Where my rocks go to die

15 May 2016

For many years I saved and collected rocks. This was especially true during my years at About.com, when I put together a large set of photos and explanations to help people learn about rocks.

When About.com dropped my contract in 2014, I’d reached “peak rocks.” My office had rocks everywhere, and my closet had still more. I’d even started putting rocks back where I collected them.

(I’d normally add a bunch of links to my old site to document those statements. However, I learned last week that About.com will take down its Geology site entirely in yet another desperate attempt to gain altitude. Some of my articles have already disappeared from Google searches. Soon my work of 17 years, and the work of my hapless successor, will disappear except from archive.org, the internet’s “Wayback Machine.” So fuck ’em.)

Some of my rocks are sentimental favorites. Other rocks, I’ll look at it and realize I can’t recall where it’s from or decide that’s too far away to visit again, and I decide to set it free.

My go-to place for that is Devil’s Slide, the sedimentary version of Orodruin, where Frodo Baggins went to destroy the One Ring. There the Pacific lies ready to grind every stone into sand. And above the old Route 1 roadway, now a county trail, are spectacular exposures of what the sand will become in the geologic future.


I went there last week. On a somber day Devil’s Slide is very fitting for this purpose. Marine haze shuts out the world. The coast is under noisy, vigorous attack. The whole place is falling into the sea, which makes the textbook “rock cycle” a visceral reality. You feel a bit wary about the roadway itself.

Offshore is Point San Pedro, made of the same rocks. Geologists have determined that those rock beds have been overturned.


Anyway, here’s where I stand. I take my rocks and fling them toward the waves. Most of them fall short, but that’s OK, they’ll make it down to the surf soon enough.


One rock was this piece of coal I once found in the yard, dating from the days when our homes had coal-fired furnaces. I think it was from Utah, because California coal wasn’t this high quality.


Given the popular feelings about shipping Utah coal through Oakland, it was the right time to make a statement by destroying this specimen.

As a geologist, I adore coal. It’s a fascinating storehouse of carbon from the distant past. As a respecter of history, I honor coal’s fundamental role in the Industrial Age. Coal saved the forests from being turned to charcoal. When I was young, my family once burned coal in the house. But its time is over. Except for scientific research and historical reenactments, coal should be left in the ground.

I support Oaklanders’ efforts to stop commerce in coal. I just don’t endorse every statement they’re making. Specifically, coal dust from rail shipments won’t cause asthma or cancer; it’s only a nuisance. Smoke from burning coal is what causes asthma and cancer, and that will happen somewhere else. I oppose burning coal, and causing asthma and cancer, anywhere on Earth.


All I ask is that people do the right thing for the right reasons. It’s important for the long term.

Conglomerate in the upper Arroyo Viejo streambed

9 May 2016

Oakland has three different bodies of conglomerate: from youngest to oldest they’re in the Orinda Formation (of Miocene age, maybe 10 million years old), the Oakland Conglomerate (Late Cretaceous, maybe 80 Ma) and the Knoxville Formation (Late Jurassic, about 165 Ma). Recently I’ve been getting fixated on the last one. I hope these photos from upper Arroyo Viejo will help you understand why.


Above I-580, Arroyo Viejo runs along the road as you drive up Golf Links Road and into the woods of Knowland Park. Halfway through, it takes a sneaky dogleg to the left, then another left turn through this tunnel under Elysian Fields Road. Beyond is the stream’s source, now in the Sequoyah Country Club golf course.


Between those two left turns, the stream valley exposes the Knoxville Formation. I haven’t walked the whole section yet, just visited both ends. The lower end has wonderful conglomerate outcrops.

High on the valley walls, the rock is heavily vegetated. It gets that way because between the pebbles and cobbles of the conglomerate, the matrix is a fine-grained sediment that supplies nutrients to plants and offers space to their roots.


As you get close to it, the conglomerate reveals the abundant well-rounded cobbles beneath its green coat. Geologists like their rocks clean, but nature prefers them this way. You have to admire them.


For the best exposures, look down into the streambed.


A little bit downstream is a section of creek choked with boulders of this stuff, some as big as sofas, that I showed you a few months ago. Those monster rocks weren’t carved out of the streambed by the creek. Instead, they rolled down the valley walls, which are very steep (35 to 40 degrees) as most are in Oakland.


The valley does have some landslide scars, so we know that those happen. But mostly I blame earthquakes.

Both events provoke the stream into washing the boulders away. Landslides create instant dams, which build up the water pressure in the lakes that pond above them. When the stream bursts through, usually in a matter of days, it makes short work of the rock pile. Earthquakes, for their part, give the whole underground a shakedown and cause a weeks-long surge of water afterward that likewise gives the boulders a good head start.

In between these disturbances, maybe once in a century, rare cloudbursts pour enough water into the watershed to roll the biggest rocks downstream and grind down the streambed an inch or so at a time.

Given enough time — and geology always provides that — the boulders break down into pebbles and clay and wash out to sea, eventually to become new rocks. The Knoxville Formation conglomerate has waited some 165 million years to start that journey. The Earth is almost thirty times older.

Rocks and views of Fairmont Ridge

2 May 2016

Fairmont Ridge is the grassy upland that forms the backdrop to San Leandro. As it happens, East Bay Regional Park District owns much of it as part of Lake Chabot Regional Park. It has some rocks, which I’ll show first, and also some fine views.

Here’s the aerial view of the ridge from Google.


And the geology of the same area is here.


We’ll look at rocks from three different units: the green area is underlain by the Knoxville Formation, a shaly sedimentary unit; the light-brown area labeled Jpb is basaltic lava; and the pink area labeled Jsv is Leona rhyolite, which you’re familiar with by now from Oakland.

The Knoxville is well exposed around Lake Chabot. Here, to the east of the access road at locality 1, it appears to be strongly sheared, suggesting that its contact with the structurally underlying Leona and basalt is a fault. This view is facing north, parallel to the contact.


The basalt unit is formally described as pillow basalt, the kind of balloon-shaped flows you’d find where lava erupts beneath seawater. But these rocks have been shoved around a lot since they were erupted in Late Jurassic time, and I have yet to see decent pillow morphology in any exposures. Still, the outcrops, like this one at locality 2, are picturesque.


The windbreak of giant, mature eucalyptus is visible in the photo. This is a naturally breezy park, and the line of trees offers some welcome shelter.

Across the ridge on the Bay side, there are more outcrops of the basalt. Around locality 3 it’s well displayed.


If you pay attention, you’ll see bits of this rock with polished surfaces, or slickensides, on them. These are caused by motion on faults, which rubs rocks against each other. Here and there, proper outcrops enable us to see that the faults are oriented vertically and parallel to the ridgeline. I interpret these as forming recently as these rocks were folded and tilted upright by motion related to the Hayward fault.


The peak of Fairmont Ridge is fenced off, but an informal trail leads north along the east side of the fence to locality 4. (Poison oak will very soon make it impassible.) That’s where this typical specimen of Leona rhyolite was.


But sometimes rocks are just rocks. Lift up your eyes from the hills and sit a spell. You can gaze upon the Bay side . . .


. . . or over the reservoir toward Mount Diablo.


Looking due east is a nice prospect of the ridge known as The Knife, overlooking San Ramon.


Its high point is named Wiedemann Hill, elevation 1854 feet, and I have a growing fixation with it.


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