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20 January 2021
Peering through
When hiking recently in my neighborhood, I saw this gleaming apparition appear in an eroded gully in a dirt road: Those multicolored stripes are varying compositions in a zone of ultramylonite: ductilely-sheared-out rock that formed in the deep equivalent of a “fault” in the Blue Ridge basement complex. We call it a “shear zone” most of the time, but a better descriptor would be “high strain zone.” These rocks are …
5 June 2020
Friday fold: Harbledown Island
Reader Christian Gronau writes with this Friday fold contribution: Greetings from Cortes Island, BC – at the opposite end of the Strait vis-a-vis Lopez Island. Your Mountain Beltway blog is always of interest, and I have been following it for several years by now. Thank you for putting the effort into this worthwhile website. Quite regularly your posts elicit “echoes” and make me go back to some of my own …
16 January 2017
Shear zones in Scottish gabbros
A quest to visit the “first shear zones” described in the scientific literature leads to an alternate location, and some GIGAmacro images of samples from the real, original spot.
25 November 2016
Friday fold: intrafolial folds in Eriboll mylonite
At the birthplace of the term “mylonite,” we can find Friday folds hidden in the foliation.
4 September 2013
Granite balls, boudins, and tadpoles in Archean schist, Laramie Range, Wyoming
A final post from my visit to the Laramie Range with Bob Bauer and Fred McLaughlin earlier in the summer -Recall that there is schist and granite at this site, and much of the granite shows cross-cutting or inclusive relationships indicating it is younger than the schist, while still younger mafic dikes cross-cut than the granite. Both exhibited folding. However, in other places, we observed the opposite: granite inclusions within …
7 June 2013
Friday fold: South Pass City, Wyoming
Two summers ago, my wife (then my very new wife) Lily and I participated in a two-week workshop for teachers on the energy resources of Wyoming. We also indulged in a visit to a mineral resource site: South Pass City, a site in the southern Wind River Uplift that was the second oldest incorporated town in Wyoming (after Cheyenne). It was a gold mining site, and today it is preserved …
8 May 2013
A comparative look at the Klingle Valley outcrop
A week ago, I featured six new GigaPans here, of the extraordinary rocks in Klingle Valley, DC. I hadn’t been able to get the next phase of imaging that site ready in time for the post, but here it is: an annotated view of the outcrop. Annotation color code: PINK = Granite contact BLUE = Sericite after staurolite pseudomorphs YELLOW = Outlines of stretched clasts within the metaconglomerate GREEN = …
30 April 2013
Strained metaconglomerate in Klingle Valley, DC
Following on yesterday’s post about the kink bands within the strained metagraywacke of the Laurel Formation in DC, let’s take the opportunity today to go to Klingle Valley, site of a different facies within the Laurel Formation: a strained metaconglomerate. Though the exposure isn’t as great as the Purgatory Conglomerate, I think you’ll find plenty to hold your attention in these rocks. Close looks will reveal sericite-after-staurolite pseudomorphs (evidence of …
29 April 2013
Kink bands in highly strained Laurel Formation, Rock Creek Shear Zone, DC
Last week before GSW, I spent several pollen-choked hours in Rock Creek Park, GigaPanning some of the rocks of the Rock Creek Shear Zone. Here are some exposures in the bed of Broad Branch that show lovely kink banding. In at least one spot, you can see a conjugate pair, so these rocks were (1) sheared out in a ductile shear zone, producing the foliation, and then (2) compressed under …
26 April 2013
Friday folds: Simple shear and the unfolding of folds
The Friday fold combines a moment of insight on a field trip in the Archean Superior Province and a new paper published this month in the journal GEOLOGY.
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.