Deep Oakland chapter 11: The Ridgeline

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.”

* * *

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

Back to Deep Oakland introduction

One Response to “Deep Oakland chapter 11: The Ridgeline”

  1. glasspusher Says:

    Very cool!

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