Fig. 48.—Same as [Fig. 47], with the valleys more developed. The dotted line represents the original outline of the island. Its area is being extended by deposition everywhere, but most at the debouchures of the streams.
Fig. 49.—A later stage of the island shown in [Fig. 48].
Fig. 50.—Diagram to illustrate the lowering of a divide without shifting it. The crest of the divide is at a, b, and c successively. If erosion were unequal on the two sides, the divide would be shifted.
Had there been several initial meridional depressions instead of one in the island, or had there been several meridional belts where the material of the surface was less resistant than elsewhere, several valleys would have been developed, converging toward the center ([Fig. 47]). If the conditions were such as to allow of the equal development of valleys on all sides of the island, each would be lengthened by head erosion until it reached the center of the island, where the permanent divide between their heads would be established. Each would be widened by all the processes which widen valleys, and their widening would narrow the intervening areas (Figs. [48] and [49]). Under conditions of equal erosion the limits of width for each valley would be the centers of the ridges on either side, and here the divides between them would be permanently established. Though erosion would continue even after the crest of the ridge had been narrowed to a line, the permanence of the divide would follow from the fact that erosion would be equal on both sides of this line, and its effect would be to lower the divide, but not to shift it horizontally ([Fig. 50]). The limits in length and width are therefore not the same where there are several valleys as where there is but one. The limit in depth, however, remains the same, and the final result of erosion, proceeding along these lines, would be the base-leveling of the land, leaving a plain but slightly above sea-level. The plain would not be absolutely flat, though its relief would be very slight, and the higher parts would be along the lines of the divides between the streams ([Fig. 51]. Compare also [Fig. 42]). Many valleys would occasion more rapid degradation than few, and the period of base-leveling would be correspondingly shortened.
Had the initial depressions which gave origin to the valleys had positions other than meridional, the valleys would have had other and less regularly radial courses, but the final result of their development would have been the same.
It is not to be inferred that the method of valley development which has been sketched is the only one. The processes of valley development are complex, and the history of some valleys has run a different course; yet the processes outlined above are in operation in all cases, and they were probably the most important ones in the development of the first drainage system on any land surface. As will be seen in the sequel the history of valleys is subject to serious accidents, and they are often of such a nature as to mask the simplicity of the more normal processes.
