Fig. [4] conveys some notion of the amount of rock which was removed from the quartzite folds about Baraboo during this long period of erosion. The south range would seem to represent the stub of one side of a great anticlinal fold, a large part of which (represented by the dotted lines) was carried away, while the north range may be the core of another fold, now exposed by erosion.

Some idea of the geography of the quartzite at the close of this period of erosion may be gained by imagining the work of later times undone. The younger beds covering the quartzite of the plains have a thickness varying from zero to several hundred feet, and effectually mask the irregularities of the surface of the subjacent quartzite. Could they be removed, the topography of the quartzite would be disclosed, and found to have much greater relief than the present surface; that is, the vertical distance between the crest of the quartzite ridge, and the surface of the quartzite under the surrounding lowlands, would be greater than that between the same crest and the surface of the sandstone. But even this does not give the full measure of the relief of the quartzite at the close of the long period of erosion which followed its uplift, for allowance must be made for the amount of erosion which the crests of the quartzite ranges have suffered since that time. The present surface therefore does not give an adequate conception of the irregularity of the surface at the close of the period of erosion which followed the uplift and deformation of the quartzite. So high were the crests of the quartzite ranges above their surroundings at that time, that they may well be thought of as mountainous. From this point of view, the quartzite ranges of today are the partially buried mountains of the pre-Potsdam land of south central Wisconsin.

When the extreme hardness of the quartzite is remembered and also the extent of the erosion which affected it (Fig. [4]) before the next succeeding formation was deposited, it is safe to conclude that the period of erosion was very long.

Thickness of the quartzite.—The thickness of the quartzite is not known, even approximately. The great thickness in the south range suggested by the diagram (Fig. [4]) may perhaps be an exaggeration. Faulting which has not been discovered may have occurred, causing repetition of beds at the surface (Fig. [6]), and so an exaggerated appearance of thickness. After all allowances have been made, it is still evident that the thickness of the quartzite is very great.

II. THE HISTORY OF THE PALEOZOIC STRATA.

The subsidence.—Following the long period of erosion, the irregular and almost mountainous area of central Wisconsin was depressed sufficiently to submerge large areas which had been land. The subsidence was probably slow, and as the sea advanced from the south, it covered first the valleys and lowlands, and later the lower hills and ridges, while the higher hills and ridges of the quartzite stood as islands in the rising sea. Still later, the highest ridges of the region were themselves probably submerged.

Fig. 6. -- A diagrammatic cross-section, showing how, by faulting, the apparent thickness of the quartzite would be increased.
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The Potsdam sandstone (and conglomerate).—So soon as the sea began to overspread the region, its bottom became the site of deposition, and the deposition continued as long as the submergence lasted. It is to the sediments deposited during the earlier part of this submergence that the name Potsdam is given.