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15 September 2023
Friday fold: Candler Formation
On a birding hike yesterday morning, I found this: This is a little slab of kinked phyllite of the Candler Formation. This metamorphic rock started as mud (and ash) deposited atop the Catoctin Formation, and it was later squeezed and heated during Appalachian mountain-building, encouraging the growth of micas at the expense of clay. A strong differential stress made sure those micas grew in one preferred orientation, and that resulted …
28 July 2023
Friday fold/fauxld duet from Glacier National Park
When is an apparent anticline not an fold? Find out on this week’s edition of the Friday Fold…
14 July 2023
Friday fauxld: Sacagawea Cirque
Found this beautiful cobble while hiking Sacagawea Peak with my family yesterday, in Montana’s Bridger Range. It’s not actually folded as much as it appears to be; the laminations are slab like and nearly parallel to the surface in this photo, with the cobble surface sectioning them obliquely to produce this pattern.
30 June 2023
Friday fold: Wissahickon block
Click to enlarge I’m writing this on the Amtrak train from New York back to Charlottesville, traveling (again) under orange-hazy skies due to Canadian wildfire smoke. I took my son to Manhattan for a concert, and we stopped off en route for an overnight in Philadelphia, visiting a former student of mine who’s now an assistant professor of geology at Rowan University in New Jersey. While walking around Philly, I …
9 June 2023
Friday fold: Pleasant Grove Park
Happy Friday, friends. Here’s a rock sample that I recently polished up: It shows crenulations in “pinstriped” schist of the western Piedmont in Pleasant Grove Park in Fluvanna County, mapped as the Mine Run Complex. Lovely stuff, eh?
17 March 2023
Friday fold: Pinto Gneiss, varnished and un-
Last week, I was lucky enough to visit Pushawalla Canyon in the Indio Hills region of southern California. There, my colleague Kim Blisniuk (San Jose State University) led our students through an exercise mapping alluvial deposits as a way of constraining offsets along the Mission Canyon strand of the San Andreas Fault. I noted that much of the alluvium was sourced to the northeast, to the Little San Bernardino Mountains. …
10 March 2023
Friday fold: Painted Canyon
Greetings from southern California! I’m running a spring break field course in the Mojave and Colorado desert. Here’s a pretty amazing fold pair in Palm Springs Formation, butted up against (darker) Mecca Formation along a fault. This is in Painted Canyon, in the Mecca Hills, a transpressional pop-up between the Painted Canyon Fault and the San Andreas Fault. Happy Friday!
10 February 2023
Friday fold: Kelvin-Helmholtz waves in granite?
Reader Christian Gronau returns with another “guest Friday fold” submission. Christian writes, Greetings from a cold and wet west coast. A good time to root through old rock samples – and let the imagination run free … The little compilation below strikes me as visually compelling (both photographs are mine) – but how likely is it that the suggested analogy has any merit ? Would it have some …
2 December 2022
Friday fold: Hidden Rock Park, Goochland County
Two weeks ago was the annual Virginia Geological Field Conference, which was centered this year on the Goochland Terrane, an interesting block of crust in the Piedmont which shows some similarities to the Blue Ridge geologic province, but also shows some differences that suggest it’s not just a mini-Blue-Ridge. One of the best exposures was in Hidden Rock Park, where a series of “whaleback” outcrops expose things like this: There, …
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.