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4 October 2019
Hey, let’s go back to Angel Island for today’s Friday fold. We’ve been there once, twice, thrice previously. The rocks in question are metaconglomerates that Jess Ball and I found at first only as float on the beach at Camp Reynolds, like these two examples: …Look at those beautiful elongated pebbles, transected by wee white veins! Where there’s float, there may be outcrop — Sure enough, with a minute or …
27 September 2019
The western edge of O’Shaughnessy Boulevard, near Glen Canyon Park in San Francisco shows beautiful examples of crumpled cherts. Here are a few dozen photos of these glorious outcrops, and instructions on how to visit.
30 August 2019
The Friday fold is a guest contribution to “Mountain Beltway” from the manager of the AGU Blogosphere, Larry O’Hanlon. It shows apparent crumpling of a few sedimentary layers at the toe of a soft sediment slump at Calafia State Beach in southern California.
31 May 2019
It’s Friday, and I’m grateful that Bret Leslie of the NRC has stepped up with a Friday fold contribution from coastal California: Bret says this outcrop is: from my trip last month to the Sonoma and Mendocino coast. The first is the cliff below the Pt. Arena lighthouse. The marine deposits (now a marine terrace) are unconformably on top of the folded Miocene sedimentary rocks in the second photo. Here …
3 May 2019
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…
26 April 2019
The Friday fold can be found in a boulder of gray chert layers on Black Sands Beach, at Marin Headlands in California.
19 April 2019
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.
12 April 2019
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.
5 April 2019
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.
29 March 2019
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.