EFFECTS OF UNEQUAL HARDNESS 95 



way. In the final stages of an erosion cycle, the ridges of hard 

 rock art- themselves brought low. Isolated remnants of hard rock 

 which remain distinctly above their surroundings in the late stages 

 of an erosion cycle are known as monadnocks, the name being derived 

 from Mount Monadnock, N. H., an elevation of this sort developed 

 in a cycle antedating the present. J/ ^1 



THE EROSION OF FOLDS 



The erosion of folded strata (anticlines and synclines) leads to 

 the development of distinctive topographic features. So soon as 

 a fold begins to be lifted, it is, by reason of its position, subject to 

 more rapid erosion than its surroundings. For the same reason, 

 the crest of a fold is likely to be degraded more rapidly than its 

 lower slopes, and must suffer more degradation before it is brought 

 to base-level. Most folds are composed of beds of unequal resist- 

 ance, and as their degradation proceeds, successive layers are worn 

 from the top, and the alternating layers of more and less resistant 

 rock are exposed. The less resistant beds are worn down faster 

 than the others, and in time the outcrops of the stronger beds 

 become ridges, distinctly above the outcrops of the weaker beds 

 "which have become valleys and lowlands (Fig. 85). 



Fig. 85. A canoe-shaped valley bordered by a ridge formed by the outcrop of 

 a hard layer in a plunging syncline. The ridge bounding the canoe-valley is 

 separated from an outer ridge by a curved valley, underlain by relatively weak 

 rock. ( Willis.) 



