Archive for the ‘oakland rocks’ Category

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.

Sandstone concretion, Joaquin Miller Park

11 January 2012

This odd tumorous-looking thing, on a sandstone boulder in the Oakland Conglomerate in Joaquin Miller Park, is a concretion.

concretion

I’ve documented concretions in Oakland before, in rocks of the Great Valley Sequence and in the nameless unit of Eocene mudstone above Shephard Canyon. This concretion is unlike the other two in (I assume) not having a siliceous matrix like the first and not being finely layered like the second. I assume that this is a typical featureless ball of extra-strong mineralization that formed slightly before the rest of the rock lithified. (And on KQED Quest Science Blogs this week, I talk about other concretions in the Bay area and California.)

By the way, I visited the lower end of Joaquin Miller Park the other day, below the Woodminster area where the Miller cottage is, and finally saw my sign about the rocks of the park. I hope that people have gotten some benefit from it.

The Holy Names hematite workings

29 November 2011

On the grounds of Holy Names College is a locality where the locals, before the Spanish moved in, would find and process the red mineral pigment of hematite.

hematite site

Today it’s the setting of a toddlers’ playground, without a sign of its former prestige. Nearby is Oakland historical landmark 51, the George McCrea House and Indian Campground. McCrea was the prominent architect who designed the house, and I find nothing online about the Indians. I don’t know if these boulders are part of the historical landmark, but they aren’t being treated like one.

I visited the site a couple weeks ago accompanied by a rockhound and a geologist. The boulders have numerous pits, much like the ubiquitous mortars where the natives once ground acorns.

hematite

The material making up the boulders appears to be ancient colluvium cemented by abundant iron oxides. This cementation would not happen at the land surface. They sit on a shoulder of land near a deep ravine of the Lion Creek drainage, evidently exposed by erosion.

The site is not far from the former sulfur mine in the hills above Laundry Canyon, and I was told that other ironstone boulders occur in the neighborhood.

Hematite is an excellent orange-red stain, useful for face paint and similar decoration. There isn’t much around here.

Lava rampart at Grizzly Peak

28 August 2011

The other week I noticed a big outcrop of basalt of Moraga Formation right next to Grizzly Peak and remarked that it would be worth a visit. And so I did, while preparing my post for KQED Quest Science Blogs last week on the Moraga Formation. It’s easy to reach from a couple of different pullouts on Grizzly Peak Boulevard. Here’s a view looking south.

moraga basalt

The rock is thoroughly covered with lichens, so you can’t tell much about it, but it appears to be a massive flow of lava that, like all the rocks in the Berkeley/Oakland Hills, has been tilted nearly upright with the top toward the northeast. Stratigraphically above it, beneath the road, it’s mapped as sedimentary rocks associated with the Moraga Formation. The area was a lake or wetland at the time, about 9 million years ago, with high hills on the west where the Bay sits today. And on the other side of the road, including Grizzly Peak itself, the bedrock is the Bald Peak Basalt, a slightly younger volcanic unit.

Here’s a better view of the west face.

moraga rampart

There’s quite a large rock face exposed on that side of the lava bed, but almost nothing exposed on the uphill side. The rock has a lot of paint on it here and there, including some marks that look like rock-climbing instructions. The rock is pretty sound for climbing, but be careful anyway because its strong exposure to sun and weather can weaken this basaltic rock without showing. (The rhyolitic rocks in Berkeley proper are better that way.)

It’s a well-used outcrop, but still, please don’t take a hammer to it as it’s located on UC Berkeley land. It’s the headwaters of Strawberry Creek.

Here’s the view back to the place where I first spotted this feature: in the row of trees just to the left of the rock.

view to claremont ridge

Hayes Creek – Dracena Park walk (#34)

10 August 2011

Last week I bought a copy of Secret Stairs East Bay, by Charles Fleming, and met the author in person at a book event at the Solano Street branch of Pegasus. As I leafed through the book and heard the author, it was clear that while the walks offer lots of insight into Oakland’s history and culture, the geologic stories to be seen on these walks were yet to be told. I thought, Well, I can try that.

Sunday my wife and I took one of the walks, number 34, to Dracena Park (featured here before) in Piedmont. It begins at Chapel of the Chimes on Piedmont Avenue and takes you on a pathway across the valley of upper Glen Echo Creek (which I’ve called Mountain View Valley). Here’s the view back toward the Chapel from the other end.

glen echo creek valley

The stream was culverted long ago, but on the 1897 topo map it’s shown as Hayes Creek. Today it’s Glen Echo Creek. The valley floor is so flat because it was graded and planted to houses. But in my unprofessional opinion, its flood hazard today is as low as anywhere.

Onward! The book directs us to the head of this little gorge, part of Pleasant Valley Creek’s watershed, and thence to the old quarry pit now known as Dracena Park.

pleasant valley creek

You should always suspect humans as a land-shaping agent in Oakland, and indeed Walter Blair, who ran the quarry and before that a dairy at this site, may have had a flume or a transport line of some sort here. But its original form appears to be intact.

We turn into the park proper, and glorious bedrock appears—Franciscan sandstone, ready-fractured for its purpose.

sandstone

Go ahead and inspect the stuff; no hammer is needed (and none allowed anyway) when it crumbles so readily. Fracturing and tectonic movements—and surely some seismic work, like a bartender’s cocktail shaker—has rubbed and even polished parts of the stone.

hand specimen

Fleming says that the stone went into the homes of Oakland, but that is not true. This is not dimension stone by any means, but rather the usual quarry of Bay Area stone hunters in general: crushed stone and aggregate for roadbeds, underlayments and concrete mixes.

What was once a noisy scene of dynamite and dust is now a green bowl punctuated by the cries of children.

dracena bowl

The walls of the park are pretty well greened over, but watch out anyway: bedrock exposures are not forever. Maybe in a marble or granite quarry, where solid rock is sawed away in blocks, but here rockfall is a continuing potential hazard.

rockfall

These are not decorative boulders emplaced by landscape designers, but fallen rock. And that ivy-covered fence at the left? It’s really a safety measure to keep landslides away from picnickers. Here’s the whole thing.

slide guard

Dracena Park is a worthy way to remake an old quarry. But if you’re here when an earthquake strikes, get away from the walls.


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