More Rockridge Rocks

Lately I’ve returned to Upper Rockridge in search of the elusive Rockridge Rock. On Bowling Drive I captured this view of Cactus Rock, which perches high above Acacia Avenue. (A reader took the same photo in July 2008.)

Cactus Rock is impressive, but I still have trouble believing that picnic parties would come up there regularly; it’s 1000 feet and almost a 200-foot climb from Broadway Terrace. The climb to “Mount Ararat” would have been shorter; but I still lack evidence that a large rock sits up there.

That leads me to 5920 Broadway Terrace, where the owners have recently finished a major upgrade to the grounds revealing some very impressive rocks. (A commenter mentioned it last year.)

Click the photo for a larger view. You can’t quite see it, but the rock beneath the porch now has a fountain installed that spills in a waterfall over it into a pool. Could this house be sitting on the original Rockridge Rock?

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.

Big Rock

I was at Temescal Regional Recreation Area yesterday—you probably call it Lake Temescal—and was happy to spot one of my favorite places.

This big hunk of Franciscan something-or-other sits near the head of the lake, displaying the ugly attractiveness that the French call jolie laide. Not a hundred feet away runs the Hayward fault, and there’s a free-running stream here too. Some classic oak trees shade picnic tables, and you can even swim where fish can nibble your toes.

Big Rock is the place I used to illustrate my post “Leave the Stone Alone.” Because for some reason, people seem to respect Big Rock.

Basalt at Joaquin Miller Park

Oakland has a long narrow strip of basalt mapped along the Hayward fault between Park Boulevard and Seminary Avenue, but it’s hard to find outcrops of it. I remember searching for it in the valley south of the Mormon temple and coming up empty. There’s supposed to be a bit of it in the hill at 98th Avenue that deflects Arroyo Viejo, though that’s so shattered that it’s hard to tell what you’re looking at. But in Joaquin Miller Park there’s a second tendril of the basalt running from under Joaquin Miller High School up the creek bed. There’s a nice exposure along the road next to the Browning monument, where this chunk sits.

The bluish color is correct; like the serpentinite around it, this rock is of Jurassic age and is part of the Coast Range Ophiolite, and a long history of burial, compression, upheaval and tectonic motions has left it rather altered from its original looks. But once it was a thick flow of oceanic lava. In fact, I may be fooling myself in seeing the vague remains of lava pillows in this roadcut:

I didn’t have the time (or the magnifier) to examine this closely—just another question to follow up on some time.

At some point in the next few days, I’ll be finalizing the contents of a new interpretive sign that will be installed at the park. Along with the usual subjects of plants and animals and human history, the rocks will get their due.

Joaquin Miller Formation, Lake Chabot

The south shore of Lake Chabot—the whole eastern side actually—is a good place to study the Joaquin Miller Formation. Outcrops of this shaly unit are hard to come by, so the shoreline and the roadcuts along the trail are useful. Even so, you realize that Earth isn’t always tidy when you come upon this gulch full of siltstone boulders.

Massive (that is, unbedded) siltstone isn’t part of the Joaquin Miller Formation’s description, which is “thinly bedded shale with minor sandstone,” grading into “thinly bedded, fine-grained sandstone near the top of the formation.” Oakland’s collection of Great Valley complex rocks, which includes the Joaquin Miller Formation, is crisply marked on the geologic map. But unlike your standard street or topo map, a geologic map is an exercise in vision and interpretation and approximation. The lines might be moved by the next geologist, with complete respect paid to the previous mappers. The photo is near the “70″ mark on the map below.

Might this belt of stones actually be in the overlying Oakland Conglomerate? I don’t think so, because there are no coarse grains in them. They’re just an unusual element in the Joaquin Miller, to be appreciated but ignored in the larger context. We can’t forget about the hundreds of meters of soft, obscure rocks that surround them.

This valley has its head in a recently populated part of northernmost Castro Valley, which means that it poses a risk of allowing urban runoff into the reservoir. Clearly it carries a lot of water at certain times, even if those occasions are rare. I hope to revisit and get to know it better, if only because living bedrock appears to crop out up there.