San Antonio hill, north side

Bella Vista hill is my name for the lobe of the big Pleistocene alluvial fan (see the geologic map) lying between Haddon Hill and San Antonio hill. This view is from the south edge of Bella Vista hill, at 13th Avenue and E. 23rd Street, looking at the steep northern side of San Antonio hill.

The valley separating the hills houses a stream called the 14th Avenue Creek on the watershed map. A long tongue of marshland once extended up 14th Avenue that is now a flat park between 14th and 15th avenues. Across that valley, San Antonio hill rises to about 225 feet at San Antonio Park, just a few blocks from the water’s edge. It is the most prominent part of the fan, as seen from BART.

The north edge of the hill is much steeper than the slope we’re standing on, the south edge of Bella Vista hill. So that’s three hills in succession that have this asymmetrical profile: Haddon, Bella Vista and San Antonio. That pattern does not continue, though.

Now that I’ve walked all over Bella Vista hill, I should present it next.

Did you see it?

Oakland is peppered with landslides, whether up in the hills like this little earthflow off Grizzly Peak Boulevard or down in the flats.

Now the U.S. Geological Survey wants to collect them all, the same way it does with earthquakes. By analogy with its “Did you feel it?” earthquake reporting site, the Survey’s Landslide Hazards Program has launched a “Did you see it?” site.

And now I have submitted this landslide to the DYSI page.

Searching for the Sobrante

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.

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 . . .

and this unnamed coarse sandstone . . .

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.

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

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.

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.

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

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.

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.

Another appearance of the Morton Gneiss

Not long after I documented the tombstone at Mountain View Cemetery made of the extremely old Morton Gneiss, I spotted the same stone—four polished disks of it—flanking the entryway of a house in the Grandview neighborhood.

This stuff is certainly a gift to the world. And yet just today I bent down by someone’s fence to caress a boulder of our own red chert, equally striking in its own way.

Slate on the water

Lately the city has been fixing up the walks along the waterfront, and they’re well worth a visit. This splendid boulder of slate isn’t on the newest part of the trail—it’s at the foot of Alice Street, just where the Jack London Waterfront leaves off and the Alice Street Mini-Park starts.

I don’t know where they get stones like this; they aren’t from Oakland or even from the Bay area. That’s OK.

Serpentine sanctuary

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.

See the boulders romping in their new space.

The hills of Mountain View

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.

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