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15 November 2011

Skolithos in the sun

Today, I share with you eight images that I took yesterday on Skyline Drive in Shenandoah National Park: Blocks of Antietam Formation quartzite (meta-quartz sandstone) of Cambrian age, used in the low rock walls of an overlook parking pull-out. They all bear lovely Skolithos trace fossils (seen end-on, and in cross-section):


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14 November 2011

Compton Peak: superb columnar jointing

After my talk Wednesday night to the Potomac Appalachian Trail Club, I got an email from PATC member Tom Johnson, with an extraordinary photo attached. It shows an exceptional outcrop of the Neoproterozoic Catoctin Formation, exposed atop Compton Peak in northern Shenandoah National Park, Virginia. The outcrop features enormous, well-preserved cooling columns from these ancient meta-lava flows. Click on it for the full-size version (posted here with Tom’s permission). You …


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11 November 2011

Friday (center)fold

At last weekend’s gala banquet to celebrate the 50th anniversary of the William & Mary geology department, I was tickled to see that the centerpiece for each table was a rock of some sort. I spotted Gore Mountain Amphibolite, Baker Mountain Kyanite, brachiopod fossils in limestone, a fossil whale vertebra, a chunk of Aquia Creek sandstone, pegmatite, and peridotite-xenolith-bearing basalt. …And on one table, there was a fold: Here’s a …


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10 November 2011

Shear bands in the Grassy Portage Sill

Continuing now with the discussion of my pre-GSA meeting field trip to examine the structural geology of the Quetico-Wabigoon subprovince boundary within the Superior Craton of southern Ontario, Canada. Our penultimate stop on the second day of the trip was a roadcut exposing the gabbro of the Grassy Portage Sill. This is what it looks like: …Except where it doesn’t. In places, it looks like this, instead: Zooming in a …


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9 November 2011

William & Mary geology: 50th anniversary

The geology program at the College of William & Mary turned 50 years old this year, and last weekend they held a party to celebrate. Of the 800 or so geology majors the department has produced in 50 years, about 100 came to this event – that’s a pretty great ratio, I think: 1 out of every 8 alumni made the trek back to Williamsburg to pay tribute to this …


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8 November 2011

Mystery mineral molds

Here’s a piece of vein quartz with a series of interesting “cubist disc” shaped holes in it. I interpret these cavities as external molds of some mineral which has now been physically removed or chemically weathered away. So only its exterior shape remains, and I wonder if that’s enough to identify the mineral that used to be here. Both images below can be enlarged by clicking on them: And the …


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7 November 2011


You may recall that I kind of like boudinage. So it piqued my interest when our field trip leaders (on the pre-GSA Minneapolis trip to examine the structural geology of the sub-province boundaries in the Superior Craton) said our next stop was to visit “pseudoboudins,” segments of granitoid pegmatites that looked like boudins but probably represented a totally different process. Here’s the idea: With regular old boudinage, the solidification of …


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4 November 2011

Friday fold: Anticline / syncline pair

Today’s fold is an anticline and its neighboring syncline, both exposed along a newly-opened stretch of New Route 55, west of Moorefield, West Virginia. The new Route 55 is a classic porkbarrel boondoggle courtesy of the late Senator Robert Byrd, but doggone if it didn’t open up some lovely new roadcuts. Here’s a stitched image of the fold, with my student Jared at center right for scale: You can click …


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2 November 2011

The Ottertail Pluton

After the awesome outcrops and pavements of strained metaconglomerates from the Quetico / Wabigoon subprovince boundaries of the Superior Craton, my pre-GSA field trip visited the most charmingly-named magma chamber I’ve ever seen, the cuddly-sounding Ottertail Pluton. This is an Algoman-type pluton which is discordant to tonalite-composition gneisses in the area. As with the Giants Range Batholith that we saw near Virginia, Minnesota, the Ottertail Pluton shows lots of cool …


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1 November 2011

Geoblogs as a device for student engagement

Here’s the talk I gave at GSA last month: [youtube=””]   It’s presented here at a slower pace than the actual talk was, since I didn’t have to run and catch a plane 45 minutes after presenting it, but there are some PowerPoint bugs with some of the animations. Oh well – recording it and putting it online is more than 99.9% of GSA presenters ever do to share their …


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