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 localities.
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. Several roadcuts display 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.
Oakland Geology 
23 July 2018 at 11:05 am
I think that this is what is found along the road-cut cum trail in Strawberry Canyon above the swimming center?
16 February 2017 at 10:14 am
I live in El Sobrante. At our neighborhood park (Lamoine Park), part of the hillside was cut away to create a flat space for the playground. At the top of that cutaway, my son and I found a layer of fragile sandstone (most of it crumbles or fractures with just a firm squeeze), and that layer is rife with small marine fossils–fan-shaped shells, spiral shells, etc. I was very curious about what this formation might be, and my amateur research led me to wonder if it was the Sobrante Formation. If you ever want to check it out, the park is located across the road from the May Valley Community Center (MVCC address: 3530 Morningside Dr, El Sobrante, CA 94803).
The lower part of the park has a large cement play structure, but the sandstone I noticed was above the newer playground at the top of the park further up Morningside Drive. The marker on the map on this FaceBook page shows more or less where I found the layer:
https://www.facebook.com/pages/La-Moine-Valley-View-Park/133671320009682
There is a fire engine play structure with a slide. Stand at the end of that slide and face up the hill. There are a couple of large pine trees slightly to your right at the top of the hill before the fence. Starting with the exposed rock around the trees’ roots and then stretching to the left for about 10 yards, we have found these rocks. It’s all exposed and eroding down the hill.
If you would like to see photos of the fossils, I can email them to you. It’s probably not the Sobrante Formation, but if it is, it is a lovely and easy spot to dig around and find fossils–shaded, not on a busy road, etc. If you do happen to know what this formation is, we’d be interested in finding out. We have some great science teachers at our elementary school who would probably love to take the kids on a quick walk up the hill for a geology lesson.
Many thanks!
Sara
[The geologic map says that locality is probably part of an unnamed sandstone unit of Miocene age. The Sobrante Formation isn’t very well defined. The rocks in Oakland were tentatively correlated with the Sobrante by Dorothy Radbruch in the 1960s. I haven’t found much about the actual Sobrante Formation, although the geologic map I use shows a Sobrante Sandstone in a few places in Contra Costa County.
If the outcrop is small, I think that fossil-hunting kids would strip it clean in short order. — Andrew]
8 June 2012 at 9:15 pm
I was just up on Thorndale taking that narrow road up to Elverton and then Skyline. Didn’t know anyone else ever used the road. I definitely noticed the crumbling rock – infact ran over a few rocks. Had no idea that this was called Sobrante.