FIG. 2.
A valley in a later stage of development.

FIG. 3.
Young valleys.

The water coming in at its head carries the head back into the land (head erosion), thus lengthening the gully; the water coming in at its sides wears back the lateral slopes, thus widening it; and the water flowing along its bottom deepens it. Thus gullies grow to be ravines, and farther enlargement by the same processes converts ravines into valleys. A river valley therefore is often but a gully grown big.

The course of a valley.—In the lengthening of a gully or valley headward, the growth will be in the direction of greatest wear. Thus in Plate [XIII Fig. 1], if the water coming in at the head of the gully effects most wear in the direction a, the head of the gully will advance in that direction; if there be most wear in the direction b or c, the head will advance toward one of these points. The direction of greatest wear will be determined either by the slope of the surface, or by the nature of the surface material. The slope may lead to the concentration of the entering waters along one line, and the surface material may be less resistant in one direction than in another. If these factors favor the same direction of head-growth, the lengthening will be more rapid than if but one is favorable. If there be more rapid growth along two lines, as b and c, Plate [XIII Fig. 1], than between them, two gullies may develop (Plate [XIII Fig. 2]). The frequent and tortuous windings common to ravines and valleys are therefore to be explained by the inequalities of slope or material which affected the surface while the valley was developing.

Tributary valleys.—Following out this simple conception of valley growth, we have to inquire how a valley system (a main valley and its tributaries) is developed. The conditions which determine the location and development of gullies in a new land surface, determine the location and development of tributary gullies. In flowing over the lateral slopes of a gully or ravine, the water finds either slope or surface material failing of uniformity. Both conditions lead to the concentration of the water along certain lines, and concentration of flow on the slope of an erosion depression, be it valley or gully, leads to the development of a tributary depression. In its growth, the tributary repeats, in all essential respects, the history of its main. It is lengthened headward by water coming in at its upper end, is widened by side wash, and deepened by the downward cutting of the water which flows along its axis. The factors controlling its development are the same as those which controlled the valley to which it is tributary.

There is one peculiarity of the courses of tributaries which deserves mention. Tributaries, as a rule, join their mains with an acute angle up stream. In general, new land surfaces, such as are now under consideration, slope toward the sea. If a tributary gully were to start back from its main at right angles, more water would come in on the side away from the shore, on account of the seaward slope of the land. This would be true of the head of the gully as well as of other portions, and the effect would be to turn the head more and more toward parallelism with the main valley. Local irregularities of surface may, and frequently do, interfere with these normal relations, so that the general course of a tributary is occasionally at right angles to its main. Still more rarely does the general course of a tributary make an acute angle with its main on the down stream side. Local irregularities of surface determine the windings of a tributary, so that their courses for longer or shorter distances may be in violation of the general rule (c, Fig. [43]); but on the whole, the valleys of a system whose history has not been interrupted in a region where the surface material is not notably heterogeneous, follow the course indicated above. This is shown by nearly every drainage system on the Atlantic Coastal plain which represents more nearly than any other portion of our continent, the conditions here under consideration. Fig. [12] represents the drainage system of the Mullica river in southern New Jersey and is a type of the Coastal plain river system.

How a valley gets a stream.—Valleys may become somewhat deep and long and wide without possessing permanent streams, though from their inception they have temporary streams, the water for which is furnished by showers or melting snow. Yet sooner or later, valleys come to have permanent streams. How are they acquired? Does the valley find the stream or the stream the valley? For the answer to these questions, a brief digression will be helpful.

Fig. 12. -- A typical river system of the Coastal type.
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