Archive for the ‘oakland rocks’ Category

Pershing knob

9 September 2012

Pershing Drive sits at the top of the south end of the Piedmont block at nearly its highest point, just under 800 feet. At one end of the road is a long exposure of rugged Franciscan chert that is exquisitely integrated into the landscaping, which includes native oaks as well as the usual plantings. It is a challenge to photograph but a delight to see in person.

chert

This is the high-grade, bleached and sometimes greenish banded chert of the Piedmont block rather than the beautiful red ribbon chert of the Marin Headlands. Its outcrop appears on the Oakland geologic map east of Piedmont’s Tyson Lake. The very steep slope on its bayward side is interpreted as a thrust fault separating the higher Franciscan melange from the lower Franciscan sandstone. Melange is not generally considered a strong stone by geotechnical engineers, but recent research shows that it depends. This melange is the best bedrock in Oakland, and worthy of its gracious and picturesque neighborhood.

Collecting the Orinda

4 July 2012

Lately I’ve been putting together a rock collection for the Chabot Space & Science Center. The last rock I needed was conglomerate, and I slapped my head and said “Duh! Use the Orinda Formation.” So it came to pass that I was on the Gudde Ridge roadcut, east of the Caldecott Tunnel, admiring that distinctive body of rock. But its exposures were off limits due to the construction.

orinda formation

There were two problems. Finding exposures of the conglomerate was difficult. And once I got close to the rock itself . . .

conglomerate

. . . this magnificent stone turned out to be rotten. You may think of conglomerate as a rugged rock, with all that grit and gravel in it. Indeed it can be. But the young, minimally processed conglomerate of the Orinda Formation doesn’t hold up to sun and weather very well. The pebbles work loose and the matrix turns crumbly in a matter of decades. The stuff in that impressive set of stairstep cuts is actually ready to give way, thudding dull under the hammer and totally useless for my purpose.

In connection with the Caldecott Tunnel work, the Fish Ranch Road offramp looks like it may have some fresh exposures. But it’s fenced off.

orinda formation

In the end, I had to find my conglomerate elsewhere. The fresh stone is beautiful, though, and being able to finish the collection made my day.

orinda formation specimen

Searching for the Sobrante

8 June 2012

A few weeks ago I went out looking for the Sobrante Formation, because it’s such a prominent rock unit in the Caldecott Tunnel. On the Oakland geologic map, it’s the tan unit labeled Tsm. This post features photos from the four numbered locations.

sobrante formation

The Sobrante is mudstone, laid down in a shallow marine setting during Miocene time. It varies between shale and fine-grained sandstone. The west half of the new Caldecott Tunnel bore goes through it, and paleo work there has found abundant fossils of fish scales. It’s not a competent (strong) rock, so you won’t see it outside of roadcuts. That’s what I went looking for.

I started in Claremont Canyon because the map shows the Sobrante just touching the road there. But between the Claremont chert . . .

claremont

and this unnamed coarse sandstone . . .

claremont

I couldn’t see anything beneath the vegetation. So I struck out there. When I checked out the winding part of Tunnel Road, though, I had better luck. There were several roadcuts that displayed the Sobrante, though not especially well.

sobrante

This is the mudstone close up, nicely laminated and fairly coherent.

sobrante mudstone

But the unit is prone to collapse, as those of you who bike here may remember. A big washout a few years ago has since been repaired.

sobrante slide

The third locality is on Thorndale Drive, a steep and narrow road heading down from north Elverton Drive. The rock is quite crumbly; indeed, the hillside may be moving. Here’s a closeup.

sobrante thorndale

And just a little lower is a large exposure where the rock spalls off so often that vegetation can’t get a foothold.

sobrante thorndale

If you can step out of the way of the shortcutting local drivers, this is a nice sheltered spot to study the unit.

But probably the most accessible roadcut is the fourth locality, along Skyline Boulevard between Snake and Shepherd Canyon roads.

sobrante skyline

This is the stuff I described a few years ago as punk shale.

The Sobrante continues southeast beyond the city line, a narrowing ribbon down Redwood Canyon. At the village of Canyon, the stream bed intersects the Sobrante and stays in it for a mile or so. It peters out in the Upper San Leandro watershed, and some day I’ll look for it down there.

Serpentine sanctuary

15 May 2012

Serpentine Prairie is looking good these days. With a large portion of the land fenced off from dogs and people, and with the exotic trees removed, this strange ground is healing.

serpentine prairie

See the boulders romping in their new space.

The hills of Mountain View

5 May 2012

A few years ago in this space I called Mountain View Cemetery “a manicured showcase of the lower Oakland hills.” While you’re visiting the dead, it is pleasant to lift your eyes to the hills and consider the living world.

cemetery hills

The cemetery’s ground reflects its variety of bedrock geology, as shown on the geologic map.

mountain view geologic map

The entrance area is young stream fill with a fringe of higher, older Pleistocene alluvium (Qpaf), then going outward and upward we have the Franciscan sandstone (Kfn) and then Franciscan melange (dark blue) with large enclaves of greenstone, or altered basalt (fg), and chert (fc) making up the highest hilltops. Their elevation corresponds with their resistance to erosion.

The result of this progression of materials is a concave hillslope, or a tilted natural amphitheater. And not only does that present clear sightlines to a range of landforms at a variety of distances, but the mountains of the Berkeley Hills also function as borrowed scenery beyond. All that is hard to gather into one photo, but we are all welcome to try.

“Mountain View” really should be spelled with two spaces between the words. The place is not just a view of mountains—although it is, to the east and across the bay to more mountains—it’s a mountain of views. Mountains and views. And Oakland has other examples all along its uplands.

Gudde Ridge

19 February 2012

The basalt of the Moraga Formation is spectacularly exposed on both sides of Route 24 east of the Caldecott Tunnel. From along the highway you can get an excellent view of its makeup and structure, but this view from Radio Tower Hill shows how the rock unit makes up Gudde Ridge.

gudde ridge

Click the photo for the 1000-pixel verson. Gudde Ridge runs just east of Round Top all the way down to Canyon Road, on the back side of Moraga. The town of Canyon is on its west flank. And it’s Moraga basalt the whole way. In this photo you can see the underlying Orinda Formation to the right of the basalt. It’s gray conglomerate as opposed to the red-brown basalt.

Davie quarry

30 January 2012

Davie Tennis Stadium is a set of courts in a former quarry, administered by Oakland but situated within Piedmont. But what about the stone, you ask.

davie chert

I haven’t checked the whole place out, but the exposures near the entrance display this dark chert or siliceous metashale. The quarry exploited the Novato Quarry terrane of the Franciscan complex, a large pod of which underlies Piedmont and its immediate surroundings. The terrane is mostly sandstone, but some fine-grained rocks occur in it too.

I’ll be leading a walk to this and three other Oakland quarries for the Oakland Urban Paths organization on February 11. Watch the OUP site for details.


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