Archive for the ‘Oakland chert’ Category

Museum-quality rocks from Oakland

30 January 2017

I keep saying that Oakland has geological features worthy of being put in textbooks. Today I’m here to show you that Oakland has rocks worthy of being in museums, and I’ve put them there.

In 2012, I was asked to put together a set of teaching rocks for the Chabot Space and Science Center. After all, other planets are made of rocks, right? It took some doing, but some of the rocks were easily available within Oakland’s borders in roadside exposures. The conglomerate of the Orinda Formation was one.

museumrocks-1

The red chert from the Franciscan Complex was another.

museumrocks-2

And of course there was our serpentinite.

museumrocks-3

All told, I made five sets of 15 rock types for the kids.

The next year I got a request from Las Positas College, in Livermore, for a boulder of blueschist. Turns out this little college teaches geology, because every citizen will benefit from a course, and students can get a head start on a 4-year degree there. I struggled one out of this streambed, where it wouldn’t be missed.

museumrocks-4

They installed it in their teaching garden as Rock J, on the left. It’s small compared to its mates, but that thing weighs a ton because high-grade blueschist is pretty dense.

museumrocks-5

My reward included a visit backstage to see their cool collections.

museumrocks-6

Then last year, I got a note out of the blue from the under-construction Maine Mineral and Gem Museum asking my help in building their collection. Maine is well known for its gemstone and mineral mines, but the state has no blueschist. I went to a quiet outcrop where it’s just lying around.

museumrocks-f

Got two nice boulders and couldn’t choose between them, so I sent them both. They told me one will go on display and the other will go in their teaching collection.

museumrocks-e

None of these are precious collectibles or gemstones. They’re just cool and educational.

I’ve pretty much stopped collecting rocks for myself because I’m not important enough. But museums are important enough.

Oakland geology ramble 2, Rockridge to Orinda

15 August 2016

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.

ramble2topo

ramble2geo

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.

ramble2-1

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.

ramble2-2

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.

ramble2-3

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

ramble2-4

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.

ramble2-5

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

ramble2-6

. . . or to the right toward Mount Diablo.

ramble2-7

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.

ramble2-8

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.

ramble2-9

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.

Shale and chert in the Claremont Shale

21 December 2015

claremont-shale

In preparation for my upcoming talk (billed as “drunken education”) for Nerd Nite East Bay, I took a three-hour tour of the Oakland hills to take fresh photos of my favorite rocks. The tour was a whirlwind one.

This is the classic exposure of the Claremont Shale in the valley of Claremont Creek, which I’ve featured here before. On this day, the light on the freshly rain-washed stone was irresistible.

In these hills, the Claremont is mostly chert — that’s the lighter colored, blocky stuff. But it’s larded with thin, soft layers of brown shale, a rock type defined as a more or less pure claystone that breaks in sheets. The layer at my hammer’s point shows how quickly the shale weathers into flakes. Most exposures don’t offer such a clear view of the shaly portion.

Clay is a sediment that comes only from the land. The sediment that makes chert is the microscopic shells of diatoms; you could say that chert is a pure precipitate of seawater. It’s intriguing to see the two so close, and so separate. Not to mention the force that so carefully tilted all of this rock upright from its original horizontal position.

With ten places to visit that afternoon, I couldn’t linger. An hour here to ponder the details would be more to my liking.