Underground Water.
In what has preceded, reference has been made only to the results accomplished by the water which runs off over the surface. The water which sinks beneath it is, however, of no small importance in reducing a land surface. The enormous amount of mineral matter in solution in spring water bears witness to the efficiency of the ground water in dissolving rock, for since the water did not contain the mineral matter when it entered the soil, it must have acquired it below the surface. By this means alone, areas of more soluble rock are lowered below those of less solubility. Furthermore, the water is still active as a solvent agent after a surface has been reduced to so low a gradient that the run-off ceases to erode mechanically.
CHAPTER IV.
EROSION AND THE DEVELOPMENT OF STRIKING SCENIC FEATURES.
The uplift following the period of Paleozoic deposition in south central Wisconsin, inaugurated a period of erosion which, with some interruptions, has continued to the present day. The processes of weathering began as soon as the surface was exposed to the weather, and corrasion by running water began with the first shower which fell upon it. The sediment worn from the land was carried back to the sea, there to be used in the building of still younger formations.
The rate of erosion of a land surface depends in large measure upon its height. As a rule, it is eroded rapidly if high, and but slowly if low.
It is not known whether the lands of central Wisconsin rose to slight or to great heights at the close of the period of Paleozoic sedimentation. It is therefore not known whether the erosion was at the outset rapid or slow. If the land of southern Wisconsin remained low for a time after the uplift which brought the Paleozoic sedimentation to a close, weathering would have exceeded transportation and corrasion. A large proportion of the rainfall would have sunk beneath the surface, and found its way to the sea by subterranean routes. Loosening of material by alternate wetting and drying, expansion and contraction, freezing and thawing, and by solution, might have gone on steadily, but so long as the land was low, there would have been little run-off, and that little would have flowed over a surface of gentle slopes, and transportation would have been at a minimum. On the whole, the degradation of the land under these conditions could not have advanced rapidly.
If, on the other hand, the land was raised promptly to a considerable height, erosion would have been vigorous at the outset. The surface waters would soon have developed valleys which the streams would have widened, deepened and lengthened. Both transportation and corrasion would have been active, and whatever material was prepared for transportation by weathering, and brought into the valleys by side-wash, would have been hurried on its way to the sea, and degradation would have proceeded rapidly.
Establishment of drainage.—Valleys were developed in this new land surface according to the principles already set forth. Between the valleys there were divides, which became higher as the valleys became deeper, and narrower as the valleys widened. Ultimately the ridges were lowered, and many of them finally eliminated in the manner already outlined. The distance below the original surface and that at which the first series of new flats were developed is conjectural, but it would have depended on the height of the land. So far as can now be inferred, the new base-plain toward which the streams cut may have been 400 or 500 feet below the crests of the quartzite ridges. It was at this level that the oldest base-plain of which this immediate region shows evidence, was developed.