocks stand- 

 ing as conspicuous eleva- 

 tions in many places. If 

 the beds are tilted highly, 

 the resistant ones may be 

 left standing as ridges 

 after the softer ones are 



FIG. 167. Diagram of rock terraces. 



worn down to valleys (Plate XVI). This is the origin of the 

 well-defined Appalachian Mountain ridges (p. 153). Smaller 

 ridges, formed in this way on the flanks of mountain ranges, 

 are sometimes called hogbacks (Fig. 168). The superior re- 

 sistance of dike rock may lead to the formation of dike ridges 

 (p. 50). If the beds are horizontal, flat-topped table moun- 



