EARTH FEATURES SHAPED BY RUNNING WATER 175 



line rocks. Within the narrows of a river the steep walls, charac- 

 teristic of youth and the turbulent current as well, are often retained 

 long after other portions of the river have acquired the more restful 

 lines of river maturity. The picturesque crag and the generally 

 rugged character of river narrows render them points of special 

 interest upon every navigable river. 



The capture of one river's territory by another. The effect 

 of a hard layer of rock interposed in the course of a stream is 

 thus always to delay the advance of the erosional process at all 

 levels above the obstruction. When a stream in incising its 

 valley degrades its channel through a veneer of softer rocks into 

 harder materials below, it is technically described as having dis- 

 covered the harder layer. Where several neighboring streams flow 

 by similar routes to their common base level, those which dis- 

 cover a harder rock will advance their headwaters less rapidly 

 into the upland and so will be at a disadvantage in extending 

 their drainage territory. A stream 

 which is not thus hindered will in the 

 course of time rob the others o'f a por- 

 tion of their territory, for it is able to 

 erode its lower reaches nearer to base 

 level and thus acquire for its upper 

 reaches, where erosion is chiefly accom- 

 plished, an advantage in declivity. The 

 divide which separates its headwaters 

 from those of its less favored neighbor 

 will in consequence migrate steadily in- 

 to the neighbor's territory. The divide 

 is thus a sort of boundary wall separat- 

 ing the drainage basins of neighboring 

 streams, and any migration must extend 

 the territory of the one at the expense 

 of the other. As more and more terri- 

 tory is brought under the dominion of 

 the more favored stream, there will come 

 a time when the divide in its migration 

 will arrive at the channel of the stream that is being robbed, and 

 so by a sudden act of annexation draw off all the upper waters 

 into its own basin. By this capture the stream whose territory has 



FIG. 184. Successive dia- 

 grams to illustrate repeated 

 river piracy and the devel- 

 opment of " trellis drainage," 

 (after Russell). 



