Deep Oakland chapter 9: Sibley Volcanic Regional Preserve

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.

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