Archive for June, 2012

San Antonio hill, north side

23 June 2012

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.

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?

11 June 2012

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

landslide

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

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.


Follow

Get every new post delivered to your Inbox.

Join 4,096 other followers