THE WORK OF STREAMS 



107 



from the elevation of the land, since when the gradient of a river is 

 increased it is able to cut a gorge in its old valley floor, leaving rock 

 terraces on the two sides. 



Stream Piracy. — Because of the more rapid headward cutting of 

 one stream than another there is a continual though usually slow 



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w 



Fig. 91. — One of three diagrams showing the development of topography in a 

 region where the underlying strata are inclined (dip) and vary greatly in their resistance 

 to erosion. The region is conceived to be reduced to a peneplain with low ridges of 

 harder strata. (Modified after Davis.) 



absorption of the tributaries of one river system by another and 

 also a struggle for existence among the tributaries of each river 

 system. A stream which has cut headward so rapidly as to divert the 

 headwaters of another stream to itself is said to behead the latter, 

 and the act is spoken of as stream piracy (Figs. 91, 92, 93). 



Fig. 92. — In this (second) diagram the peneplain has been elevated and the streams 

 have cut deep valleys and picturesque water gaps. The direction of the tributary 

 streams is determined by the strata, and a "trellised" drainage system results. 



The result of stream piracy is well shown in the difficulties experi- 

 enced by a commission appointed by the Argentine and Chilean gov- 

 ernments to determine a disputed boundary in the Andes between 



