Archive for the ‘Oakland rocks’ Category

Clay outcrop in Horseshoe Canyon

25 January 2016

The gorge of Horseshoe Creek, in Leona Heights Park, is unusually grand for its size. Its rugged rocks, mostly Leona “rhyolite,” are pretty homogeneous though.


So when a spot along the stream caught my eye with its color — reddish red and bluish gray — I went off the trail and checked it out. Notice that the surface is cut into the hillside.


This streamside lump looked just like concrete. But there was a lot of it, in different states of preservation and age, so I took it as a natural deposit.


It was hard, but a little higher up I was surprised to find soft material. Not just soft, but pure clay.


The high-silica lava and volcanic ash that makes up the Leona should weather into kaolinite (white china clay), especially under acidic conditions. We have that combination in parts of the Leona that are rich in pyrite. This mineral, with the formula FeS2, reacts with air to form iron oxyhydroxides and sulfuric acid (here’s a brief treatment).

There may be a pod of rock here with a different texture or composition from its surroundings, which might account for the purity of the clay. But I don’t actually know how pure the clay is. The way to tell would be nibbling it. Maybe on my next visit.

I think that a gradient in pH, plus interactions with air and surface water, explains the transition from gray to white to red clay as you go from depth to the surface.


Without a lab, there’s not much I can say about it, although geologists with more experience probably know this stuff cold. If so, speak up. There was another piece of evidence at the scene, though: a bit of leaking “yellowboy” from the floor of the streambed.


It means there’s a little bit of acid drainage here, not up to the level of the ex-sulfur mine just south of here. More like a geologically slow bit of natural acid drainage. It will be interesting to watch this spot during this wet winter.

Dunsmuir Ridge revisited; Irvington gravels visited

18 January 2016

Six and a half years ago, I put up a post about the special little gravel patches on Dunsmuir Ridge. On Tuesday I finally returned there to put back the rocks I collected. That’s extremely fussy of me, but a principle’s a principle and my living space refuses to stretch with the rocks I bring home.

I didn’t include a geologic map in my earlier post, so here it is now.


The gravel patches are shown in light tan, inside the “Jsv” area (which you’ll recognize as the Leona “rhyolite”). The straightest black dashed line is the Hayward fault.

Winter is a good time to walk Oakland’s hills.


The Leona makes up the country rock, distinctive yet somehow nondescript.


As you climb through the woods, the stones in the road turn rounded, and variegated. That’s the odd, out-of-place gravel.


Here’s a hunk of the laminated Claremont chert, well outside its domain.


As I encountered each lithology, I pulled out the appropriate cobble from my 2009 raid and reintroduced it to its cousin: the celadonite-bearing Leona, more abundant in the gravels than in the field . . .


and the glorious Franciscan metachert.


When all the stones were dispersed, I went up to the hilltop where the gravel had been so abundant in the roadways in 2009. I knew what to expect.


This part of the hilltop is no longer fit for science, but I’m not bothered. Real science would be done by careful excavation in undisturbed places, and those are still available. Meanwhile, you could say those rocks are now cherished stones. Rainwashed and sundried, they look their best now. Enjoy them while you’re up there.


My second purpose in revisiting Dunsmuir Ridge was to compare its gravel to the official Irvington Gravel down in Fremont, which I did that afternoon. Here’s the geology, from the Alameda County bedrock map, USGS Open-File 96-252.


It’s not a great map for field use — the geographic base layer is quite poor — but you get the gist. I-680 curves east through the northern tip of the gravel, and the original quarry is right there where the “25” is. The ex-quarry and the streambed along the north edge of the gravel are preserved in Sabercat Park.

It was an ordinary gravel quarry until the 1940s, when a treasure trove of Pleistocene fossils was extracted from the site.

More fossils are surely there, thousands of them, in the hillsides.


I took a good look at the gravel cropping out. There was a general resemblance to the Dunsmuir Hills patches that I wouldn’t have noted had I been a naive visitor. But for an initial reconnaissance visit, this was cool.


Fossil hunters adore the rain, because it gently washes dust and dirt away from even tiny fossils, allowing them to stand out on the clean surface. So I closely inspected a few spots just in case there were, like, Pleistocene mouse teeth exposed.


Just some pretty Franciscan chert again. Parks are off limits to collectors, so I would have merely photographed anything I found. I’ve come to believe that finding is a purer pleasure than keeping, anyway.

The Northern California Geological Society had a field trip here 10 years ago. Many wonderful Irvington fossils can be seen and touched at the Children’s Natural History Museum in Fremont.

Oakland, capital of lithodiversity

11 January 2016

Last month I delivered a talk to the Nerd Nite East Bay gathering titled “Diversity in Deepest Oakland: Rocks of the Town.” It was fun, and the audience was good. I made the case that per square kilometer, Oakland has more different rock types than any other city in America.

I counted off more than 25 different specific rock types (shale, schist etc.) occurring in Oakland. Although most of the rocks I showed the group have already been featured on this blog, here are a few new ones. These images from my talk are big, so I’ll try to restrain myself.

Before I start, here’s a version of the Oakland geologic map that I dubbed “Beast Oakland.” I never tried this orientation before, and I like the result.


It brings out the Hayward fault vividly. I pointed out that because it has pulled together two very different sets of rocks, the Hayward fault is a key factor in our lithodiversity. Earlier faulting has added similar complexity to the top portion, representing the high hills.

Here are the new rock types I was able to document, in the order I found them during a 3-hour reconnaissance. First was schist per se.


Schist is actually a textural term referring to the thin layers of metamorphic minerals. It gets more specific names depending on the exact mineral involved — mica schist is dominated by mica, for example.

I was able to claim marble, from a fist-sized stringer in the blueschist knocker of Contra Costa Road, along with the acid test I used to confirm it.


Next was limestone (or dolomite, which is similar), which occurs in pods in the Claremont chert.


I bagged tuff along Grizzly Peak Boulevard, where a prominent bed of this lithified volcanic ash crops out.


Both quartzite and granite appeared in a roadcut along Skyline Boulevard, where they were cobbles in the Oakland Conglomerate. That’s quartzite on the left, and I thought I’d never see either rock in this town.


And finally, up at Serpentine Prairie, I realized that the boulders included some peridotite. This one amazed me the most. It’s the deep stuff from the bottom of seafloor crust, the rock left over after basaltic lava is extracted from it by partial melting. Nearly all of it is recycled into the deep mantle by subduction, and nearly all of the rest turns into serpentine rock. It is rare.


So I ended the talk by comparing Oakland’s roster of rock types against America’s three largest cities: New York, Chicago and Los Angeles. Only asphalt (in L.A., natch) and gneiss (most of Manhattan) are unavailable in Oakland, while a bunch of our rocks are ours alone. And we are like one-tenth of their land area. Chicago? Dey got nuthin’. Los Angeles? No way Jose. New York? Fuhgeddaboutit. Oakland rocks! [mic drop]


I’m here every week folks, visit the giftshop on your way out.


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