3l8 LAND SCULPTURE 



leys and streams. In such a longitudinal valley, following the 

 strike of a mass of soft strata, the stream which occupies it will 

 tend to flow along the foot of the escarpment formed by the out- 

 crop of hard strata, and to shift its course laterally in the direction 

 of the dip, cutting away the soft beds in which it flows, and under- 

 mining the hard escarpment. Such a stream is a potent agent in 

 causing the recession of the escarpment and may remove large 

 areas of both hard and soft strata. 



The steep ridges, or " hog-backs," which occur among the foot- 

 hills of the Rocky Mountains in Colorado, show interesting ex- 

 amples of streams flowing along the strike of inclined strata, 

 though the ridges are themselves not formed quite in the way 

 already described. They are composed of the steeply dipping 

 limbs of monodinal folds, of which the upper horizontal limbs 

 have been removed by denudation (Fig. 132). 



Folded Strata. — A region of folded strata is, in the first in- 

 stance, thrown into a series of ridges and valleys, the ridges formed 

 by anticlines and the valleys by synclines. If the folding be of 

 moderate degree, so as to produce undulations of sweeping and 

 gentle curves, the tendency of denudation is to reverse the original 

 topography and convert the anticlines into valleys and the syn- 

 clines into ridges. This apparently paradoxical result is found, 

 when examined, to be natural and simple enough. The crests of 

 newly formed anticlines have been subjected to tensile stresses 

 which open the joints in the strata and render them an easy 

 prey to the denuding agents. The surface of the synclines, on 

 the contrary, has been tightly compressed, and their joints are 

 closed by crowding. Aside from this, another factor tends to pro- 

 duce the same result. In a folded series of alternating harder 

 and softer beds denudation is most rapid on the exposed anticlines, 

 and in them the hard strata are first reached and cut through. 

 When an underlying mass of soft strata is reached, they are rapidly 

 trenched into valleys which may soon be excavated below the level 

 of the synclinal troughs. 



If the folds originally made by the force of lateral compression 

 be steep and high, as in mountain ranges, the anticlines persist 



