DRAINAGE OF FOLDED AREAS 323 



The final stages of river development are reached when the 

 base-level is attained, and the drainage basin reduced to a pene- 

 plain by the combined action of the streams and weathering. The 

 flood-plain deposits may now be partially or completely removed, 

 for the main trunk no longer receives an excessive load, and 

 hence it is able to carry away some of that sediment which it had 

 previously deposited. With its drainage basin smoothed down 

 into a peneplain, the river's work is done ; it has reached old age. 



The course of river evolution above described is the ideal cycle 

 of development which, however, may be and generally is inter- 

 rupted by diastrophic movements. An elevation of the region 

 may simply rejuvenate the streams and start them afresh upon a 

 career of wearing down the land. But if accompanied by exten- 

 sive warping or folding of the rocks, the drainage system of the 

 entire region may be revolutionized. A depression of the region 

 will have the contrary effect, checking or stopping the work in 

 which the streams were engaged, drowning their lower reaches 

 and converting them into estuaries. A lowered land surface has 

 less material to lose before it is reduced to base-level, but the 

 work of denudation is accomplished more slowly. 



In a newly formed mountain region the drainage system is at 

 first consequent upon the slopes produced by folding, the principal 

 streams flowing in the synclinal troughs and passing from one 

 syncline to another at the points where, owing to the descending 

 pitch of the folds, the anticlines are lowest. The principal valleys 

 are thus longitudinal. Such a system of drainage is exemplified in 

 the Jura Mountains of Switzerland, a region where the topography 

 is still dominated almost completely by the regular folds which 

 form anticlinal ridges and synclinal valleys. In very ancient 

 regions of folded rocks, on the other hand, the original longi- 

 tudinal valleys may become altogether insignificant in comparison 

 with the transverse valleys excavated by the streams. 



When it was first suggested that rivers had cut their own valleys 

 and had not merely taken possession of ready-made trenches, it 

 was objected that such an explanation required many streams to 

 begin their course by flowing up hill. It is very common to find 



