Posted on June 15, 2010 by Callan Bentley
The Lilster & I drove out to Harpers Ferry, West Virginia, today, and crossed the Potomac River to hike up to the overlook at “Maryland Heights,” which is what they call the Blue Ridge north of the river. On the way uphill, I noticed this nice example of Harpers Formation bedding and cleavage dipping in the same direction (~east):
Note that the cleavage is dipping more gently than the bedding: this suggests that the bedding is overturned. No big shocker here: that’s the standard interpretation for the western edge of the Blue Ridge province; but it’s nice to see some meso-scale evidence of the regional structure.
Filed under: blue ridge, cleavage, geology, maryland, structure, west virginia | Comments Off
Posted on June 6, 2010 by Callan Bentley
Heat-stressed map of the Chesapeake Bay / Washington, DC region, as seen at Kenilworth Aquatic Gardens. Looks like mudcracks, eh?
Similar stresses; similar strains.
Filed under: analogies, dc, maps, maryland, primary structures, structure, virginia | Comments Off
Posted on April 7, 2010 by Callan Bentley
It’s a rough life, working in the places I have to work… here are a few photos from yesterday’s field trip on the Billy Goat Trail with my NOVA Physical Geology students. Photos are courtesy Dr. Meg Coleman, who joined us for the hike.
A post-lunch lecture on river incision (note the two prominent bedrock terraces, a.k.a. “straths” in the background):
The crew climbing the dreaded “Traverse” section of the trail:
We had a nice hot day yesterday: almost 90° F! Tragically, the snack bar was closed when we got back to the visitor’s center, so we were denied our salutatory Italian ices. Back to the trail tomorrow, for the 4th of 5 trips this week…
Filed under: geology, maryland, nova, piedmont, teaching | 7 Comments »
Posted on April 3, 2010 by Callan Bentley
Yesterday, I took my GMU structural geology class to the Billy Goat Trail, my favorite local spot for intriguing geology. Unlike last year, we managed our time well enough that we got to clamber around on the rocks downstream of the amphibolite contact. Here’s Sarah, Lara, Kristen, and Alan, negotiating a steep section:
Justin, Joe, Nik, Aaron, Jeremy, and Danny find a chunky amphibolite boudin in metagraywacke. Notice how Jeremy is gesturing about the orientation of the metagraywacke foliation wrapping around the boudin.
The thing that we found that really made me happy were these ptygmatic folds. Most of my readers will doubtless already be familiar with ptygmatic folding, but in case you’re new to this, check out this photo (ballpoint pen for scale):
Ptygmatic folding is “intestine-like” in appearance. It results where there is a particularly high viscosity contrast (viscosity is resistance to flow) between the folded layer and the surrounding matrix. The higher viscosity material makes broad lobes, while the lower viscosity material may be found in the pointy cusps between those lobes. If ptygmatic folding is well developed, the limbs become parallel to one another (isoclinal), and the visual similarity to guts is disconcerting. Here’s a smaller version, a few feet away from the first one:
I’m headed back to the Billy Goat Trail today to discuss the trail’s geology with a crew from Sigma Xi‘s D.C. chapter. I wonder what we will discover today?
Filed under: boudinage, folds, maryland, north america, ordovician, piedmont, structure, teaching | 3 Comments »
Posted on March 16, 2010 by Callan Bentley
Just got back from three days of geology conferencing at the Northeastern & Southeastern Joint Section Meeting of the Geological Society of America in Baltimore. No time to blog whilst there, though I shot a dozen or so tweets up to my Twitter feed: small beer compared to a nice meaty blog post. Apologies if it was insufficient for your needs. Great to meet up with everyone there… I’m off tonight for a two-day field trip in the Appalachians, and I’ll get back to blogging this weekend when I return.
Filed under: blue ridge, conferences, gsa, maryland, valley and ridge | Comments Off