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14 March 2014
Friday fold: rolled boudin
Howard Allen returns with a Friday fold contribution for this week. He says: Not exactly a fold, but I thought you’d like this rolled boudin(?) (quartz) in a muscovite garnet schist (note garnets above the lens cap). This specimen is also from the Shuswap Metamorphic Complex, north of Sicamous, British Columbia. Lovely. Thanks for sharing, Howard!
2 March 2014
A chip off the ol’ charnockite
We visited the Philip Carter Winery this weekend with family. Baxter and I were pleased to see outcrops of charnockite scattered over the property (located in the middle of the Blue Ridge geologic province). As any 18-month-old will tell you, charnockite is a pyroxene-bearing granitoid. It’s a distinctive and common rock type in Virginia’s Proterozoic basement complex. Here’s a close-up: The dark green is pyroxene. The white is plagioclase feldspar. …
24 February 2014
Halite casts (hoppers) from the Wills Creek / Tonoloway Formations in West Virginia
While we’re out on Corridor H, let me show you some new halite casts I found (either the Wills Creek Formation or the Tonoloway Formation): Salt casts are among my favorite primary structures. They speak very specifically to hypersaline depositional conditions.
16 January 2014
Samples from Austin: another stretched-pebble conglomerate
While at the University of Texas at Austin, where the Jackson School of Geosciences was hosting the Summit on the Future of Geoscience Education this past weekend, I was impressed to see a well-developed rock garden outside the student center. Here’s an example of a stretched-pebble conglomerate from that garden: Note the nice epidote boudins running down the middle. The way the foliation “flares” at the bottom suggests another boudin …
7 January 2014
Ice in the ‘hood
Some ice seen this morning, the coldest we’ve yet experienced at our home in the Fort Valley… 4° F when we got up this morning, with windchill around -15° F. Frost nucleated on a “petal” from a tulip tree (Liriodendron tulipifera): Frost on grass (note the different habit of the ice crystals here): Frost on some netting: Frost on some plant stems near a little creek on our land: Ice …
7 May 2013
Brallier Formation 2: tectonic structures
Yesterday we examined primary sedimentary structures (including trace fossils) at an outcrop of Devonian-aged Brallier Formation turbidites between Deerfield and West Augusta, Virginia. Today, we’ll zoom in on the tectonic structures at the site: folds, faults, and joints. Remember, you don’t have to take my word for it. You can explore it for yourself in this M.A.G.I.C. GigaPan: link One thing that’s kind of cool about that GigaPan is the …
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 …
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 …
22 February 2013
Friday fold: Martin’s metamorphics
A guest “Friday fold” from South Africa: folded gneisses, flavored with other treats of a geological nature.
28 January 2013
Asbestiform ice
I’ve mentioned this phenomenon before, but I thought this was a particularly impressive example, found in my back yard over the weekend. Pipkrakes (ice “fibers”) with iron oxides and oak leaves, Fort Valley, Virginia, January 2013. Dana saw some in Seattle, too.
Callan Bentley is an assistant professor of geology at Northern Virginia Community College. He is a fellow of the Geological Society of America and the 2018 National Association of Geoscience Teachers' James Shea Awardee. He has also won the Outstanding Faculty Award from the State Council on Higher Education in Virginia, and the Biggs Award for Excellence in Geoscience Teaching from the Geoscience Education Division of the Geological Society of America. Callan is a contributing editor at EARTH magazine and Past President of the Geological Society of Washington and the Geo2YC division of NAGT. He lives in the Fort Valley of Virginia.