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You are browsing the archive for May 2010 - Mountain Beltway.

28 May 2010

S-C fabric in meta-ignimbrite

Here’s a sample from my 2004 geology M.S. thesis work in the Sierra Crest Shear Zone of eastern California. The rock is a sheared ignimbrite (ashflow tuff) tuff bearing a porphyritic texture and a nicely-developed “S-C” fabric. With annotations, showing the S- and C-surfaces, and my kinematic interpretation: S-C fabrics develop in transpressional shear zones: ~tabular zones of rock that are subjected both to compression and lateral shear (“transform” motion). …

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26 May 2010

Strath vs. terrace graphic

There is an old Chinese aphorism that “the beginning of wisdom is to call things by their proper names.” One of the naming conventions that tends to trip up NOVA students who hike the Billy Goat Trail with me is the difference between a “terrace” and a “strath.” This morning, I created a graphic that illustrates the difference between these two landforms as I understand it: Both features are shown …

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25 May 2010

Vector maps by Eric Fischer

“The Geotaggers’ World Atlas #8: Washington, DC” by Eric Fischer If you like maps, you should go check out these images by photographer Eric Fischer. (via here) The different colors represent different modes of transportation: Black is walking (less than 7mph), red is bicycling or equivalent speed (less than 19mph), blue is motor vehicles on normal roads (less than 43mph); green is freeways or rapid transit.

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24 May 2010

The coming flood

In January, a large landslide occurred in the Hunza Valley of Pakistan’s Karakoram Range, near the village of Attabad. Like the Madison River landslide in Montana (1959), or the Gros Ventre landslide in Wyoming (1925), a river was dammed by the slide debris, and the impounded waters began to rise. At Gros Ventre, the landslide-dammed lake overtopped the debris and caused a catastrophic flood which killed 6 people in Kelly, …

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22 May 2010

New "secondary structures" display at NOVA

…. And on the other side, we have secondary (tectonic) structures, focused on folds and faults:

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New "primary structures" display at NOVA

One of the things I managed this week was to fill up a new display case in our Student Study area with a structural geology display. On one side is primary structures, both igneous and sedimentary….

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21 May 2010

Falls of the James III: river work

In today’s post, I’ll finish up with my geologic discussion of the falls of the James River in Richmond Virginia, south of Belle Isle. Previously, we’ve examined the bedrock at this location (the Petersburg Granite) and a series of fractures – some faults and some extensional joints – that deform that granite. Now we come to the final chapter in this story — the story of the river carving up …

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20 May 2010

Falls of the James II: fractures

In my previous post, I introduced you to the Petersburg Granite, as it is exposed south of Belle Isle, at the falls of the James River in Richmond, Virginia. I mentioned that it was fractured, and I’d like to take a closer look at those fractures today. The geologically-imparted fractures were exploited by human granite quarriers, and in some parts of the river bed, you can see the holes they …

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18 May 2010

Falls of the James I: pluton emplacement

Last Friday, NOVA colleague Victor Zabielski and I traveled down to Richmond, Virginia, to meet up with Chuck Bailey of the College of William & Mary, and do a little field work on the rocks exposed by the James River. Our destination was Belle Isle, a whaleback-shaped island where granite has been quarried for dimension stone for many years. The island has also served as a Confederate prison for captured …

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17 May 2010

"Geology of Skyline Drive" w/JMU

I mentioned going out in the field last Thursday with Liz Johnson‘s “Geology of Skyline Drive” lab course at James Madison University. We started the trip south of Elkton, Virginia, at an exposure where Liz had the students collect hand samples and sketch their key features. Here’s one that I picked up: Regular readers will recognize those little circular thingies as Skolithos trace fossils, which are soda-straw-like in the third …

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