THE WORK OF RUNNING WATER. 



103 



Did valleys grow in length only, competition would not destroy 

 the small ones; it would simply limit them. But valleys widen as 

 well as lengthen, and by widening, adjacent valleys may eliminate the 

 divide between them and become one. The elimination of the inter- 

 vening ridge may be by lateral planation (p. 82), or, if the valleys be 

 of unequal depth, by slope wash (see Fig. 85). By these and other 

 processes many young valleys are dwarfed, and many others are de- 

 stroyed. 



Piracy. — Streams do not always hold the courses which they es- 

 tablish for themselves at the outset. If the valley occupied by the 

 stream a, Fig. 89, is deepened more rapidly than the valley occupied 

 by h, a tributary from the former, c, may work back across the inter- 



FiGS. 89 and 90. — Diagrams to illustrate piracy. 



stream area to e and steal the head waters of that stream (Fig. 90). 

 The tributary which does the stealing is known as a pirate. Stream / 



