The first effect of the sedimentation which followed submergence was to even up the irregular surface of the quartzite, for the depressions in the surface were the first to be submerged, and the first to be filled. As the body of sediment thickened, it buried the lower hills and the lower parts of the higher ones. The extent to which the Potsdam formation buried the main ridge may never be known. It may have buried it completely, for as already stated patches of sandstone are found upon the main range. These patches make it clear that some formation younger than the quartzite once covered essentially all of the higher ridge. Other evidence to be adduced later, confirms this conclusion. It has, however, not been demonstrated that the high-level patches of sandstone are Potsdam. There is abundant evidence that the subsidence which let the Potsdam seas in over the eroded surface of the Huronian quartzite was gradual. One line of evidence is found in the cross-bedding of the sandstone (Plate XII) especially well exhibited in the Dalles of the Wisconsin. The beds of sandstone are essentially horizontal, but within the horizontal beds there are often secondary layers which depart many degrees from horizontality, the maximum being about 24°. Plates XXVII and XII give a better idea of the structure here referred to than verbal description can. The explanation of cross-bedding is to be found in the varying conditions under which sand was deposited. Cross-bedding denotes shallow water, where waves and shore currents were effective at the bottom where deposition is in progress. For a time, beds were deposited off shore at a certain angle, much as in the building of a delta (Fig. 10). Then by subsidence of the bottom, other layers with like structure were deposited over the first. By this sequence of events, the dip of the secondary layers should be toward the open water, and in this region their dip is generally to the south. At any stage of deposition the waves engendered by storms were liable to erode the surface of the deposits already made, and new layers, discordant with those below, were likely to be laid down upon them. The subordinate layers of each deposit might dip in any direction. If this process were repeated many times during the submergence, the existing complexity would be explained. [Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XII. Steamboat rock,--an island in the Dalles of the Wisconsin.] [Illustration: Fig. 10.--A diagrammatic cross-section of a delta.] The maximum known thickness of the Potsdam sandstone in Wisconsin is about 1,000 feet, but its thickness in this region is much less. Where not capped by some younger formation, its upper surface has suffered extensive erosion, and the present thickness therefore falls short of the original. The figures given above may not be too great for the latter.