160 GEOLOGY 



erosion has removed their less resistant surroundings (Fig. 121). 

 Round Valley, shown on PL X, is an example. 



Ridges and hills resulting from the unequal degradation of 

 unequally resistant strata are not equally prominent at all stages 

 in an erosion cycle. In early youth the material surrounding the 

 hard bodies of rock has not been removed; in early maturity con- 

 siderable portions of their surroundings still remain about them; 

 but in late maturity or early old age the outcropping masses of 

 hard rock have been more perfectly isolated, and are most con- 



Fig. 120. Hogbacks; Sec. 22, T. 16, N., R. 112 W., Wyoming. The rock 

 which occasions the hogbacks is the Lazeart sandstone at the base of the 

 Adaville formation. (Veatch, U. S. Geol. Surv.) 



spicuous. Most of the even-crested ridges of the Appalachian 

 system, as well as many others which might be mentioned, became 

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

 ridges of hard rock are themselves brought low. Isolated rem- 

 nants of hard rock which remain distinctly above their surround- 

 ings in the late stages of an erosion cycle (Fig. 121) 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. 



Adjustment of streams to rock structures. Valleys (gullies) 

 are located at the outset without immediate regard to the hardness 



