The horizontal beds of Potsdam sandstone may be traced up to the bases of the quartzite ranges, where they may frequently be seen to abut against the tilted beds of quartzite. Not only this, but isolated patches of sandstone lie on the truncated edges of the dipping beds of quartzite well up on the slopes, and even on the crest of the ridge itself. In the former position they may be seen on the East bluff at Devil's lake, where horizontal beds of conglomerate and sandstone rest on the layers of quartzite which dip 14° to the north. The stratigraphic relations of the two formations are shown in Fig. 5 which represents a diagrammatic section from A to B, Plate II. Plate XI is reproduced from a photograph taken in the Upper narrows of the Baraboo near Ablemans, and shows the relations as they appear in the field. The quartzite layers are here on edge, and on them rest the horizontal beds of sandstone and conglomerate. Similar stratigraphic relations are shown at many other places. This is the relationship of _unconformity_. Such an unconformity as that between the sandstone and the quartzite of this region shows the following sequence of events: (1) the quartzite beds were folded and lifted above the sea in which the sand composing them was originally deposited; (2) a long period of erosion followed, during which the crests of the folds were worn off; (3) the land then sank, allowing the sea to again advance over the region; (4) while the sea was here, sand and gravel derived from the adjacent lands which remained unsubmerged, were deposited on its bottom. These sands became the Potsdam sandstone. This sequence of events means that between the deposition of the quartzite and the sandstone, the older formation was disturbed and eroded. Either of these events would have produced an unconformity; the two make it more pronounced. That the disturbance of the older formation took place before the later sandstone was deposited is evident from the fact that the latter formation was not involved in the movements which disturbed the former. Although the sandstone appears in patches on the quartzite ranges, it is primarily the formation of the surrounding plains, occupying the broad valley between the ranges, and the territory surrounding them. The quartzite, on the other hand, is the formation of the ridges, though it outcrops at a few points in the plain. (Compare Plates II and XXXVII.) The striking topographic contrasts between the plains and the ridges is thus seen to be closely related to the rock formations involved. It is the hard and resistant quartzite which forms the ridges, and the less resistant sandstone which forms the lowlands about them. That quartzite underlies the sandstone of the plain is indicated by the occasional outcrops of the former rock on the plain, and from the fact that borings for deep wells have sometimes reached it where it is not exposed.