[Illustration: +Fig.+ 146.—A canoe-shaped valley bordered by a ridge formed by the outcrop of a hard layer in a plunging syncline. The ridge bounding the canoe-valley is separated from an outer ridge by a curved valley underlain by relatively weak rock. (After Willis.)] [Illustration: +Fig.+ 147.—A diagram to illustrate the effects of erosion on a doubly-plunging anticline made up of beds of unequal hardness.] [Illustration: +Figs.+ 148–51.—Diagrams to illustrate the shifting of rivers from a synclinal to an anticlinal position. (After Davis.)] The anticlines and synclines under consideration are assumed to have a thick hard layer at the surface, and softer beds below. This is shown in the cross-section introduced in the figure, the upper hard stratum (_m_) being indicated by the dots, while the softer one (_n_) is white. The line _oo_ represents base-level, which is below the hard layer both in the syncline and anticline, but much farther below in the latter position than in the former. Because of their higher gradients, and because of the greater fracturing to which the region they drain was presumably subject at the time of folding, the tributary streams might cut through the hard layer sooner than the main stream which they join. This done, they would enlarge their valleys rapidly in the softer rock beneath, and secondary tributaries would be developed (Fig. 149). When the condition of things represented in Fig. 149 is reached, the streams _c_ and _d_, tributary to the synclinal stream, come into competition. The former has the advantage over the latter, because it joins the main stream at a lower level. Stream _c_ will therefore be likely to capture _d_. The incipient stages of the capture are stealthy, and the later bold. At first the divide between their head waters is shifted northward inch by inch, because the gradient toward _g_ is higher than that toward _e_. The capture of the head waters of _e_ is as slow as the migration of the divide, until the divide reaches the point where _e_ joins _f_. The stream _f_ is then diverted promptly into the valley of _g_, and is at once led away to _c_ (see Fig. 150). Strengthened by its increased volume, the stream _c_ (Fig. 150) lowers its valley across the hard layer more rapidly than before, and so holds the advantage it has gained. Not only this, but the beheaded stream _d_ (Fig. 150), because of its diminished volume, sinks its valley into the hard layer less rapidly than before, and its decrease in power also works to the advantage of the stream leading to _c_. The result is that the divide between _fg_ and _d_ does not remain constant, but is driven back step by step toward _a_.