Oakland Geology

You’ve all seen this quarry, looming over Lake Chabot Road and eating its way into Fairmont Ridge. It has a long history, and like all abandoned quarries it has a long afterlife ahead.

^{Seen from south Dunsmuir Ridge}

The ridge was first opened up in 1886 by the Stone brothers, Egbert and Andrew, whose father Lysander made his fortune from the rich soil of the area now known as the Stonehurst neighborhood. Their construction firm, the E.B. and A.L. Stone Company, was a wide-ranging business that operated several quarries in the Leona Heights area.

By 1918 the quarry was owned by Joseph Costello, and in 1929 it came into the hands of the newly formed San Leandro Rock Company, which has owned it ever since although operations ceased before 2000.

Here’s the setting, from Google Earth. The quarry scar is in the upper center south of the dam at Lake Chabot.

The quarry exploited a body of rock mapped as basalt lava, of Jurassic age, that forms part of the Coast Range ophiolite. Here’s the same area of the geologic map. The Hayward fault is the solid black line just west of the quarry site.

^{Jurassic basalt (Jpb) with San Leandro gabbro to the west, Leona volcanics (pink) and Knoxville Formation mudstone (green) to the east.}

And just for context, here’s the digital elevation model giving a closer view of the quarry site and the canyon of San Leandro Creek below the Lake Chabot dam, with the active strand of the fault shown in red and older strands in yellow.

^{Lidar data from Opentopography; fault traces from USGS. Illumination from the northwest.}

The boulders that line the entryway are a good sample of what’s inside. They’re dominated by the dark, largely featureless basalt.

Here and there you can spot flow features and glassy regions that support the interpretation of this rock unit as pillow lava, the kind of blobby flows that form where red-hot lava meets freezing seawater. Hence the map symbol “Jpb” for Jurassic pillow basalt.

The quarry face itself isn’t half as well exposed, but beneath the rubble and grass the same material shows up in spots.

The thin white veins, when you can find a specimen that exposes them, appear to consist of hydrothermal quartz and olive-green chlorite. These rocks went through a few changes after the basalt first froze.

More entertaining is the view over the canyon and the Bay area beyond. You can see possibilities, whether you’re a would-be home developer or a would-be park planner.

The planner is the East Bay Municipal Utility District — the water company — which wants to buy the land and use the pit to dispose of the soil it digs up during trenching. It’s clean dirt, so no problem there; it’s useless land for anything else, so no problem there. And when they’re done, EBMUD wants to make it into a park. The problem emerges when they spell out the nitty-gritty in their project proposal: “The first stage includes using trench soils for fill operations for long-term phased placement and stabilization of approximately 3.4 million cubic yards of trench soil at the Quarry Site over approximately 40 to 80 years.” The problem is all those dump trucks over all those years on Lake Chabot Road.

The road that serves the quarry has always sparked contention. When it was the old back road between San Leandro and Castro Valley, the heavy-duty traffic from the quarry left the dirt-and-gravel road in rough shape. Eventually the route was paved, but the hillside is precarious and parts of it gave way during our latest wet season. All this before the next big earthquake.

The old road was bypassed by Fairmont Drive, a four-lane highway that was pushed over the ridge in the 1970s, but pleasure drivers, commuters and residents still use it. To preserve their quiet byway, the residents have opposed the heavy trucks of quarry traffic for fifty years, and they oppose it this time too. Today I’m fully on their side.

A possible route from the south would reach the quarry via Fairmont Drive. It would run up this valley, which is owned by the East Bay Regional Park District and is otherwise unusable because it runs right along the Hayward fault.

Considering that the park district and the water company are two trunks from the same root, maybe they could get along with a suitable easement through here.

The Gold Rush was a bust everywhere in the Oakland Hills, with one exception. That was a short-lived mine, started in 1864, on A. D. Pryal’s ranch in the northern Rockridge neighborhood. This seems most unlikely at first blush, but the source is Titus Fay Cronise’s unimpeachable book The Natural Wealth of California (1868):

“In 1864, Mr. A. D. Pryal, owner of a large ranch about four miles east from Oakland, discovered a vein of auriferous quartz in the Contra Costa hills, which cross his lands. Some of the specimens from this vein were rich in free gold, and the mine opened under the name Temescal, paid well for a short time, but the dislocation of the strata, a little below the surface, rendered its further working unprofitable.”

The only remotely likely source for such an ore is a small body of highly altered mantle material, called silica-carbonate rock (also called listwanite or listvenite), that was caught up in the Hayward Fault. Long-time readers may recall a post of mine on the subject that involved this same locality. Here’s what the area looks like on the geologic map.

That’s College Avenue running up the left side and Route 24 in purple running along the bottom, with Chabot Road to its immediate north; Lake Temescal is in the corner and the silica-carbonate is the dark blue wedge just north of it between strands of the Hayward fault. When I explored it nine years ago, I bushwhacked up its western edge and found nothing. This weekend, I bushwhacked up its eastern edge.

This part of town has been heavily built upon since Pryal first dug it up, but old photos from (I think) the 1890s show the possibilities. The first shows the Lake Temescal dam and the creek below the spillway. What would’ve been the continuation of Chabot Road (then known as Pryal Lane) runs in front of the white house at the left. The little bridge at the bottom is where the next photo is.

Notice what a mess the hillside is above the dam. Anthony Chabot apparently sluiced it all into the reservoir when he built the dam in 1868. And notice what an erosional mess the streambed is. Nobody cared back then, or nobody downstream cared enough to sue Chabot.

^{Photos courtesy Bancroft Library via Online Archive of California}

What caught my eye was the boulder at center left. Well, first, the streambank behind it looks like fault gouge, the pale powdery dirt that faults make by grinding rocks (and which I documented down at the London Road landslide). Anyway, the boulder at center left looks just like a big slickenside, the scraped-and-buffed surface that faults make by rubbing rocks.

The gist of all this is that a wide, complex fault zone like the geologic map shows could very easily carry slivers of rock from quite far away. And this is little known today, but in the early days there were curious, isolated reports of stones of gold-bearing quartz in our hills. At least two have popped up in my reading, one from north Berkeley and another from Leona Heights. So my hopes were not high as I set out on this traverse, but they weren’t zero either.

It was a real nice day. The streams had water and the ground was pretty firm and quiet. This is looking down at the head of Chabot Road, which was truncated by the freeway long ago. A strand of the fault is mapped there, but the road shows no sign of it.

I found bedrock this time. One bit was deeply weathered Leona volcanics, the same stuff that crops out uphill to the east (pink on the map).

This outcrop looked more like a strongly sheared and altered basalt, not unexpected in the Leona volcanics.

This outcrop, hard to tell; probably more of the same, brecciated.

None of what I saw appeared to be silica-carbonate rock or even leaning in that direction. But that’s what I would expect 160 years after a minor gold find petered out. I’m still not clear on what evidence led the mappers to think such a thing was here at all.

Besides, I was happy to find real bedrock at all during this visit, and there’s still a bit of the territory I haven’t set foot on yet — something for another day. After a long absence from the field, these rocks all looked beautiful to me anyway.