102 



PHYSICAL GEOLOGY 



consequent streams. As streams deepen and lengthen their valleys, 

 their tributaries may encounter new kinds of material and find that 

 some are more easily eroded than others, with the result that they 

 gradually develop valleys in the less resistant rocks. In such case, 

 the position and size of the branch streams are determined by the 

 nature of the underlying rock and not by the original slope of the 

 surface; the valleys being cut in the weaker strata, while the harder 



strata stand up as ridges 

 or even mountains. The 

 Shenandoah valley of 

 Virginia, the Lehigh val- 

 ley of Pennsylvania, and 

 the Hoosic and Hoosa- 

 tonic valleys of Massa- 

 chusetts and Connecticut 

 are examples of valleys 

 of this type. Valleys 

 formed in this way are 

 called (2) subsequent (Fig. 

 82 B), the process being 

 known as structural ad- 

 justment. 



It will readily be seen 

 that if streams drain 

 adjoining regions, the one 

 whose course is most 

 generally confined to the 



more easily eroded beds 

 Fig. 84. — Direction of drainage determined chiefly .,, ... 



by faulting and jointing, near Lake Temiskaming, wl11 S row more rapidly 

 Ontario. (After Hobbs.) and so may cut headward 



until it captures branches 

 or even the entire upper courses of streams less favorably situated. 

 Such a process is called stream piracy (p. 107). If the land is 

 warped up athwart the course of a consequent stream whose direc- 

 tion is so well established that it is able to degrade its bed as rapidly 

 as the elevation takes place, thus keeping its old course, the stream 

 is called antecedent (Fig. 83 A and B). 



(3) Another factor which sometimes determines the direction of 

 a stream is faulting (p. 261) (Pig. 84). In regions of pronounced 

 faulting, such as the Adirondacks, the courses of many streams may, 



