LAND WATERS STREAMS 



163 



phase of adjustment may be called structural adjustment. Struc- 

 tural adjustment is not uncommon among rivers flowing over strata 



Fig. 128. Diagram showing the outcrops of hard layers (shaded) on the flanks 

 of truncated folds: cd, present surface; ab, an earlier erosion surface. 



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

 hard and soft strata are most 



likely to come to the surface SUL ^ - oc 



in frequent alternation. 



The processes of adjust- 

 ment go on until the streams 

 flow as much as possible on 

 the weaker beds, and as little 

 as possible on the stronger, 

 when adjustment is complete. 

 This amounts to the same thing 

 as saying that the outcrops of 

 the hard 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 its drainage is completely 

 adjusted when as few streams 

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



Adjustment has been carried to a high degree of perfection in 

 most parts of the Appalachian system. Here, as in all other 

 mountains of similar structure, strata of unequal hardness were 

 folded into ridges. The folds were then truncated by erosion, 



sr'oo 



Fig. 129. Adjusted drainage 

 region of folded rocks. 



