WORK OF RUNNING WATER 



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

 started, and its present channel; 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 terrace. Rock terraces 

 may also result from changes of level. 



Narrows. Where a stream crosses vertical or highly inclined 

 strata of unequal resistance, 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. 81). The Appa- 

 lachian Mountains afford nu- 

 merous examples. The nar- 

 rows develop because the 

 processes which widen the 

 valley are less effective on 

 more resistant rock than on 

 less resistant. Some narrows 

 arise in other ways also. 





Fig. 80. Rock terraces due to a resist- 

 ant layer, H, of rock. 



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 elsewhere wide, narrows become pronounced. From 

 what has preceded it is clear that rapids or falls are likely to occur 

 at narrows, especially early in their history. 



Other effects. Inequalities in the hardness of rock develop 

 certain peculiarities of topography outside of valleys. The less 



Fig. ST. The Kittatinny Mountain and Delaware Water-Gap from Manunka 

 Chunk. (N. J. Geol Surv.) 



