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.
Southwest from Devil's lake and northwest from Sauk City, in the valley of Honey creek, and again in the region southwest from Camp Douglas, the topography just described is well illustrated. In both these localities, as in all others where this type of topography prevails, the intimate relations of topography and drainage cannot fail to suggest that the streams which are today widening and deepening the valleys through which they flow, had much to do with their origin and development. This hypothesis, as applied to the region under consideration, may be tested by the study of the structure of the plain.
The second type of topography affecting the plain about the quartzite ranges is found east of a line running from Kilbourn City to a point just north of Prairie du Sac. Though in its larger features the area east of this line resembles that to the west, its minor features are essentially different. Here there are many depressions which have no outlets, and marshes, ponds, and small lakes abound. Not only this, but many of the lesser elevations stand in no definite relation to valleys. The two types of topography make it clear that they were developed in different ways.
_Structure._--Examination of the country surrounding the Baraboo ridges shows that its surface is underlaid at no great depth by horizontal or nearly horizontal beds of sandstone and limestone (see Plates XVI, XXVIII, and Frontispiece). These beds are frequently exposed on opposite sides of a valley, and in such positions the beds of one side are found to match those on the other. This is well shown along the narrow valley of Skillett creek just above the "Pewit's nest." Here the swift stream is rapidly deepening its channel, and it is clear that a few years hence, layers of sandstone which are now continuous beneath the bed of the creek will have been cut through, and their edges will appear on opposite sides of the valley just as higher layers do now. Here the most skeptical might be convinced that the layers of rock on either side of the narrow gorge were once continuous across it, and may see, at the same time, the means by which the separation was effected. Between the slight separation, here, where the valley is narrow, and the great separation where the valleys are wide, there are all gradations. The study of progressively wider valleys, commencing with such a gorge as that referred to, leaves no room for doubt that even the wide valleys, as well as the narrow ones, were cut out of the sandstone by running water.