Fig. 65.
Describe the valley of stream a. Is it young or old? How does the valley of b differ from that of a? Compare as to form and age the inner valley of b with the outer valley and with the valley of a. Account for the inner valley. Why does it not extend to the upper portion of the course of b? Will it ever do so? Draw longitudinal profile of b, showing the different gradient of upper and lower portions of its course not here seen. As the inner valley of tributary c extends headward it may invade the valley of a before the inner valley of a has worked upstream to the area seen in the diagram. With what results?
The piedmont belt. As an example of an ancient peneplain uplifted and dissected we may cite the Piedmont Belt, a broad upland lying between the Appalachian Mountains and the Atlantic coastal plain. The surface of the Piedmont is gently rolling. The divides, which are often smooth areas of considerable width, rise to a common plane, and from them one sees in every direction an even sky line except where in places some lone hill or ridge may lift itself above the general level ([Fig. 62]). The surface is an ancient one, for the mantle of residual waste lies deep upon it, soils are reddened by long oxidation, and the rocks are rotted to a depth of scores of feet.
At present, however, the waste mantle is not forming so rapidly as it is being removed. The streams of the upland are actively engaged in its destruction. They flow swiftly in narrow, rock- walled valleys over rocky beds. This contrast between the young streams and the aged surface which they are now so vigorously dissecting can only be explained by the theory that the region once stood lower than at present and has recently been upraised. If now we imagine the valleys refilled with the waste which the streams have swept away, and the upland lowered, we restore the Piedmont region to the condition in which it stood before its uplift and dissection,—a gently rolling plain, surmounted here and there by isolated hills and ridges.
Fig. 66. Dissected Peneplain of Southern New England
The surface of the ancient Piedmont plain, as it may be restored from the remnants of it found on the divides, is not in accordance with the structures of the country rocks. Where these are exposed to view they are seen to be far from horizontal. On the walls of river gorges they dip steeply and in various directions and the streams flow over their upturned edges. As shown in [Figure 67], the rocks of the Piedmont have been folded and broken and tilted.