Oakland Geology

The full title of this chapter is “Leona Heights and the Southern Oakland Hills.” This area is dominated by an important and unusual set of rocks that share the same deep geological story. It’s also an area that once had a bunch of sulfur mines, which is a totally different kind of geological influence on Oakland history than those I’ve talked about before.

Leona Heights used to be a steep little hidden valley where the Ohlones came regularly — not to gather acorns or hunt game, but to mine a natural deposit of ocher. This earthy stuff, consisting of various iron oxides, is our oldest known mineral pigment. It dates from the Stone Age cave paintings and probably earlier. California tribes are sophisticated users of body paint, and ocher is so uncommon in this part of the state that the Ohlones had a lively trade in it.

The Spanish missionaries had no interest in the ocher and neither did the Mexican ranchers who took over the land, although their Indigenous ranch hands seem to have valued the site. (A small remnant is preserved at Holy Names University.) It took the Americans to dig it all up and turn it into house paint. Then underneath the ocher they found rich deposits of pyrite, which was once essential for industrial purposes (it isn’t any more).

In chapter 5 I talk about how Piedmont in the early days was a jarring combination of elegant hillside estates and noisy rock quarries. In Leona Heights too, real-estate schemes and resort hotels shared this territory with sulfur mines and Oakland’s largest quarries. The name “Leona Heights” was a developer’s coinage; it was formerly called Laundry Farm. Within a few decades after the mines closed, the area got developed in various ways, and the signs of the old days are fading but still vivid.

The rocks that contained all that pyrite are the oldest in Oakland, and they have a long history that started with volcanic eruptions under the ocean. To tell that history requires me to bring in the basics of plate tectonics, the “Earth machine” that rearranges the planet’s large-scale surface features. This three-page passage may be the steepest learning curve in the book, but it takes the reader well inside the geologist’s mindset. And with that I can proceed.

In brief, the Leona Volcanics belong to a thick slice of the Earth’s crust from the ocean floor that was hauled onto the land and stuck there, “an orphan handed off to a new parent. . . . This uncommon feature—a chunk of seafloor crust marooned on land—is called an ophiolite. Having pieces of one in Oakland is a privilege.” Oakland has three separate pieces of this ophiolite, the second being the gabbro of San Leandro and the third our lovely serpentinite, which I discuss in chapter 11.

None of this history was anything we learned about by poking around in the mines. But geologists were doing that a hundred years ago, typically graduate students working on a thesis for Stanford or UC Berkeley. I use this chapter to introduce geologists who have practiced here, what they found, and what kind of effort it took.

Though mining and quarrying ended long ago in Leona Heights, geology continues.

The pyrite-riddled rocks are collecting new coats of ocher as surface weathering breaks the pyrite down into iron oxides, which paint the rocks red, and sulfuric acid, which etches the concrete gutters and paints the creeks yellow. The past colors the present.

This entry was posted on 24 October 2022 at 8:01 am and is filed under "Deep Oakland". 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.