During the Proterozoic, Wisconsin was on the edge! A tectonic plate collision known as the Mazatzal Orogeny deformed the rocks in what are now south-central Wisconsin. The best exposures of these rocks in the region are found in and around Baraboo, Wisconsin. Here, sedimentary rocks were metamorphosed and deformed during the collision. They are now preserved as lovely pink quartzite rocks and interbedded phyllites that have been folded. The fold is a large (on the order of several kilometers) syncline and what we can see at the individual outcrops are tilted layers and associated cleavage (planar rock features that formed during deformation).
I have studied the rocks around Baraboo with colleagues and students in order to learn about 1) the kinematics of folding, 2) the small scale changes in rock cleavage that occur due to mineralogical variations, and 3) the rheological information contained within natural structures like boudin shape and cleavage refraction.
Hop on the bus to any of the stops below:
Deformed Rocks at Baraboo
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- North Limb of the Baraboo Syncline. The bedding here is tilted nearly to vertical. If you look closely, you can see thin vertical layers of phyllite. The cleavage orientation changes dramatically between the nearly massive quartzite and the finely cleaved phyllite.
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- This is the most famous rock in the Baraboo region- known as Van Hise Rock (named after the famous geologist C. R. Van Hise). It is located on the north limb of the Baraboo Syncline. In this photo, you can see an excited structural geologist marking the vertical (upturned) bedding with her torso, and marking cleavage orientations in the phyllite and quartzite layers with her arms.
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- South limb of the Baraboo Syncline. This picture looks much like the first, except here the bedding is shallowly dipping (dipping at an angle around 25 degrees). Again, you can see thin layers of phyllite. The cleavage orientation changes dramatically between the nearly massive quartzite and the finely cleaved phyllite.
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- Outcrop located near the NW end of Devil’s Lake. These are boudins- formed as the rock was extended.
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- Outcrop along Highway 12, south limb of the Baraboo Syncline. Craziness! Many interesting things are happening in this photo. Cleavage is kinked. Some boudins get so stretched out that they become unattached from the rest of the layers. When this happens, they start rotating freely during deformation (like a Pooh Stick in a stream- see A. A. Milne book for description of Pooh Sticks). Freely rotating boudins cause all sorts of interesting structures, including folded cleavage.
Baraboo scenery- this will make you want to visit Wisconsin!
I can’t imagine that you would get tired of looking ar rocks in Baraboo, but here’s what else you can see. Photos taken by me (Dyanna) or Carol Ormand.
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- Devil’s Lake – view from the bluffs in Devil’s Lake State Park. We are standing on folded quartzite!
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- Glacial Geology. Check out this terminal moraine that blocks up Devil’s Lake (it is the big mound with trees on it across the lake).
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- Glen and Glenda. Little trout swimming around in Parfrey’s Glen on a sunny October day.