San Francisco’s new transit center — with the 5-acre garden park on top — is worth a leisurely visit, no matter where in the Bay area you live. Naturally I had to see it too, because a reader sent me photos of a large, alluring boulder that’s part of the complex. He couldn’t decide what rock type it is. “Gneiss,” I typed back, refraining from adding “Nice gneiss.”
There are different ways to ornament a large structure with boulders, seems to me. You could make them identical, for that unified look. The FDR Memorial in Washington DC does that well with slabs of red granite. You could make them vary, like Ruth Asawa’s landmark “Garden of Remembrance” at SF State.
The transit center and the park on its roof (to which Salesforce has purchased naming rights for the next 25 years) are studded with about a dozen large boulders, from 4 to 6 feet tall. The designer’s scheme for them mixes unity with variety.
The variety lies in the personality of the stones and the mix of rock types. The unity lies in their source and their surfaces. Let me show you some personalities. The first two are up in the park and the rest are at ground level.
Oh, I should warn the sensitive that all of the boulders have had an opening carved into them to hold a steel sign either pointing to or announcing that you’re in “Salesforce Park.”
As for the rock types, these are mostly gabbro (basically, quartz-free granite), some with differing degrees of metamorphism. But each one is distinctive in details that geologists appreciate: petrology, deformation, texture. A selection:
Some of these photos show the texture that made me say “gneiss” — a banded appearance with dark and light layers. With closer inspection, though, they aren’t layered enough. They’re just a little smooshed, not stretched out like taffy.
The other thing I mentioned is the similarity of the boulders. First, they are similar in their provenance — where they come from. The only information I’ve been able to glean is that they were sourced in “the Pacific Northwest.” That doesn’t sound like much to go on, but Oregon and Washington are mostly volcanic, and these gabbros are not volcanic; they’re once-molten rocks that cooled slow and deep.
Only three areas have such rocks: around Medford in southern Oregon, in the Blue Hills of northeast Oregon, and in north-central Washington. I favor Washington, and my evidence is in the second similarity: their surfaces.
I notice two things about these boulders in general. First is that their shapes are all natural — they’re field stones, not chunks broken in a quarry, and they show some degree of rounding.
Boulders don’t just round themselves; it took a very vigorous environment to make these. Something like the enormous snowmelt flows that once ran down all the rivers of the Sierra Nevada, leaving streambeds like this, in the upper Stanislaus River. The largest rock here is the size of an SUV.
However, the Sierra rocks are scrubbed fresh and smooth, and that’s the second thing about the Transbay Transit Center boulders: they were all tumbled to a rough rather than polished state, and their surfaces have since then been exposed at the Earth’s surface for a particular amount of time, not long enough to decay into clay minerals or crumble apart, yet long enough to acquire rusty colors from iron oxides. A short time, geologically speaking, measured in thousands of years.
This surface staining takes the form of red-brown streaks and spots, as in the photos above, and an all-over patina in some cases.
Here are two different versions of the altered surface, a crust a millimeter or so thick, formed on a coarse-grained gabbro, and a thin bronze sheen well developed on a fine-grained version. In both photos the underlying rock is exposed by chipping.
All this evidence points me to a scenario in which a deeply exposed body of gabbro was broken into large pieces, which were tumbled briefly and left piled upon each other, perhaps in a steep talus slope along the foot of a cliff. There the wind and weather gave them their delicate earth-tone finish. I picture a locality along the Columbia River in north-central Washington that was inundated, over and over, by the colossal Ice Age Floods that ended (for now) about 13,000 years ago.
These boulders are cool. Give them a pat as you take in San Francisco’s newest public park. The NL bus takes you right to it, a world-class ride.
Oakland Geology 
5 October 2018 at 9:35 am
Standard approach in engineering reports is to put in a bunch of impenetrable igneous petrology jargon, then say “referred to hereafter as ‘granite.'”
19 September 2018 at 10:39 am
Thanks for the description and interesting bit of forensic geology.
One question: “As for the rock types, these are mostly gabbro (basically, quartz-free granite)…” I was under the impression that gabbro is, chemically, a coarse-grained (slow cooled) basalt, not related to granite.
[That’s quite true: gabbro and basalt have the same chemical composition. I have no way of chemically testing rocks like these; I can only go by the minerals. And when I describe rocks for the general public, I can’t take my readers down the rathole of igneous petrology, much as I enjoy it. People know what granite is — a light-colored mixture of hard minerals — and they would call these rocks granite. When amateurs pick up a piece of “granite” and are looking at its mineral grains, they identify it by comparison to true granite, ignoring the dark minerals. True geologist’s granite has three light-colored minerals — quartz, alkali feldspar and plagioclase feldspar — whereas gabbro has only plagioclase. And that’s what I saw in these boulders. (A two-feldspar, no-quartz “granite” is monzonite; an alkali feldspar-only “granite” is syenite.)
This is a greatly simplified explanation. — Andrew]