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29 May 2022

Accretionary wedge turbidites from the Lost Coast

The Lost Coast of northern California shows geological evidence of deep sea turbidity current sedimentation, tectonic accretion during Mesozoic subduction, and then isostatic uplift interacting with shoreline erosion. Check out a few photographs taken by Callan’s wife on a recent backpacking trip through the region.


18 September 2020

Friday fan: Death Valley, California

Working up some new images for my free, online Historical Geology textbook, I annotated a photograph I took in March of this alluvial fan in southern Death Valley. The development of desert varnish on older parts of the fan shows their age visually in a quick and easy way of determining fan deposit sequence: I’ve been making a lot of these animated annotations as a way of conserving space in …


27 March 2020

Friday fauxld: a false syncline in Titus Canyon

What day is it again? Hard to keep track in the days of raging coronavirus infections, but it is in fact Friday, which means that if you want a dose of the halcyon pre-COVID-19 days, you can enjoy this example of a false fold from Death Valley National Park’s Titus Canyon.


20 March 2020

Friday fold: more folds from lower Titus Canyon

We are living in surreal times. It hardly seems possible, but a week ago this evening, I drove down the Las Vegas strip with my students, ogling at the glitz and spectacle and crowds. Now, a mere 7 days later, Vegas has been shuttered, and it’s been shuttered for days. We traveled freely through California and now a week later, everyone in the state is ordered to stay home. What …


13 March 2020

Friday fold: Bizarro folding in Titus Canyon

Hi everyone, and greetings from eastern California’s Death Valley, where I’m leading a field geology course over our spring break. I found an excellent Friday fold for you: That’s the Cambrian-aged Bonanza King Formation, a package of limestones, as exposed in lower Titus Canyon, Death Valley National Park. Here’s the thing: the lower part of that outcrop is Upper Bonanza King Formation, while the upper part of the cliff is …


14 February 2020

Friday fold: Miocene slump in Anza-Borrego

Today’s Friday Fold comes from Edith Carolina Rojas, the dynamic geology professor at The College of The Desert in Palm Desert, California. She’s an awesome person, and also the sense of scale in this amazing image: Edith shares that this gorgeous structure is an anticline is located in Split Mountain Gorge in Fish Creek Canyon. It’s a gigantic gravity-slide fold due to soft sediment deformation in the Latrania Formation. Wow …


7 February 2020

Friday fold: Kings Canyon

The Friday fold shows disharmonic crumpling in marbles of the Boyden Cave Root Pendant in California’s Sierra Nevada. The GigaPan image displayed is part of the digital legacy of Ron Schott, who passed away a year and a half ago.


13 December 2019

Friday fold: blueschist & eclogite at Jenner, California

A pre-Fall Meeting field trip to the coast of northern California yields rare sights of garnet-bearing blueschist, plus eclogite, some pillow basalts, birds, waves, wind, and a lot of rain.


10 December 2019

‘Streetcar 2 Subduction’ is live!

This week marks the launch of a new digital revision of a field guide to the geology of San Francisco, “Streetcar 2 Subduction.” Learn more here!


22 November 2019

Friday fold: Noonday Dolomite in Mosaic Canyon

Because I’m putting together a field course for spring break 2020 to Death Valley California, I was looking through old Death Valley photos this week, from the last time I went to that special place. It was seven years ago! How time flies… This one is in Mosaic Canyon, and was taken by my student Marcelo Arispe, a talented photographer as well as a talented geologist: By the standards of …


4 October 2019

Friday fold: Angel Island “IV”

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

Friday folds: O’Shaughnessy Boulevard, Glen Canyon Park, San Francisco

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

Friday fold: Calafia State Beach

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

Friday fold: Point Arena, California

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

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…


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.


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.


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.


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.


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.