Archive for the ‘Oakland stone’ Category

Working rocks

14 September 2015

Last week I made another visit to Knowland Park in preparation for my two upcoming Wild Oakland walks on September 20 (tickets still available) and October 4 (no tickets needed). I’m holding off on posting about the park’s geology until after these events, but it’s hard to wait.

Meanwhile, here’s something different. Oakland has plenty of excellent native rocks, but the days are long past when Oaklanders could shop for Oakland stone at an Oakland quarry. Today newer places must make do with more anonymous stones from distant sources. They do their jobs with stolid competence and the occasional dash of flair.

This roadside lot on Cameron Drive uses guard rocks to discourage parking and keep runaway vehicles from breaking the wall. They’re picturesque, but kind of brusque.


Sobrante Park Elementary School has this splendid green boulder by its front entrance. It’s not serpentinite but, most likely, a beautifully chloritized basalt from the Franciscan Complex. The two neighboring boulders are sandstone. I think their job is keep runaway vehicles from the building, although neither Topanga Drive nor El Paseo Drive is a high-speed thoroughfare.


And down by Lake Merritt, the latest Measure DD improvement, the Sailboat House Shoreline Project, has made the shoreline more wildlife-friendly. The marsh vegetation will have to wait for the rainy season, but the infrastructure is in place.


These boulders are undistinguished sandstone, but they’re laid with care. I predict that the gulls will be cracking mussels on them, if they aren’t already. The rocks will also allow people to step into the marsh without kicking up the reeds and mud. And they’ll keep runaway vehicles out of the lake. Hmm, there seems to be a common thread here.

The Redwood Road boulder pile

13 July 2015


Up on Redwood Road, across from the entrance to Campus Drive, there’s a pile of boulders of Leona “rhyolite,” probably from the old quarry now occupied by Merritt College, sitting under the trees. While waiting for the 54 bus one day, they waved me over and said, Look at us. I was drawn closer, ever closer to their seductive surfaces, but kept my wits about me and escaped their clutches.









No geology lessons here, just eye candy.

Serpentinite tentacle at Merritt College

1 June 2015

My quest to get my arms around Oakland’s serpentinite patch took me to the grounds of Merritt College, where a long tentacle of this rock is mapped. It’s a complicated area, and the topography on the ground no longer matches what’s shown in the geologic map. Long story short (or in the new hip lingo, “tl;dr”), I think it’s best to show the Google Map map first, with the outline of the tentacle superimposed on it.


I’ll show photos from the four numbered locations. And then here’s the geologic map of the same area. Remember that most of Oakland’s rocks have been shoved around, crumpled, sliced by a major fault and tilted on end. It’s OK if the map doesn’t make sense.


Here’s the overall scene looking northwest from hill 1175. (Do the locals have a name for this hill? I need to know.) Redwood Peak in the background. Two ballfields and a solar array have wiped out the earlier contours of the land, which was previously a rock quarry.


The road skirting the two ballfields is cut into the serpentinite. The first locality is that light-colored patch behind the six sentinel trees. And here it is.


This is a very informative exposure showing a clear contact between the serpentinite and the adjoining shaly rocks of the Knoxville Formation (KJk). I think this is the key locality that led to this contact being interpreted as a thrust fault (signified by the black triangles, “teeth on upthrown side”). I’ll show you the evidence. First, here’s a look uphill along that contact, at the front edge of the exposure.


The rock along the contact is dark, hard and altered. Farther uphill, the serpentinite is thoroughly shattered—brecciated. The shock and shifting due to earthquakes, many thousands of them, would do this, just as they cracked the Oakland Conglomerate. The next three shots show the breccia at ever-closer range. There are lovely blue-green pebbles of the original serpentinite floating in this broken matrix.




The map’s author, Russ Graymer, interpreted this contact as a thrust fault because that’s practically the default in the East Bay hills, which have been compressed from the west for several million years and counting. He also surely noted that outside the contact zone there’s no sign of heat or chemically active fluids associated with the brecciation. That points to cool conditions near the Earth’s surface rather than some truly ancient activity when these rocks were more deeply buried. So during the recent geologic past, as our hills were being pushed upward, the serpentinite rode up relative to the Knoxville Formation mudstone, sliding on the surface between them.

The whole roadcut, wherever the rock peeks out through the grass, is serpentinite. Farther along the road is this darker example.


It’s full of swirly detail and veins of white mineral, probably calcite, that attest to hotter deep-seated activity. That’s an older story not related to the recent thrusting.

Beyond the roadcut a stream valley full of thick brush cuts through, hiding any more rock. And beyond that the ground beneath the solar array is untrustworthy, because such things are usually set upon beds of imported gravel. But on the other side, where bedrock is exposed again, there is enough dark serpentinite to justify drawing the map as Graymer did.


Next I want to trace out the uphill edge of the tentacle, which is conveniently mapped right along Fernhoff Road. Some other time.

By the way I’m finally going to subdivide the “Oakland rocks” category into a hierarchy. After nearly 8 years of this blog I’ve tagged 140 posts with that label, which is silly.


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