Oakland Geology

I haven’t mentioned it lately, but I’m writing a book on the geology of Oakland. [Note added 4 years later: It turned into Deep Oakland, but that’s quite different. This manuscript might still surface as another book.] One chapter is about a road trip that takes you to nearly every different rock unit in town — actually it’s pretty much the one I posted in 2017. One problem with that posted route is that it doesn’t pass an exposure of the Orinda Formation with its beautiful conglomerate.

The Orinda underlies the four-way intersection at the top of Claremont Canyon, but you can’t see any of it there. The intersection is in a topographic saddle, and it’s a saddle because the rock erodes more easily than the lava flows and chert beds that flank it on either side. That’s right, conglomerate — that rugged-looking stone studded with cobbles and boulders — is crummy rock.

So my road tour has to detour at this intersection, going down Claremont Boulevard a little ways to a spot where you can study the conglomerate at leisure. You’ll pass excellent exposures of it on the way, but there is literally no space to stand there.

The closeup of that rock that I posted here back in 2008 (still a favorite shot) was acquired at some peril. No, instead of stopping, you go around the hairpin turn and pull over at the entrance to UC Berkeley’s open space reserve. If you’ve followed along over the years, this is a familiar stop, the type locality of the Claremont Shale. Just uphill from that classic spot, the chert beds give way to Orinda conglomerate.

Look back across the road toward the head of Claremont Canyon, and that fire road (the Summit House Trail) is what you’ll take, just to where it enters the trees.

If you hurry, these daffodils will still be there to check your credentials and wave you through.

The uphill slope is dotted with big clean boulders. Some are showoffs . . .

and others are shy.

But all are worth a close look.

I ranged up the slope here, looking for proper outcrops of the conglomerate, but there are none to be found. It seems these boulders rolled down here the last time Claremont Boulevard was upgraded, and it’s pure serendipity they’re so nicely on display for you now.

The nearest thing to an outcrop is a spot where the fire road scrapes down to bare rock. It’s a clean fine-to-medium sandstone, also part of the Orinda Formation.

This finer-grained rock is where fossils are occasionally found, but I have to tell you I’ve never seen one myself. When the Caldecott tunnels were being bored through the hills, paleontologists were hired to collect fossils from the spoil piles. (See some of the Orinda Formation fossils here and here.) The fossils testify to a warm land of woods and year-round moisture, not much like today.

These boulders look like they’ll be around for a while. Pay them a visit some time in the next few decades.

But ultimately, rocks are perishable. Sooner or later they’ll make their way to the sea and be recycled in new rocks. The cobbles preserved in the Orinda conglomerate are all that remain of a whole landscape of rocky uplands that once existed here in Miocene times, some 11 million years ago. Not even mountains last forever.

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