If, on the other hand, the axis of the anticline or syncline to be eroded was not horizontal, that is, if it plunged, the topographic result would be somewhat different. Suppose a plunging anticline to be truncated at base-level. If either end of the fold plunged below the plane of truncation, the outcrops of a given layer on opposite sides of the axis would converge in the direction of plunge, and come together at the end. At a stage of erosion antedating planation (say late maturity) there would have been a ridge, or a succession of hills, in the position corresponding to the outcrop of a hard layer, with a canoe-shaped valley within. If two hard layers were involved, instead of one, there would be two encircling ridges, with a curved valley between them, and a canoe-shaped valley within the innermost ([Fig. 146]). If the anticline plunged both ways, the valley enclosed by the hard-layer ridge would be canoe-shaped at both ends ([Fig. 147]). In such a case there would be likely to be a low gap (water-gap) in the rim of the valley through which the drainage which degraded the surface escaped, but there would be likely to be but one, for if two or more streams had drained the area of the valley at an early stage of erosion, one would be likely to have captured the others (see [p. 138]) before late maturity. A succession of doubly-plunging anticlines and synclines might give rise to a very complex series of ridges and valleys. Illustrations of the above phenomena are found at various points in the Appalachian Mountains, especially in eastern Pennsylvania.[56]
Fig. 144.—The Natural Bridge of Virginia, from the southeast (Walcott, U. S. Geol. Surv.)
Fig. 145.—A natural bridge in development. Two Medicine River, Mont. Corresponds to the stage represented by [Fig. 142], and the view corresponds to that shown diagrammatically at the right-hand end of the figure. (Whitney.)
In the structural adjustment which goes with the erosion of folds, it often happens that the valleys come to be located on the anticlines, while the outcrops of the hard layers on the flanks of the anticlines, or even in the original synclines, become the mountains. The adjustments by which valleys come to be located on anticlines are somewhat as follows:[57] [Fig. 148] represents two doubly-plunging anticlines with a syncline between, the relative elevations being shown by contour lines. At the outset, the drainage of such a region must have followed the structural valley, and its initial course, consequent on the slope, must have been down the axial trough. Drainage from the anticlines into the synclines would have promptly developed valleys, and the valleys would soon have acquired streams.
Fig. 146.—A canoe-shaped valley bordered by a ridge formed by the outcrop of a hard layer in a plunging syncline. The ridge bounding the canoe-valley is separated from an outer ridge by a curved valley underlain by relatively weak rock. (After Willis.)
Fig. 147.—A diagram to illustrate the effects of erosion on a doubly-plunging anticline made up of beds of unequal hardness.