STREAM VALLEYS AND THEIR MEANING 493 



point, Y, farther up the valley the stream would, in the same time, 

 have cut down a much shorter distance, represented by the line 

 X-Y. The effect at the latter point, particularly on tributaries 

 entering there, would be equivalent to that of a slow uplift. We 

 should expect, therefore, to find valleys normal for such an uplift — 

 namely, in-grown meander valleys — developing at that point. 



Thus in the lower course of the master stream, valleys typical 

 of a rapid uplift would be found, while in its upper or middle course, 

 and in many of the tributaries, valleys typical of a slow uplift would 

 be encountered/ 



Fig. 10 — Diagram to illustrate the explanation of the different types of valleys in 

 different parts of a drainage system. Vertical scale greatly exaggerated. 



Valleys of the in-grown type are especially likely to develop on 

 the upstream side of resistant rock masses. These resistant masses 

 retard the progress of a wave of trenching following an uplift and 

 the stream above them, which must wait upon their down-cutting, 

 is affected as by a slow uplift. 



It follows, from considerations outlined above, that, for the 

 direct determination of the rate of uplift by means of the form of the 

 stream valleys, only the master streams, or those emptying directly 

 into the sea may be successfully used. In other cases the char- 

 acter of the rocks and the distance from the sea may play a more 

 important part than the rate of uplift. This fact does not, however, 

 destroy the usefulness of the criterion, for it still indicates the rate 

 of lowering of the local base-level of any particular stream in 

 question. 



' A. Penck, in Morphologie der Erdoberflache, Buch II, S. 348, calls attention to 

 the tendency to the formation of meanders in the middle course of a stream; also to 

 the fact that meanders are characteristic of an underloaded stream when it is so placed 

 that it cannot work downward. 



