The arrest of stream erosion by the more resistant rocks.—The capacity of a river to erode and carry away the rock material that lies along its course is dependent not only upon the velocity of the current, but also upon the hardness, the firmness of texture, and the solubility of the material. Particularly in arid and semiarid regions, where no mantle of vegetation is at hand to mask the surfaces of the firmer rock masses, differences of this kind are stamped deeply upon the landscape. The rock terraces in the Grand Cañon of the Colorado together represent the stronger rock formations of the region, while sloping talus accumulations bury the weaker beds from sight.
Fig. 183.—Plan of a river narrows.
Each area of harder rock which rises athwart the course of a stream causes a temporary arrest in the process of valley erosion and is responsible for a noteworthy local contraction of the river valley. The valley is carved less widely as well as less deeply, and since a river can never corrade below its base, a “temporary base level” is for a time established above the area of harder rock. Owing to the contraction of the valley under these conditions, the locality is described as a river narrows ([Fig. 183]). The narrows upon the Hudson River occur in the Highlands where the river leaves a broad expanse occupied by softer sediments to traverse an island-like area of hard crystalline rocks. Within the narrows of a river the steep walls, characteristic of youth and the turbulent current as well, are often retained long after other portions of the river have acquired the more restful lines of river maturity. The picturesque crag and the generally rugged character of river narrows render them points of special interest upon every navigable river.
Fig. 184.—Successive diagrams to illustrate repeated river piracy and the development of “trellis drainage”, (after Russell).
The capture of one river’s territory by another.—The effect of a hard layer of rock interposed in the course of a stream is thus always to delay the advance of the erosional process at all levels above the obstruction. When a stream in incising its valley degrades its channel through a veneer of softer rocks into harder materials below, it is technically described as having discovered the harder layer. Where several neighboring streams flow by similar routes to their common base level, those which discover a harder rock will advance their headwaters less rapidly into the upland and so will be at a disadvantage in extending their drainage territory. A stream which is not thus hindered will in the course of time rob the others of a portion of their territory, for it is able to erode its lower reaches nearer to base level and thus acquire for its upper reaches, where erosion is chiefly accomplished, an advantage in declivity. The divide which separates its headwaters from those of its less favored neighbor will in consequence migrate steadily into the neighbor’s territory. The divide is thus a sort of boundary wall separating the drainage basins of neighboring streams, and any migration must extend the territory of the one at the expense of the other. As more and more territory is brought under the dominion of the more favored stream, there will come a time when the divide in its migration will arrive at the channel of the stream that is being robbed, and so by a sudden act of annexation draw off all the upper waters into its own basin. By this capture the stream whose territory has been invaded is said to have been beheaded. By this act of piracy the stronger stream now develops exceptional activity because of the local steep grades near the point of capture, and with this newly acquired cutting power the invader is competent to advance still further and enter the territory of the stream that lies next beyond. The type of drainage network which results from repeated captures of this kind is known as “trellis drainage” ([Fig. 184]), a type well illustrated by the rivers of the southern Appalachians.
In general it may be said that, other conditions being the same, of two neighboring streams which have a common base level, that one which takes the longest route will lose territory to the other, since it must have the flatter average slope. Stream capture may thus come about without the discovery of hard rock layers which are more unfavorable to one stream than another.
Fig. 185.—Sketch maps to show the earlier and the present drainage condition about the Blue Ridge near Harper’s Ferry.