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29 March 2013

Friday fold: “V”

The Friday fold photo was taken this morning on a GigaPanning expedition, and shows a small syncline within turbidite strata of the Martinsburg Formation, Page Valley, Virginia.

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25 March 2013

Incipient boudinage in overturned Edinburg Formation

Boudinage is such a fun structure. Here’s an example from the roadcut adjacent to the quarry featured so heavily last week. The thick limestone stratum in the center of the photo has been stretched left-to-right. It exhibits pinch-and-swell structure, the first stage of boudinage. Small extensional fractures began to form in the boudin necks, accommodating the strain. Muddier strata above and below flowed into these boudin necks. This is in …

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22 March 2013

Friday fold: the case of the strangely stout stylolites

Today, we return to my field trip from last week, for a look at an odd outcrop of the Ordovician-aged Edinburg Formation: Note the car key with green lanyard, to provide a sense of scale. It’s folded, as the yellow bedding traces show in this annotated version: But what really caught my eye about this outcrop were the odd stylolites (pressure solution “seams” with a wiggly morphology and a concentration …

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21 March 2013

Slicks in Cub Sandstone

During Alleghanian deformation (late Paleozoic), the Cub Sandstone we looked at yesterday was tilted to near vertical at Catherine Furnace. The shale layers developed cleavage at this time, and there was evidently some flexural slip between sandstone layers, to judge from these fine slickensides: Students: Can you deduce the sense of motion from the orientation of the structures in this sample? (Hint: note the directions the “steps” face…)

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20 March 2013

Upper Martinsburg “Cub Sandstone” in GigaPan

Today, two GigaPans shot of the uppermost Martinsburg Formation, informally known as the “Cub Sandstone” since it crops out along Cub Run in the southern part of the Massanutten range. 10 or 15 meters upsection (west) of these two outcrops is the base of the Silurian-aged Massanutten Sandstone, the ridge-forming unit. Lower in the section: link Higher in the section: link If you explore these GigaPans, you’ll find a trend …

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19 March 2013

A closer look at the recumbent anticline from last Friday

Last week, the Friday fold was presented in GigaPan format only, which led to a concerned reader lamenting that he couldn’t see it on his mobile device. (GigaPans are Flash-based images; they don’t work on Apple devices in the standard GigaPan format, though there is a perfectly suitable workaround with two extra clicks.) So, for the sake of this poor reader, who would otherwise be denied a view of this …

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18 March 2013

Bouma ABE

The initials ABE might make you think of Lincoln, but that’s not what I’m talking about. Today, the letters mean something else: three subdivisions of the 5-part Bouma sequence of turbidites. I took these photos on a new (to me) outcrop of the Ordovician-aged Martinsburg Formation in Page County, Virginia. I visited the site for the first time last Thursday with GMU structural geologist John Singleton. We had been checking …

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15 March 2013

Friday fold: a recumbent anticline in an abandoned quarry

Yesterday, I spent a pleasant day in the field with John Singleton, the new structural geology professor at George Mason University. I was showing John a couple of sites I’ve used as field trip locations for the GMU structural geology class, and John was showing a couple of new sites to me – places he visited on last fall’s Virginia Geological Field Conference. I missed VGFC last fall, as I …

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2 January 2013

Upturned Paleozoic strata on Highway 16, Bighorns, Wyoming

Here’s a terrific outcrop to start off the new year at Mountain Beltway. We’re back in the Bighorns of Wyoming here, on highway 16, traversing the southern portion of the range en route from Buffalo to South Pass City. Click to enlarge Annotated, expanded, and Easter-egg-embedded: Click to enlarge From a different perspective (uphill a tad, looking north), consider this GigaPan I shot at the time: link The best part …

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21 November 2012

Rock Cycle I: Sedimentary → Metamorphic

An outcrop of the Ordovician-aged Martinsburg Formation is used to illustrate the development of slaty cleavage, and hence a major transition in the Rock Cycle.

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