Oakland Geology

The last chapter of Deep Oakland finally gets us where you might have thought Oakland’s geology starts: in the high hills and the rocks they expose. By now you know that geology — “the study of worlds made of rocks” — is about lots more than rocks and mountains, but in the end, rocks are where it’s at.

This chapter swings through the crest of the Oakland Hills from south to north, which almost perfectly reflects the ages of the rocks. This familiar undulating skyline formed only within the last million years or two but it exposes rocks whose ages span more than 150 million years — and the corresponding history of California.

But first I step back and simply regard them, with admiration for their beauty and affection for their deeper significance: “The hills are an arena of competing Earth energies, tectonic forces raising the heights and erosional forces cutting them down. They present a landscape always under construction, a dynamic equilibrium that comes of perpetual interplay among agents of geologic action.”

Chief among those is — well earthquakes of course, thanks to the Hayward fault, but after that it’s landslides: “The hills want to slough us off. . . . Any location in the Oakland Hills, then, can expect to be destroyed, sooner or maybe centuries later, by landslides and earthquakes.” The instability of this landscape didn’t matter until people moved up there by the thousands, starting a century ago as widespread car ownership made it inhabitable.

For their sake I add a short primer on the subject.

Before I introduce the rocks themselves, I lay out how they fit the bigger picture, which is this: for a long time (even to geologists), California was the site of a classic subduction zone, a crucial part of plate tectonics. California is special to geologists because that ancient subduction zone was interrupted and broken apart, exposing lots of different pieces that helped us first understand subduction in depth. Oakland is a microcosm of the Coast Range in that its rocks represent each of the separate parts of the old subduction zone.

With that I take readers through the slices of rock in the hills that arose from subduction and its breakup. All of them are abundantly represented here in the blog, and in fact the endnotes of the book point to these posts: the Knoxville Formation . . .

the Oakland Conglomerate . . .

the serpentinite belt . . .

the Redwood Canyon Formation . . .

and the striking chert of the Claremont Shale.

I sum it all in one sentence: “Our high hills expose snapshots from all of Oakland’s geohistorical stories, from the subduction zone’s rock factory to the later transpression that broke, chewed and smeared it all sideways—topped with real live volcanoes.”

The things I’ve talked about throughout Deep Oakland are related not just to geology but to history, policy and justice. In writing this book I’ve tried equally hard to clarify the science of the changing world and deepen Oaklanders’ understanding of ourselves. “Oakland is just one place in a wide world, but one exceptionally rich in evidence of deep Earth history, deep Earth processes and deep human changes. . . . This complex city has grown on complex ground.”

This chapter refers to the following blog posts:

Upper Arroyo Viejo: My first Oakland fossil
Oakland geology ramble 6: Chabot to Leimert via the Oakland Conglomerate reference locality
Oakland’s serpentinite patch
Redwood Ridge and the Parkridge land bridge
The Eocene mudstone, part 1: East Ridge
Searching for the Sobrante
Shale and chert in the Claremont Shale

Back to Deep Oakland introduction

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 chapter refers to the following blog posts:

Oakland ocher
Rocks of the Crusher Quarry
Clay outcrop in Horseshoe Canyon
The changing identities of the Leona Quarry
Geranium Place rocks and runoff
The mine drainage of Leona Creek, revisited
The sulfur problem of the Leona volcanics

This chapter of Deep Oakland finally takes us into traditional geology: rocks and fossils. I wanted to tackle the dynamics and features of landscapes first — the deep present — which loosens the reader’s grip on time, before dipping them into the deep end. To help with this transition, chapter 9 takes a bit of a mystical turn, because geology is a visionary science. The Sibley preserve is the best place in the Oakland area for lay people to learn to see what geologists see. I think that’s why I wrote this chapter before all the others, in 2019 after a contemplative visit here on a gray autumn day.

Sibley is well known for exposing the innards of a small basaltic volcano, now turned on edge, that was active for about a million years during the late Miocene epoch, between ten and nine million years ago. The rocks there are well exposed because they were quarried during the early 20th century — in fact, blocks from the Sibley quarries apparently line some of the old gutters in our downtown streets.

But this chapter starts in the southern extension of the park, a little valley off San Leandro Creek occupied by a family farm starting in the 1860s. It’s a simple steep-sided valley that exposes a classic sequence of sedimentary rocks belonging to the Orinda Formation. It gives me a good way to introduce the very basics of traditional geology: how sedimentary rocks form, what they represent and what features in the rocks inform geologists about the deep past. From there I take a look at truly deep time: how big it is and what it means.

Conglomerate is an easy rock to comprehend as a petrified gravel bed, the kind that can be seen at any riverbed or excavation.

It illustrates the basic rules of rocks, the ones they teach in every Geology 101 course before they teach all the exceptions. What’s cool about these rocks is that they’re the dregs of a vanished world: coastal central California as it existed in late Miocene time. What would you deduce about California today from a bucket of gravel dug up off the coast of Half Moon Bay? Not everything, of course, but something. Each pebble is a fragment of a land.

Climbing the walls of the valley, we go forward in geologic time, and at the top the rock abruptly changes to hard lava, well concealed by lichens.

Here we see one world succeeded by another.

I give more details of what we know about these past worlds as I shift to the northern part of Sibley. There quarry operations have dissected these same hills, revealing them to geologists, and excavations related to the Caldecott Tunnel have yielded a rich fossil record that tells us much about the late Miocene environment. From there I expand the geologist’s view to the larger Bay area and where Sibley fits in that setting.

From here on out, Deep Oakland stands on a bigger stage.

This chapter refers to the following blog posts:

Claremont Canyon conglomerate
Rocks of the Caldecott Tunnel
Caldecott Tunnel and the Orinda Formation
Amygdules
Oakland geology ramble 7: South Orinda to Montclair