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Flint Ridge, Ohio

Being in the middle of Ohio as a petrologist you find out that there aren’t a lot of crystalline rocks to gaze upon – at least without drilling a 5-km hole directly below one’s feet. However, there are some cool localities that exhibit mineralization that might not be igneous or metamorphic basement, but at least have some great examples of minerals for an introductory rocks and minerals course.

One of these locales near Denison is Flint Ridge, on the east side of Newark, Ohio. Flint Ridge is the name for a series of hills that are cored with chert that occurs within the Pennsylvanian-age (299-318 Ma) Vanport Limestone. There is some controversy about the origin of these chert nodules within the limestone and sandstone beds more typical for the area – did the chert form chemically from silica-rich fluids passing through the rocks after the sediment was deposited or were the beds formed from primary deposition of silica-rich skeletons, such as sponge spicules. That is one of the questions that my GEOS 211 students ponder when they get out to Flint Ridge to examine the rocks and see what they can find.

Check it out (as always, you can click on each image to see a larger version).

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Students from GEOS 211 examining the chert at Flint Ridge.

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The chert from Flint Ridge was used frequently by Native Americans to make stone tools and traded widely across the continent.

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The chert in outcrop, typically in pits dug by prospectors looking for good chert for knapping purposes.

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A piece of chert found by one of the students that shows some of the strange brecciated features of the chert.

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I found this chuck where the lower part of the rock is the chert while the top part is a sandstone. The difference in rock texture and grainsize is obvious under a handlens and merely by running your fingers across the surface

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There is also abundant quartz mineralization in the form of veins and vugs within the chert.

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More quartz mineralization

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You can see macroscopic growth banding on some of the prismatic quartz crystals as well!

Erik Klemetti

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