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6 June 2013

Percussion marks on quartzite cobbles in DC

Hey, that quartzite boulder has measles! Or is it bubonic plague? Or maybe it’s just acne? Look closer and see if you can deduce what these things are: These conical fractures are percussion marks. They form when a cobble smacks into another cobble underwater, propelled forward by a powerful current. As the two rocks knock together, the strength of the collision can overwhelm the strength of the bonds holding the …


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5 June 2013

Stone tools of the Piney Branch quarry, DC

Archaeology meets geology in this visit to the Piney Branch valley of Rock Creek Park in Washington, D.C. Cretaceous deposits of cobbles of Cambrian quartzite were quarried by Native Americans and modified into tools thanks to the fact that they break with a conchoidal fracture.


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31 October 2012

A few folds from Betty’s Bay

Here are a few folds in the quartzites of the Cape Fold Belt, exposed on the mountainsides of the Harold Porter Botanical Gardens in Betty’s Bay, South Africa. Hillside #1: Zooming in closer: Annotated (bedding traced out): Hillside #2: Zooming in on summit region: Annotated (bedding traced out): Zooming in on the central portion of the hill: Annotated (bedding traced out): The Cape Fold Belt – there it is!


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14 September 2012

Friday fold: Siim Sepp’s SS

Geoblogger Siim Sepp contributed this lovely fold. He says: I saw a nice fold in Ireland. I like it because it looks like SS to me which are my initials. If you want, you can post it as your friday fold. I haven’t used it yet in my blog. This small outcrop is in Donegal, NW part of Ireland. I accidentally stumbled upon it. The fold formed most likely during …


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6 September 2012

Ribs & hackles

Concentric ribs with hackles on a joint face, quartzite (metamorphosed fine-grained quartz sandstone stained with hematite) from Waterton Lakes National Park, southern Alberta.


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5 September 2012

Structures in the quartzite along the coast of Hermanus

Grab your coffee and join Callan & Lily for a stroll along the quartzite cliffs of Hermanus, South Africa. Bedding, cross-bedding, joints, faults, veins, folded veins, and faulted veins all await you there in the morning light.


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17 May 2012

More percussion marks

Yesterday, I introduced percussion marks to this blog space. Here are some other shots of this distinctive “shatter” structure, but in a vein of hydrothermal (milky) quartz exposed on Bear Island, between the C&O Canal and the Potomac River (about right here, just west of the trail): Sometimes percussion marks are accompanied by radial fracture sets like this one… I interpret these radial fractures (sometimes with small cone-fractures at the …


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17 April 2012

Strained Antietam Formation sandstone

I collected this sample the weekend before last on the Blue Ridge Thrust Fault field trip led by Alan Pitts. It’s a chunk of the Antietam Formation quartz sandstone, a Cambrian beach deposit. The face we photographed measures 15 cm by 12 cm. link It definitely looks best in full screen mode, so please feel encouraged to click through and explore it a bit. You’ll notice some great subtleties once …


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11 April 2012

Blue Ridge Thrust Fault field trip

One of Callan’s former students leads a field trip to examine the western edge of the Blue Ridge geologic province, attempting to answer the question of whether the Blue Ridge / Valley & Ridge contact is indeed the trace of a thrust fault. Breccias and S-C fabrics tell part of the story…


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26 March 2012

Malmesbury Group sediments on the eastern side of False Bay

In December, Callan found an outcrop of Neoproterozoic-aged turbidites in South Africa, on the eastern shore of False Bay.


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