7 September 2011
Graded bed from the Billy Goat Trail
Posted by Callan Bentley
Spotted this one Monday on the newly-rerouted section of the Billy Goat Trail’s Loop A, in Chesapeake and Ohio Canal National Historical Park. This graded bed was deposited as a turbidite in the Iapetus Ocean, sometime in the time-frame of 700 to 460 million years ago. It was metamorphosed 460 Ma during the late Ordovician Taconian Orogeny, and that’s probably when the little quartz veins formed, too. Click on it to see the whole sample at much higher resolution. I’m looking forward to taking my Physical Geology students out there in a couple of months. These are students who started class with me two weeks ago, and probably would read this post now with bewilderment as to what’s being discussed. In two short months, though, they will be able to traverse this beautiful landscape and make sense of the geological evidence it presents. That’s so cool: I love seeing people learn.
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.
mmm … graded bedding
Hey Callan,
Where has the trail been re-routed? I’m taking classes there in November.
Just the upstream-most 250 feet or so – it’s an improvement! Better views out over the Rocky Islands…
What’s the cause of the veinlets? The bedding does not look skewed, so I assume there was preferential weathering and gaps filled in by quartz-rich fluids, but why there? I would expect any weakness to be more aligned with the bedding.
I see little veins of quartz like this all the time but I’ve forgotten why.
Top to the right shearing, I reckon, more or less bedding parallel. They are standard little quartz veins – ripping of the host rock followed by sealing shut of that tear with hydrothermally-precipitated quartz.