496 JOHN LYON RICH 



SUMMARY 



A study of the form of the immediate valley or thalweg of a 

 large number of streams leads to the recognition of three distinct 

 valley types. These are: (i) the open, comparatively straight and 

 straight-walled valley through which the stream swings in more or 

 less open curves, but in which the curves of the stream do not, 

 except in the very earHest stages, correspond, necessarily, with the 

 curves of the valley as a whole; (2) that form of the incised meander 

 which we may call the intrenched meander, in which the meandering 

 stream has sunk itself with little modification into bed-rock. (In 

 this type undercut blufTs and sHp-off slopes are not well developed, 

 though they may be present to some extent. The greater part of 

 the land within the loops retains the original height of the upland) ; 

 (3) the form of the incised meander which we may designate the 

 in-grown meander from the fact that as it sinks itself into the rock 

 it is continually growing so that its final form and size may be very 

 different from that at the beginning of incision. This type of 

 incised meander does not necessitate a particularly meandering 

 course of the stream in the cycle preceding the incision — the 

 meandering course may develop as down-cutting proceeds. The 

 valley is characterized by marked undercut bluffs and slip-off 

 slopes. The evidences of growth or expansion during incision are 

 very clearly expressed in the form of the valley. 



A valley belonging to any one of these types exhibits a series 

 of characteristic valley forms as it advances in the erosion cycle 

 from the youthful to the mature and old-age stages of development, 

 but there is a strong tendency for the final stages of all the types to 

 become alike. 



A large factor in determining the form of the valley, it seems, is 

 the relative rate of down-valley sweep of the river curves. Only 

 when this sweep is subordinate to down-cutting may any form of 

 the incised meander valley be developed. 



In a large drainage system rapidly and bodily upUfted, the 

 master streams will normally develop valleys or thalwegs of type i , 

 the open valley, while in many of the headwater branches valleys 

 of type 3, the in-grown meander, may be the common form. 



