From these several facts, viz.: the horizontal strata, the ripple-marks on the surfaces of the layers, the fossils, the character of the sand, and the water-worn pebbles and bowlders of the basal conglomerate, positive conclusions concerning the origin of the formation may be drawn.

The arrangement in definite layers proves that the formation is sedimentary; that is, that its materials were accumulated in water whither they had been washed from the land which then existed. The ripple-marks show that the water in which the beds of sand were deposited was shallow, for in such water only are ripple-marks made. [1] Once developed on the surface of the sand they may be preserved by burial under new deposits, just as ripple-marks on sandy shores are now being buried and preserved.

The conglomerate beds of the formation corroborate the conclusions to which the composition and structure of the sandstone point. The water-worn shapes of the pebbles and stones show that they were accumulated in water, while their size shows that the water must have been shallow, for stones of such sizes are handled only by water of such slight depth that waves or strong currents are effective at the bottom. Furthermore, the large bowlders show that the source of supply (quartzite) must have been close at hand, and that therefore land composed of this rock must have existed not far from the places where the conglomerate is found.

The fossils likewise are the fossils of aquatic life. Not only this, but they are the fossils of animals which lived in salt water. The presence of salt water, that is, the sea, in this region when the sand of the sandstone was accumulating, makes the wide extent of the formation rational.

From the constitution and structure of the sandstone, it is therefore inferred that it accumulated in shallow sea water, and that, in the vicinity of Devil's lake, there were land masses (islands) of quartzite which furnished the pebbles and bowlders found in the conglomerate beds at the base of the formation.

This being the origin of the sandstone, it is clear that the layers which now appear on opposite sides of valleys must once have been continuous across the depressions; for the sand accumulated in shallow water is never deposited so as to leave valleys between ridges. It is deposited in beds which are continuous over considerable areas.

Within the area under consideration, limestone is much less widely distributed than sandstone. Thin beds of it alternate with layers of sandstone in the upper portion of the Potsdam formation, and more massive beds lie above the sandstone on some of the higher elevations of the plain about the quartzite ridge. This is especially true in the southern and southwestern parts of the region shown on Plate [II]. The limestone immediately overlying the sandstone is the Lower Magnesian limestone.

The beds of limestone, like those of the sandstone beneath, are horizontal or nearly so, and the upper formation lies conformably on the lower. The limestone does not contain water-worn pebbles, and the surfaces of its layers are rarely if ever ripple-marked; yet the arrangement of the rock in distinct layers which carry fossils of marine animals shows that the limestone, like the sandstone beneath, was laid down in the sea. The bearing of this origin of the limestone on the development of the present valleys is the same as that of the sandstone.

Origin of the topography.—The topography of the plain surrounding the quartzite ridges, especially that part lying west of Devil's lake, is then an erosion topography, developed by running water. Its chief characteristic is that every depression leads to a lower one, and that the form of the elevations, hills or ridges, is determined by the valleys. The valleys were made; the hills and ridges left. If the material carried away by the streams could be returned, the valleys would be filled to the level of the ridges which bound them. Were this done, the restored surface would be essentially flat. It is the sculpturing of such a plain, chiefly by running water, which has given rise to the present topography.

In the development of this topography the more resistant limestone has served as a capping, tending to preserve the hills and ridges. Thus many of the hills, especially in the southwest portion of the area shown in Plate [II], are found to have caps of the Lower Magnesian formation. Such hills usually have flat tops and steep or even precipitous slopes down to the base of the capping limestone, while the sandstone below, weathering more readily, gives the lower portions of the hills a gentler slope.