Oakland Geology

Shephard Creek flows down Shepherd Canyon, on the south edge of Montclair. At least that’s what the U.S. Geological Survey says on the Oakland topographic map. The city calls it Shepherd Canyon and Creek, and the road running up the canyon near the old railroad grade is called Shepherd Canyon Road. Anyway, Shephard Creek is culverted for most of its length, and only a little of it runs free at its bottom end, near its junction with Palo Seco Creek to form Sausal Creek.

When I was scouting out the Oakland Conglomerate, I went down to the creek bed behind the firehouse. It’s full of nondescript boulders of shale and sandstone like those in the foreground. But a tiny bit of bedrock crops out. The geologic map shows this area as the Joaquin Miller Formation, a shale/sandstone unit just beneath the Oakland Conglomerate that is of Late Cretaceous (Cenomanian, about 94 Ma) age.

Next time I’ll make sure to have my Brunton compass—that’s a heavy-duty compass and clinometer that geologists use to observe the orientation of rock beds, their strike (azimuth) and dip (inclination). The Oakland geologic map doesn’t show a strike-and-dip symbol here, so maybe I can add a data point to the big picture. Geologists prefer to make many observations at a locality and average them before drawing that symbol, though. Maybe this single small outcrop, rare though it is, isn’t significant enough. A little farther downstream, the boulders disappear and the streambank is serpentinite. The big patterns on a geologic map may cover up a great deal of variation.

This entry was posted on 15 March 2009 at 8:04 pm and is filed under Oakland sandstone/shale, Sausal Creek watershed. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.