Had the quartzite ranges not been completely buried by the Paleozoic sediments, they would have appeared as ridges on the new land surface, and would have had a marked influence on the development of the drainage of the newly emerged surface. But as the ranges were probably completely buried, the drainage lines were established regardless of the position of the hard, but buried ridges. When in the process of degradation the quartzite surfaces were reached, the streams encountered a formation far more resistant than the surrounding sandstone and limestone. As the less resistant strata were worn away, the old quartzite ridges, long buried, again became prominent topographic features. In this condition they were "resurrected mountains."

If, when erosion on the uplifted surface of Paleozoic rocks began, a valley had been located directly over the buried quartzite ridge, and along its course, it would have been deepened normally until its bottom reached the crest of the hard formation. Then, instead of sinking its valley vertically downward into the quartzite, the stream would have shifted its channel down the slope of the range along the junction of the softer and harder rock (Fig. [25]). Such changes occasioned by the nature and position of the rock concerned, are known as adjustments.

Fig. 25. -- Diagram illustrating the hypothetical case of a stream working down the slope of the quartzite range. The successive sections of the valley are suggested by the lines ae, be, ce and de.
[See larger image]

Streams which crossed the quartzite ridges on the overlying strata might have held their courses even after their valleys were lowered to the level of the quartzite. Such streams would have developed narrows at the crossing of the quartzite. In so far as there were passes in the quartzite range before the deposition of the Paleozoic beds, they were filled during the long period of sedimentation, to be again cleared out during the subsequent period of erosion. The gap in the South range now occupied by the lake was a narrows in a valley which existed, though perhaps not to its present depth, before the Potsdam sandstone was deposited. It was filled when the sediments of that formation were laid down, to be again opened, and perhaps deepened, in the period of erosion which followed the deposition of the Paleozoic series.

During the earliest period of erosion of which there is positive evidence, after the uplift of the Paleozoic beds, the softer formations about the quartzite were worn down to a level 400 or 500 feet below the crests of the South quartzite range. At this lower level, an approximate plain, a peneplain, was developed, the level of which is shown by numerous hills, the summits of which now reach an elevation of from 1,000 to 1,100 feet above the sea. At the time of its development, this peneplain was but little above sea level, for this is the only elevation at which running water can develop such a plain. Above the general level of this plain rose the quartzite ranges as elongate monadnocks (see p. [52]), the highest parts of which were fully 500 feet above the plain. A few other points in the vicinity failed to be reduced to the level of the peneplain. The 1,320 foot hill (d, Plate [XXXVII]), one and one-half miles southeast of the Lower narrows, and Gibraltar Rock (e, same plate), two miles southeast of Merrimac, rose as prominences above it. It is possible that these crests are remnants of a base-level plain older than that referred to above. If while the quartzite remained much as now, the valleys in the sandstone below 1,000 or 1,100 feet were filled, the result would correspond in a general way to the surface which existed in this region when the first distinctly recognizable cycle of erosion was brought to a close. Above the undulating plain developed in the sandstone and limestone, the main quartzite ridge would have risen as a conspicuous ridge 400 to 500 feet.

This cycle had not been completed, that is, the work of base-leveling had not been altogether accomplished, when the peneplain was elevated, and the cycle, though still incomplete, brought to a close. By the uplift, the streams were rejuvenated, and sunk their valleys into the elevated peneplain. Thus a new cycle of erosion was begun, and the uplifted peneplain was dissected by the quickened streams which sank their valleys promptly into the slightly resistant sandstone. At their new base-level, they ultimately developed new flats. This cycle of erosion appears to have advanced no farther than to the development of wide flats along the principal streams, such as the Wisconsin and the Baraboo, and narrow ones along the subordinate water courses, when it was interrupted. Along the main streams the new flats were at a level which is now from 800 to 900 feet above the sea, and 700 to 800 feet below the South quartzite range. It was at this time that the plains about Camp Douglas and Necedah, already referred to, were developed. During this second incomplete cycle, the quartzite ranges, resisting erosion, came to stand up still more prominently than during the first.

The interruption of this cycle was caused by the advent of the glacial period which disturbed the normal course of erosion. This period was accompanied and followed by slight changes of level which also had their influence on the streams. The consideration of the effects of glaciation and of subsequent river erosion are postponed, but it may be stated that within the area which was glaciated the post-glacial streams have been largely occupied in removing the drift deposited by the ice from the preglacial valleys, or in cutting new valleys in the drift. The streams outside the area of glaciation were less seriously disturbed.

At levels other than those indicated, partial base-levels are suggested, and although less well marked in this region, they might, in the study of a broader area, bring out a much more complicated erosion history. As already suggested, one cycle may have preceded that the remnants of which now stand 1,000-1,100 feet above sea level, and another may have intervened between this and that marked by the 800 to 900 foot level.

From the foregoing it is clear that the topography of the region is, on the whole, an erosion topography, save for certain details in its eastern portion. The valleys differ in form and in size, with their age, and with the nature of the material in which they are cut; while the hills and ridges differ with varying relations to the streams, and with the nature of the material of which they are composed.