THE WORK OF RUNNING WATER. 167 



The extent to which the second cycle of erosion recorded in the 

 present topography had proceeded before its interruption by uphft, 

 is indicated by the extent of the ^-alley plains (Fig. 157) below the 

 mountain ridges. While these plains were being developed on the 

 weak rocks, narrow valleys only (Fig. 158) were cut in the resistant 

 rocks which now stood out as ridges. In Fig. 158 some of these valleys 

 are shallow (c, c', c" ^ etc.), and but one of them deep. The former 

 may be either (1) the valleys of streams which crossed the hard layer 

 at the beginning of the cycle, and which were diverted before their val- 

 leys became deep ; or (2) they may represent the heads of valleys now 

 working back into the ridges. The deep valley (b) represents the work 

 of a stream which has held its course across the hard layer while the 

 latter was being isolated as a mountain ridge (compare Figs. 131 and 

 132). Deep narrows of this sort are often called water-gaps. Similar 



Fig. 159. — The Kittatinm^ Mountains and Delaware Water-Gap from Manunka Chunk. 



(N. J. Geol. Surv.) 



valleys, whether shallow or deep, from which drainage has been 

 diverted, are sometimes called ivind-gaps. The second cycle of erosion, 

 while still far from complete, was interrupted by uplift (relative or 

 absolute), and a new cycle inaugurated. This event was so recent that 

 the new (third) cycle has not yet advanced far. 



Recently it has been urged that another cycle, intermediate between 

 the first and second, is to be recognized.^ 



Some of the features just described are illustrated by Fig. 159. 

 The even mountain crest in the background is the Kittatinny Moun- 

 tain of New Jersey and its continuation in Pennsylvania. In common 



» Campbell. BuU. Geol. Soc. of Am., Vol. XIV, p. 277. 



