Rocks of Upper Rockridge III: Blueschist

At the uppermost top of Upper Rockridge is Contra Costa Road, where amid the fine homes and gardens is this knocker of real blueschist.

Knockers are the California geologist’s nickname for blocks of resistant rock in Franciscan mélange, the smorgasbord of rock types (chert, basalt, greenstone, serpentinite, etc.) bound by shaly matrix that is common throughout the Coast Ranges. Knockers are too big to be called boulders but too small to be mapped. Anyway, this knocker is a tough stone of a deep indigo color from the high-pressure mineral glaucophane, which if you remember your Greek simply means “blue in appearance.” I couldn’t resist taking home a chip.

Unlike the garnet-mica blueschist of Joaquin Miller Park, this outcrop is almost monomineralic except for some white veins, probably quartz. It gleams like leather in the magnifier, with intricate crenulations and understated foliation—not a real schisty schist, but layered enough to qualify. I’m in love with it.

Rocks of Upper Rockridge II: Brookside rock

It’s been a while, but lately I’ve returned to walking in the Broadway Terrace/upper Rockridge neighborhood, where this Franciscan knocker sits. This is a disrupted deep-sea chert that has been heavily altered, bleaching out its color.

(See the first set from upper Rockridge here.)

It’s across Ocean View Drive from the chert cluster I showed almost three years ago, at the top of the stairway called Brookside Lane. (I like that this path and West Lane have “grownup” names instead of “X Steps” or “Y Path” like all the other footpaths up here.)

I may be wrong in calling this a knocker; it is more likely to be a boulder that was moved here. But it’s certainly local. There’s a variety of highly altered rock in parts of the Franciscan called calc-silicate, and it might look like this; I haven’t gotten a handle on it yet. That would have started out as a dirty limestone, though, and this looked like a chert. I didn’t have a magnifier with me, and of course I can’t hammer it. Darn, I’ll just have to come back.

Joaquin Miller serpentinite

Joaquin Miller Park has its own nursery, run by the Friends of Sausal Creek. The east side of the nursery is cut into a hillside of pure serpentinite. That probably keeps the weeds down, but nothing can be grown in it either.

Volunteer there, and you can experience California’s state rock yourself at close range. Or join the Butters Canyon Conservancy and see some there.

I just got back from a week in New York. They have a little serpentinite in the state, on Staten Island mostly, but where I was the land is limestone, sandstone and black shale in broad, level beds. Some of it is a little bluish. But this stuff, with its unearthly color, tectonic significance and weird habitat, is pure California to me.

Earth Science Week starts tomorrow. Take a closer look at and underneath your land.