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You are browsing the archive for March 2013 - Mountain Beltway.

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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27 March 2013

Video book review: two new titles from Mountain Press

A new video book review! Under discussion today are two new books from Mountain Press: Geology Underfoot Along Colorado’s Front Range by Lon Abbott and Terri Cook, and Arizona Rocks! by T. Scott Bryan. Enjoy!

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26 March 2013

Spheroidal weathering in Laurel Formation

Another thing I saw last Friday on my DC field trip was this fine boulder exhibiting spheroidal weathering: Annotated: It’s a boulder of the Laurel Formation, a clast-poor metamorphic unit found east of the Rock Creek Shear Zone in DC. This is the first time I’ve seen it exhibit this sort of mechanical weathering. Compare it to this example from Montana or this example from the Causeway basalts of Northern …

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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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23 March 2013

Skunk Cabbage

On Friday, I took a field trip to DC with Geologic Map of the Washington West Quadrangle author Tony Fleming, City of Alexandria Natural Resource Specialist/Plant Ecologist Rod Simmons, and a host of interested folks from many different professions and localities. We were interested in looking at ecological relationships between rocks and plants, and had a pleasant afternoon hiking through Rock Creek Park. We also got in a little archaeology! …

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22 March 2013

Introducing SmartFigures

Previously, I’ve hinted that I was working on a top secret special project for Pearson Education. Now that the books have been published, I wanted to take a moment to discuss the details of the project. In the newest editions of both Earth: An introduction to Physical Geology (Tarbuck,Ā Lutgens, & Tasa, 2014) and Foundations of Earth Science (Lutgens, Tarbuck, & Tasa, 2014), you’ll find a series of videos that I’ve …

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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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