158 



GEOLOGY 



that is, to represent the height at which the water once stood. 

 In one sense this interpretation is correct, since a river has 

 stood at all levels between that of the surface in which its valley 



started, and its present chan- 

 nel; but the shelf of hard 

 rock does not mean that the 

 river was ever so large as to 

 fill the valley from its present 

 channel to the level of the ter- 

 race. Rock terraces may also 



- Rock terraces, due to resist- 

 ant layers or rock. 



Fig. 117.- 



ant lavers or rock. 



result from changes of level. 



Narrows. If a stream crosses vertical or highly inclined strata 

 of unequal hardness, its valley is usually constricted at the crossing 

 of the hard layers. If such a constriction is notable it is called a 

 narrows, or sometimes a water-gap (Fig. 118). The Appalachian 

 Mountains afford numerous examples. The narrows develop be- 

 cause the processes which widen the valley are less effective on the 

 hard layer than on the less resistant ones. Narrows sometimes 

 arise in other ways also. 



Narrows are much more conspicuous in certain stages of erosion 

 than in others. While a valley is still so young as to be narrow at 

 all points, there can be no pronounced " narrows"; but later, when 

 the valley is otherwise wide, narrows are more pronounced. From 



r 



Fig. 118. The Kittatinny Mountain and Delaware \Vater-( ia|> from Ma- 

 nunka Chunk. (N. J. Geol. Surv.) 



