Claremont chert closeup; or, Oakland hills are falling down

23 November 2015

On Grizzly Peak Boulevard, pretty much right above the Caldecott Tunnel, there’s a little old fire road that heads downhill to the west. I poked my nose down it the other day. The whole area has excellent exposures of the Claremont chert, starting with the roadside.


It’s real nice right now. The ground is moist and makes for quiet walking. Pine needles smell great. The rock is pretty.


There’s a spot where a lot of loose rock has tumbled down. The Claremont can be crumbly, because it’s so brittle, even though the stone itself is rather hard. The loose stuff is good for collecting a specimen if you’re into that. Unlike the bleached stone exposed along the ridgetop, there’s some variety here, including the black, kerogen-rich stuff that has made this formation, like its larger cousin the Monterey Shale, good petroleum source rock.




During the Caldecott Tunnel dig, this formation leaked significant amounts of oil and gas into the working space. Precautions had to be taken. The same black Claremont crops out at Alum Rock, as I showed you a few years ago, as well as at the Calaveras Dam site.

That’s all fun. But the road’s cut off by a washout ripped into the hillside, a twisted galvanized drainpipe sprawled along its path. At some risk, I scrambled across it and noted that at its floor lies the Claremont chert, which has its bedding planes oriented only slightly steeper than the gully. Treacherous ground. I don’t recommend that you follow me.


And just beyond it is another gully, somewhat bigger but not eroding as actively. Giving up on the fire road, I scrambled up to Grizzly Peak Boulevard and this is what’s at the top of that gully.


At the top of the active washout is this innocuous-looking street drain.


As far as I can tell, every one of these cute drains is carving gouges into the hillside. This one points toward the Parkwood condos.


Can’t we do better than this?

Perhaps our children can revise the old playground song to “Oakland hills are falling down.”

A lot of my outings are like this — mixtures of pleasure and concern.

Basalt at the foot of Frowning Ridge

16 November 2015

I took this photo last Tuesday, the day after our nice good rain. If you weren’t outdoors last week, you missed a brief moment in the Oakland year that lasts just a few days.


It’s the period between the first significant rain and the sprouting of the grasses.

(Before I continue, this is the last week of the scientific blog survey, to which you’re invited to respond at There are prizes, plus the good feeling of helping research. More than 100 of you have already taken part.)

The first rain drenches the ground and changes the dry, gold-brown hillsides to a rare saturated dun color. Soon afterward the hills flush green, and we’re off to a new year in the Mediterranean climate cycle that governs the Bay area. Think of it like the week between Christmas and New Years, only it’s in the calendar that plants use.

This is a special hill at the southern end of Frowning Ridge, the highland that includes Grizzly Peak and its lower, gentler neighbor Chaparral Peak. Old topo maps mark it with its elevation of 1684 feet. To the right of this photo, shot from Skyline Boulevard near Radio Tower Hill, the ground plunges to the water gap and roadcut of Route 24. On the other side of 24 the ridge resumes, under the name Gudde Ridge, and rises to the peak of Round Top. The following shots from 1684 Hill are from a visit in July 2013, during the gold season of the plant calendar.


Frowning Ridge is held up by the thick lava flows of the Moraga Formation. Like most of the rocks in the Oakland/Berkeley Hills, the Moraga Formation is tilted up to nearly vertical. You can reach 1684 Hill by an informal path off the Skyline Trail. The lower western slope of the ridge is underlain by Orinda Formation conglomerate, but basalt makes up its bulk.


Let’s look back west toward Radio Tower Hill. Last week’s photo was taken from the little saddle at the left edge.


The view north takes in the upper part of Siesta Valley. That’s Vollmer Peak in the middle, highest point in the Berkeley Hills. Grizzly Peak is just out of sight at the left, but the tip of its radio tower shows.


The view east overlooks lower Siesta Valley and Mount Diablo. On a clearer day I imagine the Sierra Nevada is visible along the left horizon.


(Again, it would be really nice if you participate in the survey of science blog readers at There are prizes, and November 20 is the last day. Thanks.

(This will be the last time I promote this study, so next week we’ll be back to normal. I’ll follow up in the Q&A/Announcements thread. Here’s the full announcement, one more time:

(Help us do science. I’ve teamed up with researcher Paige Brown Jarreau to create a survey of Oakland Geology’s readers. By participating, you’ll be helping me improve this blog and contributing to SCIENCE on blog readership. You will also get FREE science art from Paige’s Photography for participating, as well as a chance to win a $50 gift certificate, t-shirts and other perks. The survey should only take 10-15 minutes to complete.)

Read Oakland’s landscape on public transit

9 November 2015


Last Thursday I finished a long project in which I walked every block of every street in Oakland, both sides. I did this largely using public transit — our stalwart AC Transit buses and our intrepid BART cars. Both of these systems offer good vantage points to appreciate Oakland geology.

The buses give riders nice views of the hills and the bay, if you keep your eyes open and make the most of glimpses. A day pass is five bucks, and if you have a Clipper card it’s less, and if you’re of senior age it’s even less. Usually the windows are clean, and the landmarks visible en route will keep you with a sense of where you are.


The backbone routes, good for sightseeing, are the 1, the 51A/B and the 57/NL/58L. From them you can get up to the hills via several rib routes — the 11, 18, 39/339, 54, 45 and 46/46L. And that’s the real beauty of public transit, the fact that it can take you out to the land, not just in to town, work or school. And with various amounts of walking or cycling, you can then wander your fill among our rocks and scenic neighborhoods.

Then there’s BART. The windows aren’t clean often enough, but when they are the trains offer extended views of the hills with the rare advantage of continuous motion, which helps in sorting out the hills. I never tire of that.

The BART stations, conveniently, connect with the bus lines. But if you’re early for the bus, or early for the train, they’re also the best places to photograph the hills, or study them with binoculars. And here I want to give the Fruitvale station parking structure, where this shot was taken, an honorable mention.


The BART stations are the best public places to capture picture-postcard photos of the Oakland hills. The Rockridge, MacArthur, West Oakland, Fruitvale, Coliseum and San Leandro stations each have their own charms.


This winter ought to offer views that are truly more picturesque. A sky with clouds, if you get the light right, is lovelier than a big blank blue sky. And misty days can bring out the true depth of our hills, ridge upon ridge.


Urban geology, just like geology out in the country, depends on deep knowledge of the land. That comes from extended visualization, seeing the land from all angles; and from what I might call extended visceralization, walking on the land in all directions and feeling it in your bones.


Get every new post delivered to your Inbox.

Join 9,537 other followers