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. [Illustration: +Fig.+ 157.—Cross-section of a portion of the Appalachian Mountains to illustrate the phenomena of erosion cycles. (After Rogers.)] [Illustration: +Fig.+ 158.—A diagrammatic longitudinal profile of an Appalachian Mountain ridge.]