File:Flickr2015-06-03jsjgeologyVanHiseRock5.jpg
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Summary[edit]
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English: Quartzite & slate in the Precambrian of Wisconsin, USA.
The Baraboo Ranges of southern Wisconsin are dominated by a hard, erosion-resistant Precambrian metamorphic unit called the Baraboo Quartzite. These rocks were originally marine sandstones and have been subjected to metamorphism and structural folding. Original sedimentary structures are preserved, such as cross-bedding and ripple marks. Baraboo Quartzites vary in color from pinkish to dark reddish to grayish. During metamorphism, quartz overgrowths formed over the original quartz sand grains. Long-term, modern weathering can result in original sand grains being released. The large rock shown above is Van Hise Rock, which detached from a natural cliff of Baraboo Quartzite outcrop behind the photographer (the cliff has since been modified by road construction). The pinkish areas at right are quartzite. The dark gray areas at left are slate (often referred to as "phyllite" in the literature). Slate/phyllite interbeds (= originally shales) are somewhat common in certain intervals of the Baraboo Quartzite. Van Hise Rock has fallen such that the original sedimentary bedding is now vertical. The slanted layering in the slate at left is metamorphic foliation. This unit has economic significance - it has been quarried historically and in modern times. The quartzite is broken down into gravel-sized pieces for use as railroad ballast and erosion-control rip-rap. Stratigraphy: Baraboo Quartzite, upper Paleoproterozoic, ~1.7 Ga Locality: Van Hise Rock - large, cliff-detached block on the eastern side of Rt. 136, just north of the town of Rock Springs, North Range of the Baraboo Ranges, north-central Sauk County, southern Wisconsin, USA (43° 29’ 20.70” North, 89° 54’ 56.57” West) |
| Date | |
| Source | https://www.flickr.com/photos/jsjgeology/18145090214/ |
| Author | James St. John |
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