The extent to which the second cycle of erosion recorded in the present topography had proceeded before its interruption by uplift, is indicated by the extent of the valley 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 valleys 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 valleys, whether shallow or deep, from which drainage has been diverted, are sometimes called _wind-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. [Illustration: +Fig.+ 159.—The Kittatinny Mountains and Delaware Water-Gap from Manunka Chunk. (N. J. Geol. Surv.)] Recently it has been urged that another cycle, intermediate between the first and second, is to be recognized.[61] Some of the features just described are illustrated by Fig. 159. The even mountain crest in the background is the Kittatinny Mountain of New Jersey and its continuation in Pennsylvania. In common with other corresponding crests it represents the oldest recorded base-level (or peneplain) of the region. The great gap in the mountain is the Delaware Water-Gap. Below the mountain crest there is another plain, developed in a subsequent cycle of erosion, while the valley plain in the foreground represents the work of a still later cycle. [Illustration: +Fig.+ 160.—Showing certain peculiarities of Appalachian drainage. 1 = the Susquehanna; 2 = the Potomac; 3 = the James; 4 = the Roanoke; 5= the Coosa; 6 = the Tennessee; 7 = the Kanawha; 8 = head of New River; 9 = head of the French Broad.]