The Geography of the Region about Devil's Lake and the Dalles of the Wisconsin. Wallace Walter Atwood
the north, the evenness of the crest is not less distinct, but from this side it is seen to be interrupted by a notable break or notch at Devil's lake (Plates V and XXXVII). The pass across the range makes a right-angled turn in crossing the range, and for this reason is not seen from the south.
The North or lesser quartzite range lying north of Baraboo is both narrower and lower than the south range, and its crest is frequently interrupted by notches or passes, some of which are wide. Near its eastern end occurs the striking gap known as the Lower narrows (Plate IV) through which the Baraboo river escapes to the northward, flowing thence to the Wisconsin. At this narrows the quartzite bluffs rise abruptly 500 feet above the river. At a and b, Plate II, there are similar though smaller breaks in the range, also occupied by streams. The connection between the passes and streams is therefore close.
There are many small valleys in the sides of the quartzite ranges (especially the South range) which do not extend back to their crests, and therefore do not occasion passes across them. The narrow valleys at a and b in Plate XXXVII, known as Parfrey's and Dorward's glens, respectively, are singularly beautiful gorges, and merit mention as well from the scenic as from the geologic point of view. Wider valleys, the heads of which do not reach the crest, occur on the flanks of the main range (as at d and e, Plate II) at many points. One such valley occurs east of the north end of the lake (x, Plate XXXVII), another west of the south end (y, Plate XXXVII), another on the north face of the west bluff west of the north end of the lake and between the East and West Sauk roads, and still others at greater distances from the lake in both directions. It is manifest that if the valleys were extended headward in the direction of their axes, they would interrupt the even crest. Many of these valleys, unlike the glens mentioned above, are very wide in proportion to their length. In some of these capacious valleys there are beds of Potsdam sandstone, showing that the valleys existed before the sand of the sandstone was deposited.
The structure and constitution of the ridges.—The quartzite of the ridges is nothing more nor less than altered sandstone. Its origin dates from that part of geological time known to geologists as the Upper Huronian period (see p. 23). The popular local belief that the quartzite is of igneous origin is without the slightest warrant. It appears to have had its basis in the notion that Devil's lake occupies an extinct volcanic crater. Were this the fact, igneous rock should be found about it.
Quartzite is sandstone in which the intergranular spaces have been filled with silica (quartz) brought in and deposited by percolating water subsequent to the accumulation of the sand. The conversion of sandstone into quartzite is but a continuation of the process which converts sand into sandstone. The Potsdam or any other sandstone formation might be converted into quartzite by the same process, and it would then be a metamorphic rock.
Like the sandstone, the quartzite is in layers. This is perhaps nowhere so distinctly shown on a large scale as in the bluffs at
WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. V.
The Notch in the South quartzite range, at Devil's Lake.
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WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. VI.
The east bluff of Devil's lake, showing the dip of quartzite (to the left), and talus above and below the level where the beds are shown.
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Devil's lake, and at the east end of the Devil's nose. On the East bluff of the lake, the stratification is most distinctly seen from the middle of the lake, from which point the photograph reproduced in Plate VI was taken.
Unlike the sandstone and limestone, the beds of quartzite are not horizontal. The departure from horizontality, technically known as the dip, varies from point to point (Fig. 4). In the East bluff of the lake as shown in Plate VI, the dip is about 14° to the north. At the Upper and Lower narrows of the Baraboo (b and c, Plate II) the beds are essentially vertical, that is, they have a dip of about 90°. Between these extremes, many intermediate angles have been noted. Plate VII represents a view near Ablemans, in the Upper narrows, where the nearly vertical beds of quartzite are well exposed.
The position of the beds in the quartzite is not always easy of recognition. The difficulty is occasioned by the presence of numerous cleavage planes developed in the rock after its conversion into quartzite. Some of these secondary cleavage planes are so regular and so nearly parallel to one another as to be easily confused with the bedding planes. This is especially liable to make determinations of the dip difficult, since the true bedding was often obscured when the cleavage was developed.
In spite of the difficulties, the original stratification can usually be determined where there are good exposures of the rock. At some points the surfaces of the layers carry ripple marks, and where they are present, they serve as a ready means of identifying the bedding planes, even though the strata are now on edge. Layers of small pebbles are sometimes found. They were horizontal when the sands of the quartzite were accumulating, and where they are found they are sufficient to indicate the original position of the beds.
Aside from the position of the beds, there is abundant evidence of dynamic action [2] in the quartzite. Along the railway at Devil's lake, half a mile south of the Cliff House, thin
Fig. 4. -- Diagram made by plotting the different dips now at hand along a section from A to B, Plate II and connecting them so as to show the structure indicated by the known data. The full lines, oblique or vertical, represent the beds of quartzite. The continuous line above them represents the present surface of the quartzite, while the dotted lines suggest the continuation of the beds which completed the great folds of which the present exposures appear to be remnants.
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Fig. 5. -- A diagrammatic section showing the relation of the sandstone to the quartzite.
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WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. VII.
The East Bluff at the Upper Narrows of the Baraboo near Ablemans, showing the vertical position of the beds of quartzite. In the lower right-hand corner, above the bridge, appears some of the breccia mentioned on p. 18. See larger image
WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. VIII.
Vertical shear zone in face of east bluff at Devil's lake.
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zones of schistose rock may be seen parallel to the bedding planes. These zones of schistose rock a few inches in thickness were developed from the quartzite by the slipping of the rock on either side. This slipping presumably occurred during the adjustment of the heavy beds of quartzite to their new positions, at the time of tilting and folding, for no thick series of rock can be folded without more or less slipping of the layers on one another. The slipping (adjustment) takes place along the weaker zones. Such zones of movement are sometimes known as shear zones, for the rock on the one side has been sheared (slipped) over that on the other.
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