Fig. 10. -- A diagrammatic cross-section of a delta.
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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.

The Lower Magnesian limestone.—The conditions of sedimentation finally changed in the area under consideration. When the sand of the sandstone was being deposited, adjacent lands were the source whence the sediments were chiefly derived. The evidence that the region was sinking while the sand was being deposited shows that the land masses which were supplying the sand, were becoming progressively smaller. Ultimately the sand ceased to be washed out to the region here described, either because the water became too deep [3], or because the source of supply was too distant. When these relations were brought about, the conditions were favorable for the deposition of sediments which were to become limestone. These sediments consisted chiefly of the shells of marine life, together with an unknown amount of lime carbonate precipitated from the waters of the sea. The limestone contains no coarse, and but little fine material derived from the land, and the surfaces of its layers are rarely if ever ripple-marked. The materials of which it is made must therefore have been laid down in quiet waters which were essentially free from land-derived sediments. The depth of the water in which it was deposited was not, however, great, for the fossils are not the remains of animals which lived in abysmal depths.

The deposition of limestone sediments following the deposition of the Potsdam sands, does not necessarily mean that there was more or different marine life while the younger formation was making, but only that the shells, etc., which before had been mingled with the sand, making fossiliferous sandstone, were now accumulated essentially free from land-derived sediment, and therefore made limestone.

Like the sandstone beneath, the limestone formation has a wide distribution outside the area here under discussion, showing that conditions similar to those of central Wisconsin were widely distributed at this time.

The beds of limestone are conformable on those of the sandstone, and the conformable relations of the two formations indicate that the deposition of the upper followed that of the lower, without interruption.

The thickness of the Lower Magnesian limestone varies from less than 100 to more than 200 feet, but in this region its thickness is nearer the lesser figure than the larger. The limestone is now present only in the eastern and southern parts of the area, though it originally covered the whole area.

The St. Peters sandstone.—Overlying the Lower Magnesian limestone at a few points, are seen remnants of St. Peters sandstone. The constitution of this formation shows that conditions of sedimentation had again changed, so that sand was again deposited where the conditions had been favorable to the deposition of limestone but a short time before. This formation has been recognized at but two places (d and e) within the area shown on Plate [XXXVII], but the relations at these two points are such as to lead to the conclusion that the formation may once have covered the entire region. This sandstone formation is very like the sandstone below. Its materials doubtless came from the lands which then existed. The formation is relatively thin, ranging from somewhat below to somewhat above 100 feet.

The change from the deposition of limestone sediments to sand may well have resulted from the shoaling of the waters, which allowed the sand to be carried farther from shore. Rise of the land may have accompanied the shoaling of the waters, and the higher lands would have furnished more and coarser sediments to the sea.