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3 May 2019

Friday fold: Monarch Canyon

When a dike is subjected to shear, how does it deform? A guest Friday fold from Darrel Cowan of the University of Washington teaches a nice little lesson. Join the field trip in Monarch Canyon of Death Valley National Park…

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26 April 2019

Friday fold: Black Sands Beach

The Friday fold can be found in a boulder of gray chert layers on Black Sands Beach, at Marin Headlands in California.

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19 April 2019

Friday fold: Angel Island “III”

For the third Friday in a row, “Mountain Beltway” features folds from Angel Island, in northern San Francisco Bay, California, This time, it’s meta-cherts on Perles Beach, showing impressive metamorphic recrystallization reactions.

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12 April 2019

Friday fold: Angel Island “II”

Happy Friday, all. Here are a few shots of crinkled, thin, multicolored cherts from Kayak Beach on northern Angel Island, California, piggybacking on the folds I showed you last week.

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5 April 2019

Friday fold: Angel Island “I”

The Friday fold is on the north shore of Angel Island, San Francisco Bay, showing blueschist-facies high-contrast metalliferous cherts with folds on many scales.

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29 March 2019

Friday fold: Plunging Purisima

AGU’s Centennial year is also the 35th anniversary of the publication of Clyde Wahrhaftig’s unique field guide to the geology of San Francisco. A team of geologists is updating “A Streetcar to Subduction” for a modern digital audience, and recently did some California field work to visit key sites. Check out one of them here with the Friday fold, as we visit a plunging syncline in the Purisima Formation on the coast near San Mateo.

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14 December 2018

Friday fold: Titus Canyon Anticline

A guest Friday fold brings us to a position in the sky somewhere over eastern Death Valley National Park. Join us in contemplating the Titus Canyon Anticline.

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22 August 2018

Q&A, episode 5

A reader asks: “What is foliation and what makes it so important to the structure of rock?”
Callan answers with a lot of images of beautifully foliated rocks.

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10 August 2018

Friday fold: Gnarly chert from Spy Rock

The Friday fold is an outcrop of folded chert from the Franciscan mélange in Mendocino County, California. (It should look familiar to anyone who’s ever visited Marin Headlands near San Francisco.)

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6 July 2018

Friday fold: twisted turbidites at Usal Beach

Last week, I spent two perfect days camping with family at Usal Beach, in Mendocino County, California. Along the beachside cliffs there, I spotted plenty of lovely turbidites: graywacke and shale and a little bit of conglomerate that had been scraped off the subducted Farrallon Plate to help contribute to the bulk of the Franciscan complex. That accretionary process imparted some stresses on these deep-sea deposits, and in many places …

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