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1 March 2012
Sea Point Migmatite
Callan visits the Sea Point migmatite, a contact between intrusive granite and older metasedimentary rocks, along the west coast of South Africa near Cape Town. His guide? None other than AGU Blogosphere blogger Evelyn Mervine of Georneys!
24 February 2012
Friday fold: Veach Gap anticline, in GigaPan
GigaPan by Alan Pitts, as part of the M.A.G.I.C. project that we are working on. Here’s another one from a few meters away. Here’s a shot from 2.5 minutes after Alan saw this anticline for the first time. Here’s some shots of my own early visit to the site, after I first found it.
26 January 2012
The mixed-up quartzites of Cape Agulhas
Callan and his wife journey to Africa’s southernmost point, and find a geological mystery there.
11 January 2012
Splotchy Liesgang banding
At the Three Rondavels overlook in the northern Drakensburg Mountains of South Africa, I saw this chunk of quartzite with a peculiar variety of Liesegang banding (iron oxide staining of the rock by groundwater): A short distance away, I found another example: In one key way, I liked this second example better, even though the Liesegang banding wasn’t as well expressed here. The improvement is that it also shows bedding, …
23 December 2011
Friday fold: a wrinkled mountain in Hermanus
While I was away in South Africa, both Brian Romans of Clastic Detritus and Evelyn Mervine of Georneys posted pictures of folds in quartzite of the Cape Fold Belt in southern South Africa. Well, I’m not going to be left out. Here’s a belated Friday fold for December 23, showing a bunch of sweet folds in a mountain just east of the coastal whale-watching town of Hermanus: Zooming in: And …
8 November 2011
Mystery mineral molds
Here’s a piece of vein quartz with a series of interesting “cubist disc” shaped holes in it. I interpret these cavities as external molds of some mineral which has now been physically removed or chemically weathered away. So only its exterior shape remains, and I wonder if that’s enough to identify the mineral that used to be here. Both images below can be enlarged by clicking on them: And the …
7 September 2011
Graded bed from the Billy Goat Trail
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 …
8 June 2011
A rose quartz gravestone
On Monday when I went to Powell’s grave, I noticed a few “boulder with a plaque” gravestones, the most distinctive of which was this lovely chunk of rose quartz: Quartz is a good choice, since it’s very very very stable at Earth surface conditions, and thus will stick around a lot longer than, say, a marble tombstone. I still think I’d rather opt for a rock bearing tectonic structures or …
2 June 2011
Conglomerate from Dolly Sods
While my focus was more on bugs and plants than geology this time around, I did hike a new trail at Dolly Sods this past weekend, and on that trail I found several nice boulders of conglomerate. These stood out as being much coarser than the pervasive quartz sandstone of the Pennsylvanian-aged Conemaugh Group which otherwise dominates the outcrops and float of Dolly Sods. Therefore, they were photo-worthy. The conglomerate …
27 May 2011
Friday fold(s): the Outdoor Lab
Today’s Friday fold takes me back 25 years, to when I visited the Outdoor Lab with my science class in Arlington County Public Schools. I revisited this exemplary outdoor education facility on Tuesday, at the invitation of its director, Neil Heinekamp. Neil wanted a geology “expert” to take a look at their rocks, and I wanted a chance to check out their rocks as part of my expanding examination 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.