492 JOHN LYON RICH 



which a region may have been subjected. In applying this criterion 

 in practice, however, we immediately encounter the difficulty that 

 not all regions have master streams flowing directly to the sea. 

 The region we are studying may be located in the interior of a conti- 

 nent and on a relatively small stream. In such cases, can we use 

 the form of the valley in arriving at conclusions as to the rate of 

 uplift of the region ? Obviously not without qualification. 



In order to put this criterion on a basis where it may be applied 

 in practice it is only necessary to bear in mind that the equivalent 

 of uplift as used in this connection, is any condition which will cause 

 a stream or any section of a stream to behave as it would if it were 

 independent and subjected to uplift. In other words the equivalent 

 of uplift maybe thought of as any condition which lowers the local 

 base level of a stream. Thus a lowering of the lake level in the 

 case of a stream flowing into a lake or the deepening of a master 

 stream, in the case of a tributary, is equivalent to uplift in an 

 independent stream. 



VARYING TYPES OF VALLEY FORM IN DIFFERENT PARTS OF A SINGLE 



DRAINAGE SYSTEM 



Turning our attention once more to our hypothetical drainage 

 system uplifted after having reached the old-age stage of develop- 

 ment, we may readily see that, even in this simple case, the same 

 type of valley will not, as a rule, develop in all parts of a drainage 

 system. Take, for example, the case of rapid uplift. We have seen 

 that the master stream would trench rapidly and would form an 

 open valley of type i or, where the original course was meandering, 

 the intrenched meander valley of type 2. What would happen 

 meanwhile in the distant headwater branches ? 



Let the profile of the stream before upHft be represented by the 

 line a of Fig. 10. The final profile of equilibrium after uphft may 

 be represented by the line c. The trenching resulting from uphft 

 would proceed upstream in such a way that at some time between 

 uplift and final grading the profile would take a form somewhat like 

 the line b. Now at a point W in its lower course the master stream, 

 at the time represented by the profile b, would have cut down a 

 distance represented by the length of the line V-W. At another 



