19 October 2013
Halite casts from Tonoloway Formation under the GIGAmacro lens
Posted by Callan Bentley
The work of team M.A.G.I.C. continues. This is a lovely sample quartet of salt cast samples from Silurian-aged Tonoloway Formation limestone. I collected these samples on Corridor H’s newly-opened section west of Moorefield, West Virginia, last spring. The big one at the bottom was collected by my friend Leigh Henry, who graciously loaned it to M.A.G.I.C. for imaging purposes.
This GigaPan turned out well in some regards, and poorly in others. The lighting, I think, is just about perfect. The size and resolution are awesome (almost 4 billion pixels). However, the most obvious flaw is that the samples are not imaged in totality – the top and left sides of the GigaPan got clipped off. We don’t understand why, and it frustrates us. Making an image like this one is pretty much a four-day-long affair. When something as basic as the edges of the imaged area ends up askew, it’s maddening. Furthermore, the scale bar that we always put in these images somehow didn’t get photographed… I can compensate for that by typing “Width of field of view is 27 centimeters,” but the text isn’t as intuitive as simply having a scale bar there. Argh. Furthermore, what’s not apparent as we set up the shot becomes apparent when you explore the GigaPan image: little chunks of foreign sediment or hairs, dust particles, etc. on the surface of the specimens. It seems that no matter how much attention we pay to preparing and cleaning the specimens (we blast them with compressed air before a shoot), we always miss some of this offending “litter”.
The samples were collected at this roadcut:
link
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.
The Tonoloway is Silurian. This material looks a lot like something from the underlying Wills Creek Formation which is notorious for its seriously mixed lithology including evaporites, small stromatolite mounds, shales, brachiopod shoals, dolostone, and calcite-cemented quartz sandstones.
Oh, dear. You’re absolutely right. Upper Silurian, and I mistook it for Devonian. I’ll fix the post. Ronald, I’d love to visit some of the outcrops on Corridor H with you and have you orient me to the stratigraphy in person – I feel like there’s a disconnect between the broad scale geologic map of the state and these gorgeous new exposures.
Also, while I was editing the post, I inserted a second GigaPan, of the site where these samples originated. This may aid you in identifying the unit more precisely. Thanks for correcting me!
Can you tell if these samples are limestone or dolomite? According to stratigraphy from I-68 in Maryland by Southworth et al [1], both Wills Creek and Tonoloway contain evaporites. But Wills Creek’s portions with evaporites are all dolomite, while Tonoloway has limestone with evaporites. So if these samples are limestone (which look to me they are), they probably are part of Tonoloway.
[1] Southworth et al, Geology of National Capital Region: Field Trip Guidebook, p 112