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31 October 2014
Here are four folds from the Potomac Terrane (or maybe an “exotic unit” that looks like the Potomac Terrane, butted up against the Potomac Terrane) that I saw on the 2014 Virginia Geological Field Conference back in October. It was an overcast, rainy day, so I apologize for the relatively low quality of these images. Happy Friday!
28 May 2014
Last week, the “Border to Beltway” field exchange team went to Dora Kelly Park in Alexandria, Virginia. There, a ravine reveals the boundary between the Coastal Plain and the underlying metamorphic rocks of the Piedmont… My NOVA colleague Ken Rasmussen joined us for the day, leading us deep below the soil profile to examine the basal nonconformity beneath our youngest geologic province: The ravine was dark, so the next few …
11 April 2014
My student James O’Brien took this image of a kink band along the Billy Goat Trail, downstream of Great Falls in Maryland’s metamorphic Piedmont province. A lovely little structure, don’t you think? Thanks, James! Happy Friday, all.
24 March 2014
Here’s a sweet little sample of migmatite (~470 Ma late Ordovician Taconian Orogeny, U/Pb date from zircon), that my students and I spotted last week on the Billy Goat Trail, downstream of Great Falls in Maryland’s metamorphic Piedmont province: Note the white translucent quartz, the orangey (partially kaolinitized and rusty stained) opaque potassium feldspar, and the shreds of biotite torn and tangled between them: I love the fact that I …
22 November 2013
Here’s a sample that my Physical Geology students see on their field trip to the Billy Goat Trail:
30 April 2013
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 …
1 April 2013
The Friday before last, I was in DC for a fun geology/botany field trip, and I got the opportunity to stroll around the barn at historic Peirce Mill, a historical grain mill along Rock Creek in Rock Creek Park, Washington, DC. The barn is immediately south of Tilden Street NW. It appears to have been constructed from local stone: metamorphic rocks of the Rock Creek Shear Zone, a ductile fault …
26 March 2013
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 …
14 December 2012
On Duck Creek (Ellendale, NC quadrangle), you can see folded xenoliths within the Toluca Granite (383 Ma, 378 Ma, or 368 Ma, depending on which mineral you ask). The granite there contains xenoliths that contain pre-exisiting fabrics and structures, and we stopped at Duck Creek on the pre-GSA-Charlotte field trip I took to the Neoacadian Inner Piedmont to check out these xenoliths. Here are some less impressive xenoliths, strung out …
16 November 2012
The Friday fold comes from a stop on the “Neoacadian Inner Piedmont” field trip that Callan attended prior to the GSA meeting in Charlotte, North Carolina, last week.