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24 April 2015
Friday fold: Bloomsburg Formation at lime kilns on C&O Canal
Milepost 127.4 (High, 2001) on the C&O Canal: …Cool if you’re into history. …Cool if you’re into economic geology. …Cool if you’re into Friday folds! Reader Eric Fulmer sent me this photo (along w/ two others you’ll see in weeks to come). Thanks, Eric!
8 December 2014
Fluvial geomorphology of Mather Gorge
What hath the Potomac River wrought on the rocks of Mather Gorge? Some interesting shapes to the land surface reveal a fascinating history.
10 November 2014
Root wedging: a recent example
What is Kenny pointing at here? Why, it’s a boulder. Where did it come from? Look uphill: This is as perfect an example of root wedging as I’ve seen! Spotted it last Friday along the C&O Canal towpath.
12 September 2014
Friday fold: Quartz vein in Catoctin Formation, Point of Rocks, Maryland
I took this image in 2005, when I was working up a geologic history of the C&O Canal National Historical Park. It’s a vein of quartz, gracefully folded within the Catoctin Formation. The exposure is along the railroad tracks at Point of Rocks, Maryland, easternmost extent of the Blue Ridge province on the north shore of the Potomac River. The Culpeper Basin begins about 100 meters to the east of …
25 April 2014
Friday fold: Paw Paw Tunnel anticline
As with last week, the Friday fold comes from my Field Studies in Geology class to Sideling Hill and the Paw Paw Tunnel. This is a view of the downstream (north) entrance to the Tunnel, with students highlighting the trace of bedding in the turbidites of the Brallier Formation: Can’t make it all out? Let’s try zooming in, and laying on some annotation: Happy Friday!
18 April 2014
Friday fold: Sideling Hill
Here’s what the Sideling Hill road cut looked like last month: It’s a terrific example of a syncline. Usually I show folds in profile view, but here, the view is essentially perpendicular (not parallel) to the axis of the fold: Sideling Hill’s rocks are early Mississippian in age, made of debris shed off the late Devonian Acadian Orogeny, and they were folded during Alleghanian deformation in the Pennsylvanian-Permian.
11 April 2014
Friday fold: kink from the Billy Goat Trail
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.
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 …
24 March 2014
Monday migmatite
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 …
4 December 2013
The Smithsonian Castle and the Seneca Quarry, by Garrett Peck
The Smithsonian Castle is one of the most striking buildings on the National Mall in Washington, DC. One reason for this is its distinctive architecture, but a second reason is its color: a bright, deep red. This color comes from the rock from which the Castle constructed: the Triassic-aged “Seneca Sandstone,” a part of the formation technically known as the Manassas Sandstone. It is overlain by the Balls Bluff Siltstone, …
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.