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.