Vollmer Peak and the Bald Peak Basalt

At 1905 feet above sea level, Vollmer Peak is the highest point on San Pablo Ridge and in the Berkeley/Oakland Hills. It doesn’t stand out from below — you know it by the two widely separated towers on it, to the right of Grizzly Peak — but it sure stands out when you’re on it. Here’s a view of Grizzly Peak from its upper flank.

griz-from-vollmer

Vollmer Peak used to be known as Bald Peak, which accounts for the name of the rock unit that holds it up. The Bald Peak Basalt is the youngest volcanic rock in the Oakland area, the well-defined reddish blob in the geologic map labeled Tbp (for Tertiary Bald Peak).

vollmer-geomap

I haven’t seen a lot of this rock, but it’s described as “massive basalt flows.” Here’s an example from nearby Chaparral Peak. Notice the dark color and the light-brown weathering rind, both of which are typical.

BPbasaltfromchap-peak

Its age, about 8.4 million years, is distinctly younger than the lava flows you’ll see at Sibley Volcanic Reserve, which are part of the Moraga Formation (Tmb) and date from 9 to 10 million years ago. The two volcanic units are separated by sedimentary rocks of the Siesta Formation, and apparently the Bald Peak and Siesta interfinger with each other in outcrops in the upper part of the Siesta Valley.

It’s nice and quiet up there, and the wildflowers are in progress. The peak used to be unforested, like all the high hills, and it has remnant populations of many different meadow plants.

Here’s a shot overlooking Briones Reservoir on a moist day. In clear weather the Sierra Nevada takes precedence.

vollmerviewE

And here’s the view southeast. There’s a lot to point out in it.

vollmerviewSE

Left to right on the horizon, we see the flank of Mount Diablo, the twin humps of Las Trampas Ridge and Rocky Ridge with Bollinger Canyon between them, the Diablo Range hills beyond the Livermore Valley, and Round Top on Gudde Ridge. The green valley in the middle is Wilder Valley (or Gateway Valley) in Orinda, now being developed. It’s the counterpart of Siesta Valley on the north side of route 24, which we can’t see because we aren’t high enough. The tree-studded hilltop in the middle is Eureka Peak.

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