=Narrows.=—Inequalities in hardness occasion another peculiarity common to valleys. If a stream crosses vertical or highly inclined strata of unequal hardness, its valley is usually constricted at the crossing of the harder layers. If such a constriction be notable it is called a _narrows_, or sometimes a _water-gap_ (Figs. 121, 159, and Fig. 2, Pl. XII). The Appalachian Mountains afford numerous examples. The constriction arises because the processes which widen the valley are less effective on the hard layer than on the less resistant ones on either hand. Though most narrows are due to the superior resistance of the rock where they occur, they are sometimes the result of other causes. [Illustration: +Fig.+ 121.—Lower narrows of the Baraboo River, Wis. The even-crested ridge is Huronian quartzite. The surroundings are of Cambrian sandstone. (Atwood.)] [Illustration: +Fig.+ 122.—A hog-back, Jura-Trias. Colorado City, Colo. (Russell, U. S. Geol. Surv.)] Narrows are much more conspicuous in certain stages of erosion than in others. While a valley is still so young as to be narrow at all points, no narrows will be conspicuous; but at a later stage in its history, when the valley is otherwise wide, narrows are more pronounced. At a still later stage, when the hard strata themselves approach base-level, the narrows again become inconspicuous. From what has preceded it is clear that rapids or falls are likely to occur at narrows, especially in the early part of their history. =Other effects on topography.=—Inequalities in the hardness of rock develop certain peculiarities of topography other than those of valleys. The less resistant portions of a land area more or less distant from streams are worn down more readily than those which are more resistant. If great areas of high land be capped with hard rock they are likely to remain as plateaus after surrounding areas of less resistance are brought low. If the hard capping affects a small area instead of a large one, the elevation is a butte, a hill, or a mountain, instead of a plateau (Fig. 110). Many buttes and small mesas are but remnants of former plateaus (Mesa Lauriano, N. M., Fig. 1, Pl. XII). A feature of buttes and mesas capped by hard rock is the steep slope or cliff corresponding to the edge of the hard bed (Figs. 78 and 109). [Illustration: +Fig.+ 123.—A ridge due to the outcropping edge of hard Jurassic rock. Wyoming.]