STREAM VALLEYS AND THEIR MEANING 491 



occupy the greater part of the width of the necks of land between 

 the loops.' 



Conclusions from deductive study. — -From the foregoing considera- 

 tions it becomes apparent that a slow uplift, of such a master stream, 

 unless it be so slow that sweep dominates, will tend toward the 

 formation of the in-grown type of meander, whether or not the 

 stream meandered widely in the cycle preceding the uplift, and that 

 a rapid uplift will result in the intrenchment of the stream in 

 whatever course it may happen to be holding at the time. If this 

 original course happens to be relatively straight, a valley of the 

 open type will develop; if it is meandering, the meanders will be 

 intrenched but will present characteristics sufficient to distinguish 

 them from the in-grown meanders resulting from slow uphft. 



It appears also — -and this fact should be emphasized — that, for 

 each of the hypothetical cases studied, the form of the valley goes 

 through a definite and characteristic evolution as it advances in the 

 erosion cycle, but that the end product, namely the open valley, 

 wider than the meander belt and with meander-scalloped sides, 

 is the same in all cases. 



New terms might, perhaps with advantage, be invented to desig- 

 nate the different stages in this evolution, or it might, perhaps, be 

 wise to apply the familiar, though somewhat overworked terms, 

 "youthful," "mature," and "old." If this were done "youth" 

 might be made to cover the period up to the time when active 

 down-cutting ceases; "maturity" the period from the latter until 

 wide, meander-scalloped flood plains develop and until the spurs 

 of incised meanders have been cleared away by sweep, and "old 

 age" the remainder of the cycle. 



Significance of uplift. — -In the preceding discussion we have 

 assumed rapid or slow uplift of the basin of a master stream flowing 

 directly to the sea. We concluded that rapid uplift should result 

 in one type of valley form; slow uplift in another. If this proposi- 

 tion holds true, we have a valuable criterion for determining from 

 the form of the river valleys or thalwegs the rate of the uplift to 



' C. F. Marbut, Physical Features of Missouri, p. 104; "Meanders," Missouri 

 Geological Survey Bull., X, 94-109. This is a very excellent discussion of meanders; 

 both the flood plain and incised types. 



