Steven Dutch, Natural and Applied Sciences, University
of Wisconsin - Green Bay
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NE 1/4 SW 1/4 Sec 7 T29N R10E A mylonite zone along the Eau Claire River, with spectacular examples of jointing, chemical weathering and fluvial erosion as well. |
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Left: Entrance to the county park. Below: fall foliage. |
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 | Below: the bridge across the river seen from the west. |
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 | Left and below: views of the rapids west of the bridge. |
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 | Left and below: this rocky knob exhibits marvelous parallelogram jointing in three directions, |
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 | Below: parallelogram jointing in three directions is characteristic of these rocks. |
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 | Left and below: fluvially polished outcrops with scalloped surfaces and small potholes. |
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 | Left and below: many of the potholes show chemical weathering at the water line in the form of etching and, in some cases, actual overhangs. |
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 | Left and below: the rocks west of the bridge are felsic mylonite, probably originally a rhyolite. |
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 | Left and below: macroscopic views of the mylonite, with foliation accented by weathering. |
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 | Left: foliated mylonite and potholes. |
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Left: outcrops south of the stream. Below: dark green veins cut the mylonite. Some of the veins parallel to foliation may be altered pseudotachylite, or frictional melt produced during fault slippage (some people regard them as fossil earthquakes). The cross-cutting veins are more problematic. |
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 | Left: view of the bridge from the east. |
 | Left and below: east of the bridge the rocks become coarser, less mylonitic, more mafic, and more variable. They may originally have been mafic volcanic rocks. |
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 | Below: an old dam crosses the stream a couple of hundred meters east of the bridge. |
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Left: lath-like crystals in the schist near the dam. Below: small garnets in the schist. |
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Created 19 March 2007, Last Update 18 April 2007
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