Skillett Falls, in the Potsdam formation, three miles southwest of Baraboo. The several small falls are occasioned by slight inequalities in the hardness of the layers.
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the vertical face (Fig. [16]). The result of wear at this point is the undermining of the hard layer above, and sooner or later, portions of it will fall. This will occasion the recession of the fall (profile d e and f e). As the fall recedes, it grows less and less high. When the recession has reached the point i, or, in other words, when the gradient of the stream below the fall crosses the junction of the beds of unequal hardness, as it ultimately must, effective undermining ceases, and the end of the fall is at hand.

When the effective undercutting ceases because the softer bed is no longer accessible, the point of maximum wear is transferred to the top of the hard bed just where the water begins to fall (g, Fig. [16]). The wear here is no greater than before, though it is greater relatively. The relatively greater wear at this point destroys the verticality of the face, converting it into a steep slope. When this happens, the fall is a thing of the past, and rapids succeed. With continued flow the bed of the rapids becomes less and less steep, until it is finally reduced to the normal gradient of the stream (h e), when the rapids disappear.

When thin layers of rock in a stream's course vary in hardness, softer beds alternating with harder ones, a series of falls such as shown in Plate [XVI], may result. As they work up stream, these falls will be obliterated one by one. Thus it is seen that falls and rapids are not permanent features of the landscape. They belong to the younger period of a valley's history, rather than to the older. They are marks of topographic youth.

Narrows.—Where a stream crosses a hard layer or ridge of rock lying between softer ones, the valley will not widen so rapidly in the hard rock as above and below. If the hard beds be vertical, so that their outcrop is not shifted as the degradation of the surface proceeds, a notable constriction of the valley results. Such a constriction is a narrows. The Upper and Lower narrows of the Baraboo (Plate [IV]) are good examples of the effect of hard rock on the widening of a valley.

Erosion of folded strata.—The processes of river erosion would not be essentially different in case the land mass upon which erosion operated were made of tilted and folded strata. The folds would, at the outset, determine the position of the drainage lines, for the main streams would flow in the troughs (synclines) between the folds (anticlines). Once developed, the streams would lower their beds, widen their valleys, and lengthen their courses, and in the long process of time they would bring the area drained nearly to sea-level, just as in the preceding case. It was under such conditions that the general processes of subaerial erosion operated in south central Wisconsin, after the uplift of the quartzite and before the deposition of the Potsdam sandstone. It was then that the principal features of the topography of the quartzite were developed.

In regions of folded strata, certain beds are likely to be more resistant than others. Where harder beds alternate with softer, the former finally come to stand out as ridges, while the outcrops of the latter mark the sites of the valleys. Such alternations of beds of unequal resistance give rise to various peculiarities of drainage, particularly in the courses of tributaries. These peculiarities find no illustration in this region and are not here discussed.

Base-level plains and peneplains.—It is important to notice that a plane surface (base-level) developed by streams could only be developed at elevations but slightly above the sea, that is, at levels at which running water ceases to be an effective agent of erosion; for so long as a stream is actively deepening its valley, its tendency is to roughen the area which it drains, not to make it smooth. The Colorado river, flowing through high land, makes a deep gorge. All the streams of the western plateaus have deep valleys, and the manifest result of their action is to roughen the surface; but given time enough, and the streams will have cut their beds to low gradients. Then, though deepening of the valleys will cease, widening will not, and inch by inch and shower by shower the elevated lands between

WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XVII.