Archive for the ‘“Deep Oakland”’ Category

The geo-flaneur

26 December 2022

I’ve always been a walker. As a child I was prone to wandering off; as a teen I learned that if I walked long enough I’d arrive in another town. My legs take my head to a special place. Since moving to Oakland, more than thirty years ago, I’ve gradually transformed into a flaneur: someone who enjoys just strolling around my city, or any city, no particular place to go, free to “lean and loaf at my ease, observing a spear of summer grass.” I love to revisit Oakland’s places and watch them change along with me.

My special joy and erudition centers on our special geology. The more I see of it, the more it speaks to me. Some times it even shouts. This blog began with an urge to share what I was perceiving, and it nurtured a private dream that the urge might somehow evolve into a book. The work involved was daunting; it started late and took years of one step at a time. It was not something a flaneur would do. But last week the final text was sent to the printer, and the publisher has a page with preliminary reviews.

One of those reviewers, our own Jenny Odell, says the book “has turned me into a newcomer to my own city, but has also changed the way I will view any landscape.” She gets what it’s about.

Oakland is full of flaneurs like Jenny, and for them all I want to do is add a new dimension to their pleasures. For other Oaklanders, I hope to tempt them into the same habit. Deep down, it’s a form of citizenship.

Winter is a good time to look around while the street trees are bare. The Oakland Hills are beautiful all year round, but early winter brings them a special palette between green and brown. In Deep Oakland I say this about them: “Seen from the city below, especially from the Bay shore, the hills may seem like an even wall, or a wave of rock carpeted with woods. But look again when the air thickens with marine haze and they resolve into a series of en echelon ranges, a set of waves with profiles as crisp as if cut from paper.” This time of year, there have been quite a few days like that.

The geo-flaneur takes walks no one else would think of. I seek out the neighborhood’s eminences and declivities, its peaks and valleys. I also relish unheralded details in the views we all see.

I put in my book the geological things and places I love to visit. That doesn’t discount the joys of the everyday views: handsome buildings, landscaping large and small, the state of the sky and the season, the people along the way. I see and celebrate them too.

This city has everything.

This year marked the fifteenth anniversary of this blog. The actual date of my first post was 25 September 2007, but this year I was too busy at the time rolling through the contents of Deep Oakland to make note of it. And I don’t look back much: I’ve observed the fifth anniversary and my 500th post, and that’s about it. But fifteen years, that’s pretty good. The blog will continue as long as I do.

Deep Oakland chapter 11: The Ridgeline

31 October 2022

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.”

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I’ll be back on my regular schedule in two weeks, 14 Nov.

Back to Deep Oakland introduction

Deep Oakland chapter 10: Leona Heights

24 October 2022

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.