Yesterday I led a walk for the group Wild Oakland that took in the rocks of lower Shepherd Canyon, which are the westernmost outcrops of the Great Valley Sequence. These are the same kinds of rocks that make up the monumental set of ranges marching up the western side of the Central Valley from Taft to Redding. The map below shows our planned route. The red dots mark the beginning and end of the route plus mileages. (In fact, for lack of time I cut the walk short where the 3-mile mark is, so we didn’t see the loop on the right side. I leave that as an exercise—and it is exercise—for the reader.)
This is the geology along that route.
The starting point, where we climbed up to the unpaved start of the Montclair Railroad Trail, offers a nice view over the valley of the Hayward fault, here at Montclair Playground . . .
. . . and looking northwest up the fault valley.
This outcrop, above the curved cut in the railbed, shows the Oakland Conglomerate to advantage.
And maybe 100 feet away, the rock abruptly changes to shale of the Shephard Creek Formation.
This spot corresponds to the symbol on the geologic map with the number “73” on it, which means that the bedding here is tilted 73 degrees from the horizontal.
Farther up the valley, we examined this outcrop of the Redwood Canyon Formation.
I pointed out the thin set of shale beds running up the center of the image and showed how the sandstone beds on the left side had been laid down on top of the shale—that is, the internal evidence shows that this whole set of rocks here is tilted up beyond vertical and is upside-down. This spot corresponds to the symbol on the geologic map labeled “78”.
The last spot is in the Shephard Creek Formation where a large sandstone bed sits amid the shale. The location is just about where the word “Park” is along the walk route. On the underside of that sandstone bed is a splendid set of sole marks. This shot shows how the underlying shale is bent by the pressure of the overflowing sand avalanche that built this sandstone bed.
And this shot looks up at the underside. When there are a sufficient number of these marks, the geologist can work out what direction the avalanche flowed.
As I said, we cut the walk short at this point and came down through Shepherd Canyon Park along this stream valley, which is filled with a peculiarly flat deposit that I strongly believe is landfill. It forms the higher terrace in this view looking back from the soccer field.
Does anyone know the history of this piece of land?
If anyone would like a copy of the handout I prepared, I’ll send you the doc file. Just write to geology at andrew-alden dotcom.