The changes in the courses of streams, by means of which they come to sustain definite and stable relations to the rock structure beneath, are known as processes of _adjustment_.[54] Since streams and valleys adjust themselves to other conditions as well, this phase of adjustment may be called _structural adjustment_. Structural adjustment is not uncommon among rivers flowing over strata which are vertical or highly inclined, since in these positions the hard and soft strata are most likely to come to the surface in frequent alternation. The smaller streams suffer capture and adjustment first, since, as a rule, they have shallower valleys. It often happens that main streams, because of their deeper valleys, hold courses not in adjustment with structure (the Delaware, the Susquehanna, etc.), while tributary streams are captured, diverted, and adjusted. The capture of a tributary, however, leads both to the diminution of its main and to the increase of its captor, and the weakened stream may ultimately fall a prey to the one which is strengthened. The processes of adjustment go on until the streams flow as much as possible on the weaker beds, and as little as possible on the stronger, when adjustment is complete. This amounts to the same thing as saying that the outcrops of the hard layers tend to become divides. In many cases an area is so situated that there is no escape for its drainage except across resistant rock. In this case its drainage is completely adjusted when as few streams as possible cross the resistant rock, and these by the shortest routes. Adjustment has been carried to a high degree of perfection in most parts of the Appalachian system. Here, as in all other mountains of similar structure, strata of unequal hardness were folded into ridges. In this case, the folds have been truncated by erosion, exposing the more and the less resistant beds (_H_ and _S_ respectively) in alternate belts along the flanks of the truncated folds (_ab_ and _cd_, Fig. 133). The streams, especially the lesser ones, now flow along the strike of the softer beds much more commonly than elsewhere, and where they cross the hard layers it is usually at right angles to the strike. This is shown in Fig. 134, where the arrows indicate the direction of strike. In the history of these rivers, however, a factor is involved which has not yet been considered, and these streams will be referred to later. [Illustration: +Fig.+ 133.—Diagram showing the outcrops of hard layers on the flanks of a truncated fold. _cd_ represents the present surface; dotted lines above, earlier surfaces.] [Illustration: +Fig.+ 134.—Example of adjusted drainage in a region of folded rocks, Va.-W. Va.] [Illustration: +Fig.+ 135.—Diagram to illustrate readjustment of drainage, as base-level is approached.]