MATURE DRAINAGE SYSTEMS 



329 



ing the portion a" and reversing the portion a'. Similarly, m has 

 captured and divided e, n has done the same with b, and p with d, 

 while g must eventually suffer the same fate. Wind gaps will be 

 left in the ridges where the captured streams once crossed them. 



In regions of folded rocks thrown into a series of parallel anticlines 

 and synclines, the process of adjustment may become exceedingly 

 complicated. Suppose an original consequent stream flowing in 

 a syncline of hard rock considerably above base-level, whose sub- 

 sequent branches have opened out valleys in softer rocks along the 



Fig. 134. — Evolution of a river system, 

 first stage. The shaded lines represent 

 escarpments of hard rock. (De Lap- 

 parent.) 



Fig. 135. — Evolution of a river 

 system, second stage. (De Lap- 

 parent.) 



crests of the anticlines, where the harder surface stratum is first cut 

 through. The extension and junction of these subsequent branches 

 may offer a more advantageous course than the hard syncline, and 

 cause the latter to be wholly or partially deserted. The streams 

 originally flowing in the synclinal troughs may gradually be shifted 

 to the degraded anticlines which, as we have seen, are wasted 

 away more rapidly. 



A thoroughly mature drainage system is characterized by a com- 

 plete adjustment of its streams to the structure of the rocks. The 

 rivers as finally established are thus apt to be a patchwork of streams 

 captured and diverted, and the result of adjustment is the produc- 



