Oakland geology ramble 2, Rockridge to Orinda

The second geology ramble — my name for a long walk that starts in one place and ends in another — is a long and rugged one, just to show you I’m not kidding about these. From the Rockridge BART station to the Orinda BART station is a walk of more than 9 miles with a thousand-foot climb in the middle.

There are several ways to do this. This summer I’ve pioneered what I’ll call the middle route on two separate outings. Here they are, first on Google Maps and then on the geologic map (both images are 1000 pixels). The photos are a mixture from both traverses.

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From the Rockridge station, the route along the south side of Route 24 is more direct while the alternative, up Chabot Road to Roanoke Road to The Uplands to Tunnel Road (dashing across Tunnel to the uphill side), takes you through more shade and past more rocks, starting on Roanoke.

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The mixed lithologies of Franciscan melange (KJfm) give way to rugged outcrops of the Leona rhyolite (pink color) as you cross Vicente Creek on Tunnel Road. Admire them at the century-old estate called The Rocks. Beyond the Fire Garden is a short stretch without sidewalks that passes a long, excellent exposure of Leona Rhyolite. This was quarried in the 1930s and again in the 1950s during construction of upper Broadway, the Caldecott Tunnel and Route 24.

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The rocks change to mudstone of the Great Valley Sequence (Ku), then the much younger Sobrante Formation (tan color), as you ascend Tunnel Road’s steady, gentle grade.

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The change to the Claremont chert is dramatic as you near the ridgetop and enter Sibley Preserve.

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Cross the park on the Round Top Loop trail, which goes through the coarse-grained sedimentary rocks of the Orinda Formation (Tor), although you won’t see much of them. Take the Volcanic Trail left, which leads into the structurally overlying basalt of the Moraga Formation (Tmb). The quarry that carved up these golden hilltops extracted that basalt. In 0.2 miles, at the right edge of this photo, is a road with a cattle gate that exits the park.

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The views change dramatically on the east side of the hills, whether you’re looking to the left up Siesta Valley . . .

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. . . or to the right toward Mount Diablo.

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Straight ahead lies the unbuilt Wilder Ranch subdivision of Orinda. The valley it sits in is the continuation of Siesta Valley, and it’s underlain by nonmarine sedimentary rocks of the Siesta Formation.

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Both valleys owe their shape to the large fold, or syncline (“sloping together”), in the Siesta Formation that’s noted on the geologic map. The Moraga Formation basalt is also downfolded by this syncline, and it crops out again in the hill with the quarry scar.

This subdivision looks bleak, but the developers are doing the job right. The lots are gray because they’re sealed with some tough, pliant substance that prevents all dust and weeds. And as you cross, the route takes the dirt road running from the intersection of Wilder and Bigleaf Roads to the big bend in Rabble Road. You’ll pass several vegetated catch basins designed to hold the extra runoff from the new properties.

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This is another example of the flood-control practices I mentioned last week.

The route goes from Rabble Road to Boeger Ranch Road, but take the straight spur between them and follow it to the end, where a footpath connects with the end of Oak Road. All of this area is mapped as mudstone of the Mulholland Formation, of which I know nothing beyond its (young) age, Miocene and Pliocene. From Oak, take Stein Way down to busy-busy Moraga Way and from there head to the BART station. Sidestep as much of Moraga Way as possible by taking Camino Encinas.

If time permits, stop for a beer at The Fourth Bore in Theatre Square. If you take this ramble the other way, stop for a beer at Ben & Nick’s. Either way, you’ve earned it.

The first time I made this trek, many years ago, I took the northern route: up Claremont Canyon, north on the Skyline Trail, then down through the Lomas Cantadas maze to Camino Pablo. That was work. I’ve hiked up the canyon on Claremont Avenue several times, but the traffic is nerve-racking. The alternatives, through the Hiller Highlands or Grandview neighborhoods, are steep, sunny trudges. On the Orinda side it would be more fun to descend through the East Bay MUD land from the Skyline Trail (for which you need a hiking permit). I plan to attempt the northern route again when the weather cools. I have a vague scheme for a southern route, too.

See ramble 1 here.

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2 Responses to “Oakland geology ramble 2, Rockridge to Orinda”

  1. ✡ Dyann Jaxon™ (@XJaxon5150) Says:

    In laymen’s terms, is there an “earth shattering” synopsis here?

  2. Andrew Says:

    The synopsis is that you cross a lot of different rock units and see a lot of different views!

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