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13 November 2014
Gypsum casts? You be the judge — UPDATE: Syneresis cracks!
Silurian aged mud cracks feature small lensoidal features: are they casts of ancient gypsum crystals?
9 November 2014
Silurian mudcracks in cross-section
Spotted these cross-sectioned mudcracks yesterday on Corridor H, on the GSW fall field trip: They are in the Tonoloway Formation, a batch of tidal flat carbonates with lots of evidence of shallow arid conditions.
10 October 2014
Friday folds: ploudin trio from Corridor H
Three folded sandstone slab-blobs will serve as today’s Friday folds. Meet the ploudins!
6 October 2014
Non-bedding-parallel stylolites in Tonoloway limestones, Corridor H
What can a little squiggle in a rock tell us? Stylolites in West Virginian limestone cut across the bedding plane, implying Alleghanian tectonic stresses.
3 September 2014
Rugose corals in the Clearville member of the Mahantango Formation
Here are some rugose coral fossils (along with some cross-sectioned articulate brachiopod shells) to be seen in the Clearville member (~80 feet thick) of the Mahantango Formation, exposed on the north side of route 55, just west of the West Virginia / Virginia border. These fossils are cool in their own right (what fossils aren’t?) but here they’re serving another purpose – they’re letting us know where we are in …
2 July 2014
Fault breccia in the Helderberg Group, Corridor H
Here’s a breccia that Dan Doctor and I found in a tabular zone within the Helderberg Group (Devonian limestones) in one of the massive new roadcuts along Corridor H. link Is it a fault breccia or a sedimentary breccia? The breccia was bedding parallel, which suggests it could be just another bed, but it’s so darn coarse and angular (unlike the rest of the Helderberg) that we were skeptical. Indeed, …
5 June 2014
Palimpsest tales from a Silurian limestone
My favorite rocks are those that tell multiple stories – rocks that are “palimpsest” with subsequent “chapters” of their biography capable of being teased out, based on different features to be observed in the rock. Click to enlarge What can we see in this small sample of the Silurian-aged Tonoloway limestone, from Corridor H, West Virginia? To start with, it’s sedimentary, and stratified. There are multiple layers of fine-grained gray …
4 June 2014
Faults in the Tonoloway Formation, Corridor H
Here’s something fun: Click to enlarge Those strata are Silurian-aged Tonoloway Formation carbonates. There are plenty of dessication cracks to be seen, as well as salt casts, among the layers exposed. But more eye-catching at this distance is the faulting that disrupts the high-contrast layers… Both (apparent) normal and reverse faults can be seen in this road cut. Exciting stuff! We visited it two weeks ago on “Border to Beltway’s” …
30 May 2014
Friday fold: subthrust syncline in lower Helderberg limestones, Corridor H
Click to enlarge You owe it to yourself to click through and make this bigger. Check out the prominent lower left to upper right thrust fault, and the shattering in the shorter limb of the underlying syncline. Is that another one to the right? Happy exploring, and happy Friday!
29 May 2014
A new diamictite exposure (Devonian?) along Corridor H
While on Corridor H last week with Team “Border to Beltway” (and USGS research geologist Dan Doctor), we stopped at the putative mass transport deposit. We still haven’t figured out which unit this is (It’s not the Foreknobs), but as we approached it, Dan wondered aloud, “I wonder where the top of the Devonian is. Maybe we could find some of Dave Brezinski’s glacial deposits.” If you’re not aware of …
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.