142 



GEOLOGY. 



all points, no narrows will be conspicuous; but at a later stage in its 

 history, when the valley is otherwise wide, narrows are more pro- 

 nounced. At a still later stage, when the hard strata themselves 

 approach base-level, the narrows again become inconspicuous. 



From what has preceded it is clear that rapids or falls are likely 

 to occur at narrows, especially in the early part of their history. 



Other effects on topography. — Inequalities in the hardness of rock 

 develop certain pecuharities of topography other than those of valleys. 



Fig. 123. — ^A ridge due to the outcropping edge of hard Jurassic rock. Wyoming. 



The less resistant portions of a land area more or less distant from 

 streams are worn down more readily than those which are more re- 

 sistant. If great areas of high land be capped with hard rock they 

 are likely to remain as plateaus after surrounding areas of less resistance 

 are brought low. If the hard capping affects a small area instead of 

 a large one, the elevation is a butte, a hill, or a mountain, instead of a 

 plateau (Fig. 110). Many buttes and small mesas are but remnants 

 of former plateaus (Mesa Lauriano, N. M., Fig. 1, PI. XII). A feature 

 of buttes and mesas capped by hard rock is the steep slope or cliff 

 corresponding to the edge of the hard bed (Figs. 78 and 109). 



If the rock of a region be stratified and the layers tilted, the re- 

 moval of the softer beds leaves the harder ones projecting above the 

 general level in the form of ridges or ^^ hog -backs" (Figs. 122 and 123). 



