STREAM VALLEYS AND THEIR MEANING 497 



Any barrier of hard rock in the course of a stream may, by 

 retarding down-cutting, allow the development of valleys of type 3 

 above the barrier at the same time that below it type i is the 

 prevailing form. 



In conclusion, it appears that it is the ratio of the rate of vertical 

 down-cutting to that of lateral cutting and sweep which determines 

 the form of the thalwegs. When down-cutting predominates, 

 valleys of types i or 2 are formed; when sweep is dominant the 

 open valley, type i , is the result. It is only when conditions lead 

 to continuous down-cutting so slow that it is equaled or exceeded 

 by lateral cutting, yet rapid enough to make sweep subordinate, 

 that the in-grown meander type of valley may be produced. 



These conclusions, if correct, establish valuable criteria for the 

 interpretation of the physical history of a region from the form of 

 its valleys. 



