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You are browsing the archive for 2013 April.

30 April 2013

Strained metaconglomerate in Klingle Valley, DC

Following on yesterday’s post about the kink bands within the strained metagraywacke of the Laurel Formation in DC, let’s take the opportunity today to go to Klingle Valley, site of a different facies within the Laurel Formation: a strained metaconglomerate. Though the exposure isn’t as great as the Purgatory Conglomerate, I think you’ll find plenty to hold your attention in these rocks. Close looks will reveal sericite-after-staurolite pseudomorphs (evidence of …

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29 April 2013

Kink bands in highly strained Laurel Formation, Rock Creek Shear Zone, DC

Last week before GSW, I spent several pollen-choked hours in Rock Creek Park, GigaPanning some of the rocks of the Rock Creek Shear Zone. Here are some exposures in the bed of Broad Branch that show lovely kink banding. In at least one spot, you can see a conjugate pair, so these rocks were (1) sheared out in a ductile shear zone, producing the foliation, and then (2) compressed under …

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26 April 2013

Friday folds: Simple shear and the unfolding of folds

The Friday fold combines a moment of insight on a field trip in the Archean Superior Province and a new paper published this month in the journal GEOLOGY.

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25 April 2013

Deformation in the Lake Vermillion Formation

Today, let’s go back to the Pike Dam, where we spent some lovely moments last week, agog at the lovely graded beds and flame structures visible there. In contrast, today we want to examine the deformational structures seen elsewhere at this same outcrop. There are folds and faults and joints and more exotic fare: tension gashes and Riedel shears. The deformation here is the youngest to affect the Vermillion District …

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24 April 2013

Native copper from the Catoctin Formation

Another new insight from last week’s visit to the Outdoor Lab was that they have several fine examples of native copper found in float of the Catoctin Formation on their property. Here are a few examples: Classic examples – a bit of malachite in there too, it looks like. I wasn’t totally shocked when I saw these items, since just a month or two ago, I was shown a similar …

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23 April 2013

Return to the Outdoor Lab

Two years ago, I took a trip to the Phoebe Hall Knipling Outdoor Lab, which is Arlington, Virginia’s outdoor education facility in the Pond Mountains (southern continuation of the Bull Run Mountains), on the eastern edge of the Blue Ridge geologic province. I was invited back last week to look at some new exposures. I brought the GigaPan along. There had been additional erosion on the saprolitic exposure of Harpers …

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22 April 2013

Monday macrobug: Carpenter bee

These guys are the bane of my existence lately. Now that the ladybugs are gone, we’ve got a dozen carpenter bees orbiting the house, seeking an opportunity to drill holes in it. Here’s one after an encounter with a tennis racket I keep on the porch expressly for the purpose of controlling their population: Look at those amazing eyes! Ventral view, rotated to make it look like he’s flying above …

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21 April 2013

Sleeping Inn with the Martinsburg Formation

Three new GigaPans I shot last Friday east of Staunton, Virginia, at a semi-legendary exposure behind the Sleep Inn at the 250 / 81 intersection. link link link Students: Which way is up? Which criteria did you use to make that determination?

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19 April 2013

Friday fold: Another from the depths of Mosaic Canyon

More folds from the charismatic Noonday Dolostone in Mosaic Canyon, Death Valley National Park, California:   Annotated: Happy Friday!

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17 April 2013

Four new GigaPans from an intriguing contact

Callan and his colleague Jay Kaufman (University of Maryland) go to extraordinary lengths to document an intriguing block of rock in northern Virginia’s Blue Ridge province. Great images and a lot of fun result – but what do these rocks tell us?

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