If the strata involved be horizontal the determination of cycles is sometimes less easy. Thus in [Fig. 156], it is not possible to say whether a and a′ represent remnants of an old base-level, or whether they represent the original surface from which degradation started. So, too, the various benches below a, such as b, b′, and b″ may readily be the result of the superior hardness of the beds at this level. For the determination of successive uplifts in the field it is necessary to consider areas of considerable size, and to eliminate the topographic effects of inequalities of hardness, and of certain other factors to be mentioned presently.
Fig. 156.—Diagram to illustrate cycles of erosion where the beds are horizontal.
The inequalities in the depths of the young valleys in Figs. [155] and [156] may be explained on the supposition that the deeper ones belong to main streams, and the shallower ones to tributaries. Such a valley as that shown at e, [Fig. 155], suggests rejuvenation at this point; but farther up the stream which occupies this valley, rejuvenation might not be apparent. In this case, the main streams might be flowing in new valleys, d, d′, etc., while the heads of their tributaries are still flowing in the older valleys of the preceding cycle (compare [Fig. 154] and [Fig. 1, Pl. XIII]).
It is by the application of the preceding principles that it is known that the Appalachian Mountains, after being folded, were reduced to a peneplain ([p. 76]), throughout their whole extent from the Hudson River to Alabama. The peneplain level is indicated by the level crests of the Appalachian ridges, shown in cross profile by the high points of [Fig. 157]. The system was then uplifted, and in the cycle of erosion which followed, broad plains were developed at a new and lower level, corresponding in a general way to the plains b, b′, and b″ of [Fig. 155]. The plains were located, for the most part, where the less resistant strata come to the surface. Above them rose even-crested ridges, the outcrops of the resistant layers, which had been isolated by the degradation of the softer beds between. They constitute the present mountain ridges (the high points of [Fig. 157]). The evenness of their crests, testifying to the completeness of the first peneplanation, is shown in [Fig. 158], which represents, diagrammatically, a longitudinal profile of an Appalachian Mountain ridge. The evenness of the crest is interrupted by (1) notches (b, c, etc., [Fig. 158]) cut by the streams in later cycles, and (2) by occasional elevations above the common level (monadnocks, a, a′, [Fig. 158]). The monadnocks are generally rather inconspicuous, but there is a notable group of them in North Carolina and Tennessee. Mount Mitchell and Roane Mountain are examples. When long distances are considered, the ridge crests depart somewhat from horizontality. This is believed to be due, in part at least, to deformations of the old peneplain during the uplift which inaugurated the second cycle of erosion.
Fig. 157.—Cross-section of a portion of the Appalachian Mountains to illustrate the phenomena of erosion cycles. (After Rogers.)
Fig. 158.—A diagrammatic longitudinal profile of an Appalachian Mountain ridge.
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.