Horizontal or very slightly undulating strata, especially if the upper beds are harder than those below, give rise by erosion to flat-topped ridges or table-mountains (Fig. 39). But if the strata be softer and of more uniform texture, erosion yields rounded hills, often very steep, and sometimes passing into pinnacles, as in the Bad Lands of the west. Broad, open folds, as we have seen, give, normally, synclinal hills and anticlinal valleys (Fig. 22), when the erosion is well advanced. But in more strongly, closely folded rocks the ridges and valleys are determined chiefly by the outcrops of harder and softer strata, as shown in Fig. 40, the symmetry of the reliefs depending upon the dip of the strata. This principle of unequal hardness or durability also determines most of the topographic features in regions of metamorphic and crystalline rocks, in which the stratification is obscure or wanting. [Illustration: Fig. 39.—Horizontal strata and table-mountains.] [Illustration: Fig. 40.—Ridges due to the outcrops of hard strata.] The boldness of the topography, and the relation of depth to width in valleys, depends largely upon the altitude above the sea; but partly, also, upon the distribution of the rainfall, the drainage channels or valleys being narrowest and most sharply defined in arid regions traversed by rivers deriving their waters from distant mountains. That these are the conditions most favorable for the formation of cañons is proved by the fact that they are fully realized in the great plateau country traversed by the Colorado and its tributaries, a district which leads the world in the magnitude and grandeur of its cañons. But deep gorges and cañons will be formed wherever a considerable altitude, by increasing the erosive power of the streams, enables them to deepen their channels much more rapidly than the general face of the country is lowered by rain and frost. This is the secret of such cañons as the Yosemite Valley, and the gorge of the Columbia River, and probably of the fiords which fret the north-west coasts of this continent and Europe. For a full description and illustration of the topographic types developed by the action of water and ice upon the surface of the land, and of the various characteristic forms of marine erosion, teachers are referred to the larger works named in the introduction, especially Le Conte’s Elements of Geology, and to the better works on physical geography. We will, in closing this section, merely glance at some of the minor erosion-forms, which are not properly topographic, but may be often illustrated by class-room and museum specimens. Mere weathering, the action of rain and frost, develops very characteristic surfaces upon different classes of rocks, delicately and accurately expressing in relief those slight differences in texture, hardness, and solubility, which must exist even in the most homogeneous rocks. Every one