STRUCTURAL ADJUSTMENT 97 



In the structural adjustment which goes with the erosion of 

 folds, it happens in many cases that valleys come to be located on the 

 anticlines after the latter have been worn dosvn, while the outcrops 

 of the hard layers on the flanks of the anticlines, or even in the 

 original synclines, become the mountains (Fig. 87). 



ADJUSTMENT OF STREAMS TO ROCK STRUCTURES 



Valleys (gullies) are located at the outset without immediate 

 regard to the hardness and softness of their beds. It is primarily 

 the slope about the head of a gully which determines its line of 

 growth, and, once established, streams tend to hold their courses; 

 but the streams on the weaker rock will deepen their valleys more 

 rapidly than others, and have an advantage over them. Being 

 deeper, their tributaries may be lengthened until their heads reach 

 the other valleys, with the results shown in Figs. 88-90. Even 

 where several streams cross the same resistant bed, piracy is likely 

 to take place among them, for some are sure to deepen their valleys 

 faster than others, because of inequalities of volume, load, or hard- 

 ness. This is illustrated by Figs. 91-93. (See also P'igs. 52 and 53.) 

 P.iracy may take place where streams do not flow over rock of un- 

 equal resistance, but it is more common where they do, for greater 

 resistance of rock puts the stream which crosses it at a disadvantage 

 as compared with the stream which crosses less resistant rock. 



The changes in the courses of streams by means of which they 

 come to sustain definite and stable relations to the rock structure 

 beneath, are known as processes of adjustment. 1 Since streams 

 and valleys adjust themselves to other conditions as well, this 

 phase of adjustment may be called structural adjustment. Struc- 

 tural adjustment is not uncommon among rivers flowing over strata 

 which are vertical or highly inclined, since in these positions, strata 

 of unequal resistance are most likely to alternate with one another 

 at the surface. The processes of adjustment go on until the streams 

 How as much as possible on the weaker beds, and as little as possible 

 on the stronger. Adjustment is then complete. This amounts to 

 the same thing as saying that the outcrops of resistant layers 

 tend to become divides. In many cases an area is so situated that 

 there is no escape for its drainage except across resistant rock. In 

 this case drainage is completely adjusted when as few streams as 

 possible cross the resistant rock, and these by the shortest routes. 



1 Campbell, Jour, (leol., Vol. IV., pp. 567, 657. 



