Archive for the ‘oakland rocks’ Category

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.)

Knowland Park knockers II: Rocks other than chert

2 November 2015

The distinctive landscape of Knowland Park owes much to its large exposure of Franciscan melange, in which lumps of various rock types stick out of the ground like raisins in pudding (or whatever culinary simile you prefer). A few weeks back I featured the chert knockers, because there are so many, and this week’s subject is the ones that aren’t chert.

Before I go further, a reminder of the scientific blog survey. The deadline for participating has been extended until November 20. Here’s the info.

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. You can find the survey here:

OK. Here’s the geologic map showing the Franciscan area, labeled KJfm (for “Cretaceous/Jurassic Franciscan melange”). The places featured in this post are numbered 1 through 8 from north to south.


Knocker 1 isn’t really a knocker, but an exposure in the fire road, of greenish serpentinite.


I include it because there are relatively few in this piece of melange. Other melange areas, like those in San Francisco or Marin County, may be mostly serpentinite, but not here.

Knocker 2 is at the edge of a cul-de-sac overlooking the gorge of Arroyo Viejo. It’s a lovely greenstone.


A closeup shows the greenish rock, which is a metamorphosed lava, along with its iron-rich weathering rind and the carbonate veins that are evidence of its deep-sea origin (more here).


Knocker 3 is exposed along the road just above here, a nice graywacke, or dirty sandstone.


The Piedmont block, Oakland’s other body of Franciscan melange, is largely graywacke.

Knocker 4 is the big one, which caught my eye the first time I set foot in the park.


Its bluish color stands up to close inspection. This is a classic high-grade block, a body of rock that was carried deep into the Earth and returned to the surface quickly enough that the high-pressure blueschist minerals it turned into were preserved.


You have to look closely at these rocks to see past the lichens that tend to cover every exposed surface. Geologists carry hammers to ensure fresh exposures, but rocks in the park should not be hammered.

Knocker 5 is just up the hill. I haven’t given it a good look yet, but my initial impression is that it’s lava.


Knocker 6, across a small gully from knocker 4, is populated by a clump of trees. I think there’s a reason for that because the rock fractures nicely enough for the roots to reach deep.


I interpret it as metamorphosed lava, from its greenish color, extremely fine grained (aphanitic) character and massive fabric.


The next two knockers are outside the park — the Franciscan doesn’t honor property lines, and the Chabot Park neighborhood once looked just like Knowland Park.

Knocker 7 is on posted land at the end of the public part of Kerrigan Drive. I think it’s serpentinite . . .


. . . because that’s what’s underfoot here.


Knocker 8 is exposed along lower Lochard Drive and is too large to photograph easily. Looming over the road, it looks like basalt lava.


But a fresh exposure shows some cryptic internal features, plus extensive deposition of iron oxides from weathering below ground.


I’ve visited this site twice and am still not sure what to call it.

There are more knockers to be found in Knowland Park and south of the park. I plan to keep up my search to the south end of the Franciscan, along Chabot Reservoir.

Once again, I hope you’ll take part in the blog survey between now and November 20. It has prizes.


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