Archive for the ‘Oakland rocks’ Category

Twenty Oakland rock types in a 30-mile drive

2 October 2017

As far as I can tell, Oakland has more rock types within its boundaries than any other city in America. When I added them up for a talk I gave at East Bay Nerd Nite, I counted more than 25, from limestone to blueschist. This 30-mile road trip will take you to most of them, with the Hayward fault as a bonus. It starts above the UC Berkeley campus and runs south the length of the high hills, then goes back north through Montclair and Piedmont to end at Mountain View Cemetery.

That’s 1200 pixels wide if you click on it, but don’t worry, I’ll show it in pieces below for more detail. In fact, let’s go to part 1 right now. It goes from Grizzly Peak through Joaquin Miller Park on Grizzly Peak and Skyline Boulevards.

The numbered segments correspond to the formations on the geologic map, as follows:

  1. Moraga Formation (basalt, andesite, tuff)
  2. Orinda Formation (conglomerate)
  3. Claremont Shale (chert, shale, dolomite limestone)
  4. Sobrante Formation (mudstone, shale)
  5. Unnamed mudstone/sandstone
  6. Redwood Canyon Formation (sandstone, siltstone)
  7. Shephard Creek Formation (sandstone, mudstone, siltstone, shale)
  8. Oakland Conglomerate* (conglomerate, sandstone)
  9. Joaquin Miller Formation (sandstone, shale)

Then there’s part 2, from Joaquin Miller Park to the edge of Montclair on Skyline, Grass Valley Road, Golf Links Road, Keller Avenue, Campus Drive and Redwood Road.

  1. Serpentine (serpentinite, blueschist)
  2. Oakland Conglomerate* (conglomerate, sandstone)
  3. Knoxville Formation (conglomerate, shale)
  4. Leona volcanics (metatuff, metabasalt)

Part 3 takes the freeway to Montclair Village, where you won’t see any rocks, then goes down Moraga Road to Piedmont’s Dracena Park and over to Mountain View Cemetery, where rocks are abundant.

  1. No rocks to be seen, but do stop on Medau Place and spot the offset curbs where the Hayward fault crosses it
  2. Franciscan melange (argillite, metachert, greenstone)
  3. Franciscan sandstone (sandstone, siltstone)
  4. Franciscan melange

The cemetery’s melange has many bodies of hard rock (knockers) that stand above the ground. They have their own blog category. Search this site, or check the category list on the right, for posts I’ve written about these rock units.

*The cobbles embedded in the Oakland Conglomerate offer more rock types, including granite, quartzite, gneiss and schist. That’s how I get up to 20.

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Sibley sights: Lapilli tuff

18 September 2017

Sibley Volcanic Regional Preserve is the site of a small volcanic center that was last active about 10 million years ago. After it fizzled out, the whole thing was gradually buried in younger sediment. Within the last few million years, the action of the Hayward fault squeezed, folded and uplifted this sequence of rocks and tilted it almost to perpendicular. Then erosion of the uplifted hills — and quarrying by a Kaiser company — exposed a good cross section to view.

Stop number 10 on the self-guided geology tour is an out-of-the-way spot where a rare and striking example of lapilli tuff is exposed. Each time I pass by — three times so far — I can’t resist photographing it. There was May 2005:

There was June 2009:

And there was just last month.

I need to unpack the name “lapilli tuff.” Tuff is a rock type consisting of ash — volcanic material that’s been explosively erupted and then lithified. It’s formally called volcaniclastic material: pulverized rather than solid lava. Lapilli is the name for ash particles of the same size range as gravel, or 2 to 64 millimeters across.

These lapilli (a single particle is called a lapillus) are very consistent in size and texture. They suggest that a spray of red-hot lava was erupted from a volcanic vent nearby and fell together in a neat pile. Perhaps there were strong winds at the time that sorted the droplets by size. Whether the lapilli were still so hot that they fused together before they fully cooled — an agglutinate — or fused together later when cold — an agglomerate — is not clear to me.

Whatever the circumstances were, they were unusual enough that only wide-ranging geologists and professional volcanologists are likely to have seen more than one example of rock quite like this. It merits the specialized name lapillistone, because it appears to contain very little material other than lapilli.

Oddly, it seems I never photographed the same rock twice during my visits to this spot, although it’s possible the rocks eroded beyond recognition over that 12-year span. Will have to keep coming back.

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.

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.