How useful is the Orinda Formation?

Walking along the paved trail north of Inspiration Point, I was brought up short by a splendid outcrop of conglomerate.

It’s strongly reminiscent of the Orinda Formation conglomerate exposed to the south in Claremont Canyon, in Sibley Volcanic Preserve and along Route 24 east of the Caldecott Tunnel.

Naturally I fired up the geologic map (I keep USGS map MF-2342 on my tablet) to see how the locality is mapped. It’s the little hill northwest of Inspiration Point, right above the word “Nimitz” where a power line runs.

But instead of Orinda Formation (the orange unit labeled “Tor”), which underlies Inspiration Point, it’s mapped as “Tus,” or “unnamed sedimentary and volcanic rocks (late Miocene).” Turns out there’s a major fault that separates two big blocks of young East Bay rocks — that thick black line with the teeth that represent the upthrown side — and even though the rocks look the same, we can’t say for sure they are.

The area of “Tus” rocks is rather large; in fact it’s the largest single rock unit on the geologic map.

I poked around the literature and found that the Orinda Formation has drastically shrunk over the years. As one example, here’s part of a 1973 map of the Lafayette area (Calif. Div. Mines & Geology Map Sheet 16) that classified a bunch of rock as Orinda Formation, drawn with the exquisite attention that emanates authority.

But the details are quicksand. First, the map is not about bedrock per se, but landslide hazards. Second, the author’s citations are generally very old, not that there’s anything wrong with that, but still. Third, the author’s idea of the Orinda is a unit that mixes lava beds (lumped today in the Moraga Formation) with the coarse sediment that defines the Orinda today. A long footnote explaining his thinking shows that he basically made an arbitrary choice of stratigraphic nomenclature to match the informal usage of local engineering geologists, who tend to talk about “Orinda-type” materials (like I was thinking at my outcrop) without making sure the stuff actually matches all the criteria for the Orinda Formation.

I’ve read my share of geologic engineering reports; any large construction project has to have one prepared. They’re good for their purpose — ensuring that the work is suited for the ground — but they don’t critically assess all the details of the science. And they probably shouldn’t. Instead, they line up the existing literature, outmoded and current alike, and discuss or dismiss it on the way to reaching their conclusions and advising their client.

Maps like Sheet 16 propagate obsolete or informal nomenclature, and thus stratigraphic concepts that are outmoded or discredited persist in the geotechnical literature like zombies long after research scientists have moved on. But I don’t blame people. The old idea of the Orinda Formation, widespread and simple, was very handy. The current idea of the Orinda, constricted and specific, is less handy because it leaves a large area of bedrock with the mumbly label “unnamed sedimentary and volcanic rocks (late Miocene).”

Geologic maps aren’t written in stone. Only stone is, and we’re still learning to read it.

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One Response to “How useful is the Orinda Formation?”

  1. mpetrof Says:

    I love your ending comment. We’re experiencing the same thing reading Raymond, Wakabayashi and Bero on Ring Mountain where even the terminology of terranes, blocks and formations is in dispute.
    Several people are working assiduously in their retirement to improve the detailed mapping of the East Bay Hills. I hope they get published before were all gone.

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