Oakland Geology

The Lincoln Square shopping center, on Redwood Road next to Route 13, has textbook exposures of serpentinite. Last week, five years after my first quick visit, I gave it a more searching look. Here’s a view of the terrain.

A century ago this site was the confluence of three first-order streams forming East (Lion) Creek. The largest of these comes down from due north, the western branch descends south-southeast from Holy Names, and the third branch flows due west from just above the “Crest” in “Crestmont” on the map. The Alma Mine, a set of over 5000 feet of tunnels in the hillside that was active until 1921, was about a hundred yards to the east.

In the early 1960s the shopping center was built on fill at least 20 feet deep, with the streams culverted beneath it, and its footprint was excavated into the surrounding bedrock. There are two serpentinite exposures, one above the eastern parking lot by the gas station and the other behind the back of the building next to the Safeway.

The first exposure, across from Sparky’s burger place, displays horizontally streaked rock, an intimate mixture of dark-blue and greenish-yellow serpentine with lumps in it like this.

This is rock that’s clearly been squeezed and stretched, but I see no indicators of the exact direction. Either it went both ways, top-to-the-left and top-to-the-right, or the motion was perpendicular to the rock face such that any indicators would be invisible.

On to the other exposure, which looks a lot fresher.

Here the matrix around the lumps is much better exposed, and lumps of all sizes are easily seen. They range in size up to a meter; this one is more like 20 centimeters long. These are generally elongated and indicate top-to-the-right deformation.

Some of the largest lumps appear to show relict texture — that is, traces of the mineral grains in the original peridotite before it was turned into serpentinite (see my backgrounder on serpentinization).

The matrix is very soft underfoot. Right now the footing is good because the rock is wet. There’s so much moisture coming out of the slope that the drain behind the building has steady running water in it. Soil doesn’t accumulate on it, and only the stubbornest plants, like pampas grass and patches of moss, can get any purchase.

I brought along my acid bottle, as I do, and the matrix fizzes vigorously indicating that it’s full of lime. And patches of moss, like that at the top of this photo, appear to be the site of a chemical reaction that forms little white “popcorn” balls of calcite.

These accumulate where they wash out of the soil. Most measure about a centimeter, but some are several centimeters across.

I took some home for closer study. They dry as light and hard as blackboard chalk, have no internal structure or crystallinity, and fizz away to nothing in acid leaving just a breath of grayish residue, probably a touch of clay. Whether it’s true calcite or an amorphous version of calcium carbonate, I’m not competent to say. Mineral chalk is how I’ll think of it.

The Lincoln Square exposure is a minor part of Oakland’s serpentinite patch, the little ribbon of purple crossing the Golden Gate Academy on the geologic map.

I actually don’t fully trust this map; I’m suspicious of the thin green stripe of Knoxville Formation (KJk) and the exact extent of the pink Leona volcanics (Jsv). But a borehole record from farther up the hill, in the fat part of the serpentinite, describes the rock as “serpentine with lime.” I don’t associate lime, or calcium in general, with serpentinite, but in fact the minerals in the precursor rock, peridotite, do include some (clinopyroxene in particular) with calcium. Clearly I have more to learn.

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