This is the coolest thing I’ve seen this week: a graded bed metamorphosed via Acadian mountain building: The graded bed starts at the Swiss army knife at left, where you see an abrupt transition between coarse grained metamorphic porphyroblasts (“pseudo-andalusites”) and finer grained quartzite. This was once a mud to sand transition when these were … Read more
Here’s a sample from my 2004 geology M.S. thesis work in the Sierra Crest Shear Zone of eastern California. The rock is a sheared ignimbrite (ashflow tuff) tuff bearing a porphyritic texture and a nicely-developed “S-C” fabric. With annotations, showing the S- and C-surfaces, and my kinematic interpretation: S-C fabrics develop in transpressional shear zones: … Read more
In the 1987 comedy Planes, Trains, and Automobiles, John Candy and Steve Martin have a funny experience. It involves a cozy hotel room (one bed only) and the two travelers are huddled up for warmth. As he wakes up, John Candy thinks he is warming his hand “between two pillows.” At hearing this, Steve Martin’s … Read more
The first time I went to the Billy Goat Trail (Potomac, Maryland) with geology as the goal (as opposed to mere recreation), it was 2002. The trip was led by a professor at the University of Maryland. I was a graduate T.A. then, and didn’t know anything about the local geology. I remember at the … Read more
Searching through my photo archives this morning for something suitably “Eastery”… something in pastel colors, perhaps? … a petrified lagomorph? … how about an egg, or something egg-shaped? This is as close as I got: This is in the Owens Valley of eastern California, showing a boulder of the Mesozoic Sierra Nevada Batholith bearing a … Read more
A quick post to share a few images of an outcrop I visited last September out in California’s Owens Valley. This is a spot where alluvial fans coming off the eastern Sierra Nevada were overrun by a basaltic lava flow (Jeff, Kim, Fred, and Kurt for scale): The unofficial term for these conglomerates deposited by … Read more
Sunday morning, NOVA adjunct geology instructor Chris Khourey and I went out to Sugarloaf Mountain, near Comus, Maryland, to poke around and assess the geology. Sugarloaf is so named because it’s “held up” by erosion-resistant quartzite. It’s often dubbed “the only mountain in the Piedmont,” which refers to the Piedmont physiographic province. Here’s a map, … Read more
Courtesy of my scanner, and my generous student Saadet M., whose family runs a building stone business here in northern Virginia… behold a lovely metaconglomerate: This is a really cool rock. I would rather have this than diabase for my countertop. It doesn’t show up well in this scan, but the rock has a moderately … Read more
Thursday is ‘fold day’ here at Mountain Beltway. Let’s take a look at some folds I saw last weekend in New York City. We’ll start with a bunch seen in the Manhattan Schist in Central Park. Here’s an example of the foliation in the schist. It’s got finer-grained regions and coarser, schistier regions with big … Read more
New York City has some cool geology: Paleozoic metamorphics scraped by Pleistocene glaciers.
S.E. Central Park:
The Manhattan Schist with its well-developed alignment of muscovite mica:
One of the cool things about being the local geoblogger is that people get in touch with you about local geology. Sometimes this even leads to meeting up for field trips. Here’s two quick photos from a recent (January 2010) field trip to a creek near Springfield, Virginia. My host was Barbara X, a local … Read more
A few photos from last May’s spring field trip with the Geological Society of Washington… Here’s the group at Chain Bridge Flats (far westernmost-Washington, D.C.), looked at the metamorphic rocks there — a metagraywacke melange known as the Sykesville Formation. Another group shot, with field trip leaders Tony (khaki shirt) and Gary (red jacket) Fleming … Read more
Check out these cool structures in one of the amphibolite bodies exposed along the Billy Goat Trail (C&O Canal NHP, near Potomac, Maryland): Those are shear bands — basically small shear zones that are discretely localized within a larger body of less-deformed rock. Note the grain-size reduction visible in the shear bands, their dextral sense … Read more