If the rock of a region be stratified and the layers tilted, the removal of the softer beds leaves the harder ones projecting above the general level in the form of ridges or “hog-backs” (Figs. 122 and 123). Dikes of igneous rock, harder than the beds which they intersect, likewise become ridges after the degradation of their surroundings. The plugs of old volcanic vents and other igneous intrusions of limited area often constitute conspicuous hills or mountains after erosion has removed their less resistant surroundings (Fig. 124). Inequalities of hardness are therefore responsible for many hills and ridges. In the isolation of the hills and ridges picturesque coves are developed, where the attitude and distribution of the weak and strong rocks are propitious. The bottoms of the coves are located on the weak rocks, and above them rise the precipitous slopes of the resistant ones. Round valley (Fig. 1, Pl. XVII, High Bridge, N. J., quadrangle, U. S. Geol. Surv.) and the coves about the head of Hiawassee River (Dahlonega, Ga., quadrangle) are examples. [Illustration: +Fig.+ 124.—Matteo tepee, Wyo. Mass of igneous rock exposed by erosion, and preserved because of its superior resistance. (Detroit Photo. Co.)] Ridges and hills resulting from the unequal degradation of unequally resistant terranes are not equally prominent at all stages in an erosion cycle. In early youth the material surrounding the hard bodies of rock has not been removed; in early maturity considerable portions of their surroundings still remain about them; but in late maturity or early old age the outcropping masses of hard rock have been more perfectly isolated and are most conspicuous. Most of the even-crested ridges of the Appalachian system, as well as many others which might be mentioned, became ridges in this way. In the final stages of an erosion cycle the ridges of hard rock are themselves brought low. Isolated remnants of hard rock which remain distinctly above their surroundings in the late stages of an erosion cycle (Fig. 124) are known as _Monadnocks_, the name being derived from Mount Monadnock, N. H., an elevation of this sort developed in a cycle antedating the present. [Illustration: +Figs.+ 125–27.—Diagrams illustrating piracy, where the stream which does not flow over rock of superior hardness captures those which do. Fig. 126 represents a further development of the drainage shown in Fig. 125, and Fig. 127 represents a still later stage.] [Illustration: +Figs.+ 128–30.—Diagrams to illustrate piracy, where the competing streams all cross a hard layer. The diagrams represent successive stages of development.]