After the head of a tributary has worked back from the immediate slope of its main every condition which determines the course of a gully is likely to affect it, and it is by no means certain that it will continue to lengthen in the direction in which it started. Since the general slope of the surface into which the tributary works is likely to be seaward, more water is likely to enter from the landward than from the seaward side of its head, so that, except where there are notable irregularities of slope, its tendency will be to turn more and more toward the direction of its main (efg, [Fig. 62]).
In depth the tributary is always limited by its main. The principles which determine the length and width of a main valley determine also the length and width of a tributary (see [p. 67] et seq.).
A CYCLE OF EROSION. ITS STAGES.
From what has preceded it is clear that the topography of a region undergoing erosion will change greatly from time to time. The first effect of erosion is to roughen the surface by cutting out valleys, leaving ridges and hills. The final effect is to make it smooth again by cutting the ridges and hills down to the level of the valleys.
Fig. 63.—Diagram showing three parallel valleys in a land surface.
Fig. 64.—Diagram to illustrate the lowering of the surface by valley erosion. The successive cross profiles of the valleys are represented by the lines 1–1, 1–1′, 2–2, 2–2′, etc.
The base-level of erosion has already been defined; but the mode of its development may now be illustrated in the light of the preceding discussion. Suppose a land surface affected by a series of parallel young valleys without tributaries ([Fig. 63]). Between them there is a series of upland plateaus. The profile of the surface between two adjacent valleys is represented in section by the uppermost line in [Fig. 64]. As the valleys are widened from 1–1 and 1,′–1′, to 2–2 and 2′–2′, the intervening plateau is correspondingly narrowed. When the valleys have attained the form represented by 3–3 and 3′–3′, the intervening upland has been narrowed to a ridge, a, and the valley flats have become wide. With continued erosion the ridge will be lowered (to b and below), and in time the surface will approach a plain. In this condition it is known as a peneplain (an “almost-plain”). Finally, when running water has done its utmost, the ridges will be essentially obliterated and a base-leveled plain (e, e′, e″) results. The figure expresses the fact that the base-level develops laterally from the axis of the valley. It also develops headward from the seaward end of the valley. Similarly, taking into account all the valleys which affect it, the seaward margin of a base-leveled plain is developed first, and thence it extends itself inland.
