Rocks and views of Fairmont Ridge

Fairmont Ridge is the grassy upland that forms the backdrop to San Leandro. As it happens, East Bay Regional Park District owns much of it as part of Lake Chabot Regional Park. It has some rocks, which I’ll show first, and also some fine views.

Here’s the aerial view of the ridge from Google.

fairmontaerial

And the geology of the same area is here.

fairmontgeomap

We’ll look at rocks from three different units: the green area is underlain by the Knoxville Formation, a shaly sedimentary unit; the light-brown area labeled Jpb is basaltic lava; and the pink area labeled Jsv is Leona rhyolite, which you’re familiar with by now from Oakland.

The Knoxville is well exposed around Lake Chabot. Here, to the east of the access road at locality 1, it appears to be strongly sheared, suggesting that its contact with the structurally underlying Leona and basalt is a fault. This view is facing north, parallel to the contact.

fairmont-1

The basalt unit is formally described as pillow basalt, the kind of balloon-shaped flows you’d find where lava erupts beneath seawater. But these rocks have been shoved around a lot since they were erupted in Late Jurassic time, and I have yet to see decent pillow morphology in any exposures. Still, the outcrops, like this one at locality 2, are picturesque.

fairmont-2

The windbreak of giant, mature eucalyptus is visible in the photo. This is a naturally breezy park, and the line of trees offers some welcome shelter.

Across the ridge on the Bay side, there are more outcrops of the basalt. Around locality 3 it’s well displayed.

fairmont-3

If you pay attention, you’ll see bits of this rock with polished surfaces, or slickensides, on them. These are caused by motion on faults, which rubs rocks against each other. Here and there, proper outcrops enable us to see that the faults are oriented vertically and parallel to the ridgeline. I interpret these as forming recently as these rocks were folded and tilted upright by motion related to the Hayward fault.

fairmont-4

The peak of Fairmont Ridge is fenced off, but an informal trail leads north along the east side of the fence to locality 4. (Poison oak will very soon make it impassible.) That’s where this typical specimen of Leona rhyolite was.

fairmont-5

But sometimes rocks are just rocks. Lift up your eyes from the hills and sit a spell. You can gaze upon the Bay side . . .

fairmont-6

. . . or over the reservoir toward Mount Diablo.

fairmont-7

Looking due east is a nice prospect of the ridge known as The Knife, overlooking San Ramon.

fairmont-8

Its high point is named Wiedemann Hill, elevation 1854 feet, and I have a growing fixation with it.

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