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15 May 2012
Diabase dike in diabase
Seen in rip-rap on the side of Naked Creek, a week ago yesterday: This boulder is exotic to its current location. It is typical of medium- and coarser-grained diabase from the Culpeper Basin, a Triassic rift valley east of the Blue Ridge. The main minerals are plagioclase (light-colored) and pyroxene (dark colored).
22 September 2011
More moki marbles
More moki marbles: little concretions in sandstone, kind of like the ones I showed you Tuesday from Illinois. But these ones are from the Navajo Sandstone, a late Triassic or early Jurassic erg deposit from the Colorado Plateau. These photos were taken in Zion National Park, near Springdale, Utah (real close to the cross-beds I featured a month ago). They are tougher than the sandstone in which they formed, and …
21 August 2011
The best cross-bedding you’ll ever see
Setting aside the lack of scale, it really doesn’t get any better than that. Click through to make it huge. This is the Navajo Sandstone, early Jurassic (or late Triassic?) in age. It’s in Zion National Park, Utah. Wind direction was from the right towards the left, as these preserved slip-faces of ancient dunes indicate. The beds are right-side-up, and they have been differentially eroded, causing the cross-beds to jump …
27 June 2011
Goose Egg / Tensleep contact
West of Shell, Wyoming, on route 14, there is a lovely exposure showing the tilted stratigraphic contact between the lower Tensleep Formation (purple; Pennsylvanian period) underneath Goose Egg Formation (orange/tan; Permian to Triassic in age). The contact dips to the west because it has been deformed during Laramide mountain-building (uplift of the Bighorn block, and downdropping of the Bighorn basin). Here’s a gigapan to show the contact:
23 May 2011
Geology LOLcats 3
This morning, my cat Lola (a.k.a. “LOLa”) had squirmed herself in between the sheets, and it reminded me of something: If this pose inspires you to another LOLcat caption, you can click through for the original image file, unadorned. Modify it as you like, and give us a link to your creation in the comments on this post! Have fun.
26 February 2011
Waterfall Formation outcrops
I first mentioned the Waterfall Formation on this blog in May of 2008, but this was my first time visiting it in situ. Previously, including the May 2008 visit, was memorable time with charismatic boulders of float. The occasion to see this geologic unit in outcrop was the field trip 6 days ago with the George Mason University GeoClub, when we took a hike to Thoroughfare Gap. After looking at …
21 October 2010
Tavşanlı Zone field trip, part 2
Yesterday, I shared a few thoughts about the first couple of stops on the field trip I took earlier this month from Istanbul to Ankara, prior to the Tectonic Crossroads conference. Today, we’ll pick up with some images and descriptions from the next few stops. After lunch, our next stop brought us to a relatively low-metamorphic-grade outcrop of sheared graywacke (dirty sandstone) and shale. As you can imagine, it wasn’t …
22 June 2010
At the edge of the intrusion
Mountain Beltway reader Greg Willis attended my colleague Ken Rasmussen’s Triassic Rift Valley field course last weekend, and sent me this photo of the view inside the Luck Stone diabase quarry in Centreville, Virginia: Here’s an annotated version: Both photos are enlargeable by clicking on them (twice). This quarry chews into rock right along the contact between a mafic igneous intrusion and lake sediments that formed when water pooled in …
4 March 2010
Triassic rifting in the Capitol
My girlfriend’s mom was in town in January, and we took her down to visit the Capitol Building. The tour had a good bit of history, but definitely missed the opportunity to talk geology. I was particularly struck by the columns in the Hall of Statuary: Close up of one column, with my hand for scale: That’s the Leesburg Conglomerate, a Triassic-aged deposit found in the western part of the …
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.