LAND WATERS STREAMS 159 



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 peculiarities of topography outside of valleys. 

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

 streams are worn down more readily than those which are more 

 resistant. If great areas of high land be capped with hard rock, 

 they are likely to remain as plateaus after surrounding areas of 



Fig. 119. r- The Enchanted Mesa. A striking butte in New Mexico. The 

 name mesa is not commonly applied to elevations of such small summit 

 area. (R. T. Chamberlin.) 



less resistance are brought low. If the hard capping remains over 

 a small area instead of a large one, the elevation is a butte, a hill, 

 or a mountain, instead of a plateau (Fig. 119). Many buttes and 

 small mesas are but remnants of former plateaus. A feature of 

 buttes and mesas capped by hard rock is the steep slopes or 

 cliffs corresponding to the edges of the hard beds (Figs. 101 and 

 119). 



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. 120). 

 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 

 limited area often constitute conspicuous hills or mountains after 



