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.
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.
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.
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.
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.
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.
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.)
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 … Read more
Sonora Pass, California, is a lovely place to examine a volcanic-on-plutonic nonconformity that spans about 80 million years of missing time. Let’s check it out on a photo-rich virtual field trip!
Fifteen years after mapping deformed rocks of the Sierra Crest shear zone system in the high Sierra, a family vacation brings Callan back to the pre-batholith metasedimentary rocks which show a pronounced strain.
A detailed examination of an elegant photo of the eastern front of California’s Sierra Nevada, from the perspective of the Alabama Hills. How many different geologic phenomena can be packed into a single image? Let’s find out!
The guest Friday fold comes to us from the Miocene of California, deposited in a releasing bend basin along the San Andreas Fault.
My friend Bill Burton (USGS, Reston) shared today’s Friday fold — Chevron folds in marble, Kings Canyon, California. Nice! Thanks for sharing, Bill!
Another week, another batch of new images produced on my home-based Magnify2 imaging system from GIGAmacro. Leptaena brachiopod in (Mississippian?) limestone from Montana: Link Here’s the flip side of the same sample, with a lot of fenestrate bryozoans to see: Link Fault breccia from the Corona Heights Fault of San Francisco: Link Amygdular metabasalt from … Read more
Happy Friday – sorry to have not shared any folds with you last week. I hope these beautiful folds in Catalina Island meta-cherts will make up for it: As with the previous couple of Friday folds, this image is courtesy of Sarah Penniston-Dorland (University of Maryland, College Park).
Happy Friday! Here’s another fold from Catalina Island, California – an antiform in metasediments, courtesy of Sarah Penniston-Dorland of the University of Maryland:
My friend Sarah Penniston-Dorland, of the University of Maryland, supplied this week’s Friday fold. it comes from Catalina Island, California, where Sarah just wrapped up some field work with two of her students. All three of them have given me permission to post the images here: The Catalina Schist is a suite of subduction-related metamorphic … Read more
The week before the AGU Fall Meeting, San Francisco experienced heavy rain and strong coastal wave action. These two phenomena liberated a big boulder of serpentinite on Marshall’s Beach. As it moved downhill, it opened up a scarp with views into the colluvial soil horizons.
South of the Golden Gate Bridge in San Francisco, there are beachside exposures of serpentinite mélange: tectonically sheared-out former oceanic crust accreted to western North America as part of an accretionary wedge.
At the end of the AGU Fall meeting, Callan visits the Corona Heights “mirror” fault, renowned for its gorgeous slickensides. Explore the site in photos in GigaPans.