94 



WORK OF RUNNING WATER 



by hard rock is the steep slopes or cliffs corresponding to the edges 

 of the hard beds (Figs. 70 and 82). 



If the rock of a region is stratified and the layers tilted, the 

 removal of the softer beds leaves the harder ones projecting above 

 the general level in the form of ridges or " hog-backs" (Fig. 83). 

 Dikes of igneous rock, harder than the beds which they intersect, 

 likewise become ridges after the degradation of their surroundings. 

 The plugs of old volcanic vents and other igneous intrusions of 



Fig. 84. A monadnock; a mass of intruded igneous rock isolated by erosion, 

 and remaining high because of its superior hardness. Matteo Tepee, Wyo. (De- 

 troit Photo. Co.) 



limited area may constitute conspicuous hills or mountains (Fig. 

 84) after erosion has removed their less resistant surroundings. 

 Cushetunk Mountain, PI. VIII, is an example. 



Ridges and hills resulting from the unequal degradation of 

 unequally resistant strata are most prominent in the late maturity 

 or early old age of an erosion cycle. The outcropping masses of 

 hard rock are then more perfectly isolated than at earlier stages. 

 Most of the even-crested ridges of the Appalachian system, as well 

 as many others which might be mentioned, became ridges in this 



