Rock Springs

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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The Upper Narrows of the Baraboo River, sometimes called Ableman's Gorge, is one of the classic geological field localities in Wisconsin if not the entire world.


This figure, redrawn from a diagram in Dalziel and Dott (1970) is a composite section projected onto a north-south plane running along the gorge. The section south of the bridge appears generally as shown, but the section north of the bridge is actually a mirror image of its appearance in the field.

Ripple marks in a vertical bedding surface of Baraboo Quartzite, south side of the Old Quartzite Quarry.



Two views of basal Cambrian cobble and boulder conglomerate unconformably overlying vertical beds of Baraboo Quartzite at the top of the Old Quartzite Quarry.

A sample of basal Cambrian conglomerate with cobbles of Baraboo quartzite. It fell from the top of the cliffs.

The Breccia Zone

Most of the Precambrian quartzites in Wisconsin show brecciation with angular quartzite surrounded by white vein quartz, often with cavities and quartz crystals. The blocks shown here are typical of the breccia zone in the Upper Narrows. Apart from the obvious conclusion that the brecciation is due to explosive hydrothermal activity, no clear explanation for the brecciation has been proposed.

The Polished Outcrop

The outcrop shown here has been a mystery for years. Explanations include the tongue-in-cheek suggestion that it was polished by mastodons using it for a scratching post. More prosaic explanations include wind polishing and possibly coating by silica dissolved and redeposited by ground water.

Van Hise Rock

Van Hise Rock is a classic locality. It was here that Van Hise and his associates developed many of the relationships between bedding and foliation that structural geologists use routinely. They were inspired to do so by the scanty outcrop in Wisconsin and the need to formulate techniques that would allow them to extrapolate observations in the field. In a final irony, it turns out that much of the Precambrian basement in Wisconsin is dominated by structures where this sort of analysis cannot be applied effectively

Tablet on the north side of Van Hise Rock. It reads:

Van Hise Rock

The material of this rock was once sand on the sea bottom and has since hardened into quartzite. It was tilted to the present position by a slow earth movement and then separated from the adjacent cliff by erosion. The vertical light and dark bands represent the original layers. The inclined cracks in the dark layer were caused by the readjustment in the layers during the tilting.

This rock is pictured in geologic books as a type illustrating important principles of structural geology and has been a point of special interest to many investigators in geology visiting this region. President Charles H. Van Hise of the University of Wisconsin was one of the first and foremost of these.

Please do not deface.

Tablet presented by friends of Van Hise at the University of Wisconsin, 1923.

Tension gashes on the north face of Van Hise Rock, indicating left-lateral shear. Since the view is approximately south, the rocks above the tension gashes moved down (east) relative to those below.

North of the Bridge

North of the bridge, the contact between Cambrian sandstone and Precambrian quartzite can be seen. The unconformity between the two is a nearly vertical buried cliff (sometimes termed a buttress unconformity). The sandstone immediately to the north is massive and lacks bedding, then grades into cross-bedded sandstone. The crossbeds are a meter and more in size. The massive sand is interpreted as wind-blown sand banked up against a cliff face, grading into cross-bedded dune sand further away. The photos above show massive sandstone in the lower right and cross-bedded sandstone to the left.

The very top of the cliff is covered by horizontally bedded sandstone interpreted as a marine transgression. This photo shows massive sandstone below grading into bedded sandstone at the top. The buttress unconformity is in the ravine just outside the picture to the right.

References

Dalziel, I. W. D., and Dott, R. H., 1970; Geology of the Baraboo District, Wisconsin, Wisconsin Geological and Natural History Survey Information Circular 14, 164 p., 7 plates.


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Created 19 May 1999, Last Update 24 June 1999

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