As the valleys increase in width and depth, and as the tributaries become more numerous and wider, the topography of which the valleys are a feature, becomes more and more broken. At first the tracts between the streams are in the form of ridges, wide if parallel valleys are distant from one another, and narrow if they are near. The ridges wind with the valleys which separate them. Whatever the width of the inter-stream ridges, it is clear that they must become narrower as the valleys between them become wider, and in following down a valley a point is reached, sooner or later, where the valleys, main and tributary, are of such size and so numerous that their slopes constitute a large part of the surface. Where this is true, and where the valleys are deep, the land is of little industrial value except for timber and grazing. When, in descending a valley system, this sort of topography is reached, the roads often follow either the valleys or the ridges, however indirect and crooked they may be. Where the ridges separating the valleys in such a region have considerable length, they are sometimes spoken of as "hog backs." Still farther down the valley system, tributary valleys of the second and lower orders cross the "hog backs," cutting them into hills. By the time this sort of topography is reached, a series of flats is found bordering the streams. These flats may occur on both sides of the stream, or on but one. The topography and the soil of these flats are such as to encourage agriculture, and the river flats or alluvial plains are among the choicest farming lands. With increasing distance from the heads of the valleys, these river plains are expanded, and may be widened so as to occupy the greater part of the surface. The intervening elevations are there relatively few and small. Their crests, however, often rise to the same level as that of the broader inter-stream areas farther up the valleys. The relations of the valleys and the high lands separating them, is such as to suggest that there are, generally speaking, two sets of flat surfaces, the higher one representing the upland in which the valleys lie, the lower one representing the alluvial plains of the streams. The two sets of flats are at once separated and connected by slopes. At the head of a drainage system, the upland flats predominate; in the lower courses, the river plains; in an intermediate stage, the slopes are more conspicuous than either upper or lower flat.