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26 October 2014
Bedding / cleavage relations in the Stephen Formation, Yoho NP
Good afternoon! Here are a few photos, both plain and annotated, showing the relationship between primary sedimentary bedding and tectonic cleavage in the “tectonised Stephen” Formation atop the Cathedral Escarpment (in Yoho National Park), just northeast of the Walcott Quarry where the (thicker, basinward) Stephen Formation hosts the Burgess Shale. Weathering exploits both these planes of weakness… Here, the cleavage is more planar at the bottom of the sample, and …
8 August 2014
Friday fold: Buckle vs. passive folding in the Chancellor Group slates
The Friday fold is an outcrop in Yoho National Park that showcases differences between buckle folding and passive folding.
17 June 2014
Weathering on Old Rag Mountain 2: Opferkessel
Yesterday, I pointed out an example of differential weathering on Old Rag Mountain, in Shenandoah National Park, in Virginia. Today, I’d like to shine the spotlight on another example of weathering to be seen along the trail there: little weathering pits that occur on the top of the granite outcrops. These are opferkessel. Some people call these “potholes,” a term I do not approve of in this context. To me, …
16 June 2014
Weathering on Old Rag Mountain 1: feeder dikes
Old Rag Mountain is a distinctive mountain in the eastern Blue Ridge of Virginia, contained in a little lobe of Shenandoah National Park. It’s a great hike on several levels: (1) it’s got no trees on the summit, so you can actually get a decent view from on top, (2) it’s got a great section of full-body rock scrambling on the Ridge Trail, and (3) it’s long (9.2 miles round …
10 June 2014
Diabase intrusion at the Vulcan Manassas quarry
When I was flying back from Phase I of “Border to Beltway” in Texas this past March, I was delighted to photograph a bunch of local geology from the air, including this prominent diabase quarry in Manassas: I had never been to this particular quarry before, but as it turned out, it was one of our final destinations on Border to Beltway’s second phase, last month. Here’s a closer view …
6 June 2014
Friday fold: differential weathering of carbonate intraclasts in mudstone
Howard Allen is the documentarian of this week’s fold: Howard writes that this is: Middle Cambrian Chancellor Formation rock with recessive weathering intraclasts(?). Hamilton Lake trail, Yoho National Park, British Columbia. My interpretation of this one is a little shaky–it was raining when I took the photo (in 1982) and I was hiking with a non-geologist friend, so I didn’t linger at the outcrop or record the precise location. I …
15 April 2014
Bloomsburg Formation exposed near Elizabeth Furnace
As noted previously, I live in a regional scale fold: the differential erosion of the Massanutten Synclinorium has produced the ridge of Massanutten Mountain, which separates the Fort Valley from the Shenandoah and Page valleys on either side. The Fort is “fort” like because the strata which underlie it are relatively friable, soluble, or otherwise erode-able. The ridge-forming layer is the Massanutten Sandstone, a Silurian-aged quartz arenite. Here’s a boulder …
26 March 2014
The Great Unconformity in the Franklin Mountains
Good morning! Let’s take a walk up the east side of the Franklin Mountains, north of El Paso, Texas, to walk across the Great Unconformity. The basement rock exposed here is the Red Bluff Granite, a 1.1 Ga felsic magma that intruded the columnar basalts of the Mundy “Breccia” and the Castner Marble. (It is unknown what substrate the Castner Marble was deposited upon.) This is what the Red Bluff …
25 March 2014
Root wedging: examples from Maryland and Texas
While I’m showing photos from last week’s Billy Goat Trail field trips (3 in total), let me share a striking example of root wedging from Olmstead Island, on the walkway out to see Great Falls: And, since I meant to get back to blogging about west Texas this week, here’s another example of the same process, seen with a different kind of tree in different sorts of rocks, in the …
17 February 2014
A marine incursion in the Hampshire Formation?
I went out last Tuesday to Corridor H, the exemplary new highway cutting through the Valley and Ridge province of eastern West Virginia. Joining me was former student Alan Pitts, a devotee of Corridor H from way back in the early days when we just called it “New Route 55.” The boondoggle highway is now open all the way west to the Allegheny Front, practically into the Canaan Valley. On Tuesday, …
Callan Bentley is Associate Professor of Geology at Piedmont Virginia Community College in Charlottesville, Virginia. He is a Fellow of the Geological Society of America. For his work on this blog, the National Association of Geoscience Teachers recognized him with the James Shea Award. 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. In previous years, Callan served as a contributing editor at EARTH magazine, President of the Geological Society of Washington and President the Geo2YC division of NAGT.