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.

This entry was posted on 20 February 2023 at 8:01 am and is filed under Hayward fault, Quarries and mines. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.