174 



EARTH FEATURES AND THEIR MEANING 



landscapes may thus be brought into striking contrast with the 

 straight lines of youthful canons which with V-sections descend 



from their lowest levels 

 (Fig. 182). The full 

 cross section of such a 

 valley shows a central V 

 whose sharp shoulders 

 are extended outward 

 and upward in the soft- 

 ened curves of later ero- 

 sion stages. 



The arrest of stream 

 erosion by the more re- 

 sistant rocks. The ca- 



FIG. 182. A rejuvenated river valley (after a 

 photograph by Fairbanks). 



pacity of a river to erode and carry away the rock material 

 that lies along its course is dependent not only upon the ve- 

 locity of the current, but also upon the hardness, the firmness 

 of texture, and the solubility of the material. Particularly in 

 arid and semiarid regions, where no mantle of vegetation is at 

 hand to mask the surfaces of the firmer rock masses, differences 

 of this kind are stamped deeply upon the landscape. The rock 

 terraces in the Grand Canon of the Colorado together represent 

 the stronger rock formations of the region, while sloping talus 

 accumulations bury the weaker beds from sight. 



Each area of harder rock which rises athwart the course of a 

 stream causes a temporary arrest in the process of valley erosion 

 and is responsible for a noteworthy local contraction of the river 

 valley. The valley is carved less widely as well as less deeply, 

 and since a river can never corrade 

 below its base, a " temporary base 

 level " is for a time established 

 above the area of harder rock. 

 Owing to the contraction of the 

 ^valley under these conditions, the 

 locality is described as a river 

 narrows (Fig. 183). The narrows 

 upon the Hudson River occur in 

 the Highlands where the river leaves a broad expanse occupied 

 iby softer sediments to traverse an island-like area of hard crystal- 



*& 



FIG. 183. Plan of a river narrows. 



