Archive for the ‘Oakland rocks’ Category

Oakland building stones: Serpentinite

26 June 2017

In a modest West Oakland neighborhood on Market Street is the modest West Grand Shopping Center. Its ordinary building is clad in rough stone, an exterior treatment similar to the Kaiser Building and many other examples.

But at the West Grand Shopping Center, the cladding consists of fist-sized pieces of beautiful serpentine rock.

The front side of the building is pristine. The rear side, on Myrtle Street, is a full block long and completely faced with serpentinite. Unfortunately the bottom seven feet or so has been painted over.

The mutable color of this stone, blue-green in the shade and olive-green in the sun, gives the building a real Oakland look. I don’t know where the stone came from. Our own serpentinite is usually bluish and not of this quality, except maybe in small outcrops in the Franciscan melange. Perhaps it’s from a quarry in the Mother Lode country. It must have taken a few carloads of rock and a crew of skilled artisans to put this together.

A few months back, when I was presenting the building stone verd antique, serpentinite’s dressed-up cousin, I said “You can’t do much with California serpentine except admire it.” Makes me happy to be proved partly wrong — you can always admire it, and sometimes build with it.

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Mountain View Cemetery: The Bay area’s best landscape

1 May 2017

Although I’m tempted just to let the photos in this post stand on their own, let me make a case that Mountain View Cemetery offers the best landscape in the Bay area.

First there’s the cemetery itself. The managers have been putting a lot of effort into improving the ground — see the excellent new stonework and gravel path in the first photo — and this winter’s abundant rainfall has abetted it by giving the hills a coat of green that ought to last longer than usual before turning gold, then brown.

Unlike your typical cemetery, Mountain View is very large and occupies the rolling terrain of the Piedmont block, consisting of Franciscan melange. In the photo below, all of the land in sight lies within the cemetery’s property.

Long ago the operators arranged for Cemetery Creek (headwaters of Glen Echo Creek) to fill three ponds, where the water can be parceled out over the dry season to help keep the turf lush. Right now they’re brim full and support a few waterfowl. The open hilltop on the left side will be turned into a new section of graves under a proposed plan.

The Franciscan outcrops or “knockers” in the cemetery’s hills echo the finished stone displayed so touchingly in the grave markers. Many historical Oaklanders famous and obscure rest here. A random walk in any direction will bring up names that ring a bell if you’ve spent significant time in the East Bay. Yes, that’s what cemeteries are for, but without the graves this territory would be just another busy park. The dead ensure that the living visitors stay on their best behavior.

And this is important — the delights of the cemetery don’t stop at its edge. The east side has had its huge hedge of overgrown eucalyptus removed, opening the crest of the high hills and the well-tended neighborhoods of Broadway Terrace to view.

A few eucalyptus trees remain on the hilltop hosting the cemetery’s high staging area. In manageable numbers, their trunks are attractive as they frame views of tempting places.

With the view east restored, there’s now a postcard vista in every direction you look. To the west you can see the Golden Gate, San Francisco, St. Mary’s Cemetery and Oakland’s outer harbor.

To the southwest is downtown Oakland and the ridges of the San Mateo Peninsula. These views, as well as those to the north and south, will always be unspoiled. From Mountain View, one can take in surroundings that encompass a large share of the greater Bay area from the midst of a setting that’s both attractive and historic.

So that’s my main case for this being the Bay area’s best landscape. But there’s more — there are rocks. I always make sure to visit this outcrop of red-brown radiolarian chert on the hillside behind the garden mausoleum, plot 3.

Other parts of the cemetery consist of shale, like this bit left behind from an excavation in plot 9.

The road up to the top of the cemetery exposes some of the well-bedded mudstone that underlies much of the grounds, but look in the gutter for the freshest exposure.

And once up there, make your way bayward from the northern tip of plot 77 to this outcrop of green and red chert.

I’m glad to entertain arguments that one place or another might be superior to our cemetery. For sheer viewshed, Mount Diablo is a candidate, as is Tamalpais. Twin Peaks in San Francisco gives excellent views of Oakland. Mount Livermore, on Angel Island, is worth a special mention. The South Bay and North Bay have many more picturesque places, not to mention the Peninsula. Lots of these spots survey a more spacious territory, but Mountain View surveys the most gracious territory, a viewshed of singular integrity that extends from infinity to your feet. In a region full of landscapes, this one offers as much elegance as it does grandeur.

East Bay diatomite

3 April 2017

The geologic map I rely on for this blog — U.S. Geological Survey map MF-2342 — extends north to Pinole, where it shows this little pod of rocks labeled “Tsa” and “Tdi” between Pinole, El Sobrante and Richmond.

Both units are of early Miocene age: Tsa stands for sandstone and Tdi stands for diatomite. The T stands for Tertiary, the catch-all term for Cenozoic rocks older than Quaternary, which — OK, you don’t need the whole lecture just now. The point is, I had to go see this diatomite because I didn’t know it existed in the East Bay. I’ve seen it in the Central Valley, but never around here.

Going north on I-80 you take the Appian Way exit right and immediately turn left on Sarah Drive. Down at the bottom of a valley is Pinole’s little, undeveloped Sarah Drive Park.

On the way to the hilltop, you start seeing this odd rock in the road. Pick up a piece and you’ll find it’s very light. That’s the diatomite.

The trail becomes very steep, exposing the bedrock. The hilltop affords nice views. I was especially taken with the view north.

And the view east looks up Pinole Valley toward Mount Diablo on the horizon. If you’re riding toward Sacramento on the Capitol Corridor Amtrak train, there’s a moment just east of Point Pinole where you can catch this same view of the mountain.

And there were butterflies.

So that’s all great. But here’s what’s cool about the diatomite.

Diatomite is composed of diatoms, the microscopic algae that make shells of silica. As an industrial commodity it’s also called diatomaceous earth, or DE, or kieselguhr if you’re feeling smart. As the stabilizing agent for nitroglycerin in dynamite, it made Alfred Nobel’s fortune, and that’s why we have the Nobel Prize.

As its silica content slowly turns into the crystalline mineral quartz, diatomite becomes the rock called chert. As it happens, the Pinole diatomite is about the same age as the chert in Oakland’s Claremont Shale. By some tectonic accident, it avoided being converted, and you can enjoy its lightweight charm without a trip to Los Banos.

Origins of Oakland ocher

27 March 2017

Before Europeans came into this country, the locals treasured the ocher deposits in the East Oakland hills. Ocher is the name for a variety of clay-like, iron-rich minerals with a color range from yellow to red to brown. For tens of thousands of years, we’ve used ocher as pigments and preservative coatings. Some cultures would bury their dead in it.

Our ocher deposits formed exclusively in the Leona volcanics, because that body of rocks was permeated with pyrite by hydrothermal springs as it rode on the seafloor toward North America, back in the Late Jurassic. Pyrite is pure iron sulfide (FeS2) and looks like this.

You can get nice crystals of it at any rock shop.

In the Leona volcanics, you’ll sometimes see pyrite in fresh exposures, like this roadside boulder along Campus Drive. It’s gray because the crystals are so small.

Oxygen, in air or in water, breaks pyrite down. The sulfur turns into sulfuric acid and leaches away while the iron oxidizes into a range of minerals on the ocher spectrum. This process reliably turns the surface of the Leona orange and red, like here in the former Crusher Quarry.

Pure, straight iron oxide (Fe2O3) is the mineral hematite, or red ocher. It can look black, but when powdered it turns the lovely color shown on the streak plates.

Between pure FeS2 and pure Fe2O3 is a range of hydrated iron oxides that form ochers of different colors. The roadcut on lower Redwood Road, at the site of the former Alma Mine, shows off some of them well. Here’s a hematite crust, which is right near a piece of concrete pavement that’s eaten out by acid.

And here’s a beautiful brown crust.

Most likely this is goethite (“GUUH-tite”), or brown ocher or sienna, an iron oxyhydroxide with the formula FeO(OH). Here’s a specimen I collected in Wisconsin, with a glittering crust of hematite on it.

Yellow ocher has even more water associated with it — the formula is FeO(OH) · nH2O. That’s what I would call this crust in the Crusher Quarry.

There are wild cards in this scheme, namely manganese oxides and jarosite. Manganese oxide, the mineral psilomelane (“sigh-LOW-ma-lane”), is black. Just a few percent turns ocher into umber. (So does carbon.) Jarosite is a hydrated iron sulfate that can form if some of the sulfur lingers instead of turning to acid. It has yellow to brown colors.

So really good ocher, in chunks worth the effort of digging, is hard to find. Oakland once had a large body of it that had slowly gathered on top of the Leona volcanics as the rock beneath was etched away by acid. Such an iron-oxide cap is called a gossan. A little bit of the deposit is preserved on the Holy Names University campus.

All of these ocherous minerals are important ingredients in soil, especially in dry regions. Rarely are they pure, though. Oakland’s ocher patch was the center of a widespread trade, back in the day. But in the late 1800s, Americans mined it out and turned it into red paint.