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 … Read more
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.
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 … Read more
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, … Read more
Concentric ribs with hackles on a joint face, quartzite (metamorphosed fine-grained quartz sandstone stained with hematite) from Waterton Lakes National Park, southern Alberta.
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.
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 … Read more
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. [gigapan id=”103122″] link It definitely looks best in full screen mode, so … Read more
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…
In December, Callan found an outcrop of Neoproterozoic-aged turbidites in South Africa, on the eastern shore of False Bay.