Geologizing on the bus: The 14 and 62 lines

28 November 2022

I gave up owning a car ten years ago and I’m big on our bus system. The 33 line, as I’ve written, is good for access to the hills of Piedmont and Montclair. The 14 and the 62 will give you a good tour through, not to, the lower levels of Oakland’s geology.

Both lines run from the West Oakland BART station to the Fruitvale station, but on separate routes that share just one six-block stretch and one intersection. I like them both because they’re residential, not arterial routes; they connect neighborhoods, not endpoints (unless a roundabout ride is just what you want). People get on and off all the way. Here’s the 14, as shown on the street map over at AC Transit.

And here’s the 62.

And here they are superimposed on the geologic map.

Their west ends cross the plain of former ice-age sand dunes that underlies West Oakland and downtown (Qms on the map, for Merritt Sand). It has a very low, almost imperceptible dome shape that you can perceive as you look down side streets. The hills are nice to watch, too.

The West Oakland plain from the 14

The downtown platform from the 62

Both routes dip as they cross the outlet of Lake Merritt, crossing artificial fill (af on the map) laid down on what was once a wide marshy slough.

The E.18th Street landing from the 14

Then they rise onto another very flat plain, just a couple meters lower than downtown. This is the same marine terrace (Qmt on the map) that underlies Lakeside Park, built up when the sea sat extra high for a while in the late Pleistocene.

The marine terrace’s east end on E. 14th from the 62

The two routes cross at 8th Avenue and E. 18th Street, and soon they both leave the terrace and climb into the long set of intricate low hills I call the Fan (Qpaf on the map, for Pleistocene alluvial/fluvial). These are made of million-year-old stream gravel that was uplifted, in my interpretation, along with the bedrock hills of Piedmont by interactions on the Hayward fault.

In the views from the bus, the topography competes with a set of interesting buildings that represent the whole twentieth century, plus glimpses of the hills and the Bay, especially at the higher elevations.

Old palm allee of the Smith estate on the 62 route

Topography near San Antonio Park. The 14 runs through this valley.

They also go up and down some of the stream valleys in the Fan. Both routes touch parts of Brooklyn (14th Avenue) Creek (though the 96 is the best line for that) and they share a stretch of 23rd Avenue Creek. The 14 also follows the valley of Courtland Creek along High Street.

Look for the edge of the Fan’s gravel hills wherever you cross Foothill Boulevard. The 40 line runs along Foothill, following an old Ohlone footpath. I like watching the Fan go by when I ride it.

The east end of both routes, at the Fruitvale station, sits at the northern extreme of an arc of coastal plain that extends unbroken around the Bay to South San Francisco (Qhaf on the map, for Holocene alluvial/fluvial).

At High Street, E.14th heads across the flats as seen from the 14

Once these flats were all orchards and farmland that made the Bay area (and Oakland) an agricultural powerhouse, and Fruit Vale, the floodplain of Sausal Creek, was one of the earliest nuclei of that industry.

Other bus routes go through the Fan and offer similar rides, but the 14 and 62 really focus on it. The 57 and NL are classic arterial routes that cross the Fan on its high inner side. If you happen to catch one of the plush transbay commuter buses, the NL can’t be beat but the 57 will give you Oakland color from end to end. The Fan ends just before the Eastmont Center at 73rd Avenue; Evergreen Cemetery sits on its tip.

The last thing I want to say is that AC Transit is going to be taking a hard look at its routes soon. I intend to enjoy the 14 and 62 when I can and speak up to preserve them.

Oakland stone landmarks: The Lakeshore henges

14 November 2022

There are three stone circles — little henges — on Lakeshore Boulevard, one near El Embarcadero by the library and two up near Mandana. They all have the same stones, supposedly from the same source.

The first one we all know, but maybe not its name: Astro Circle.

It’s a large ring of stones in Eastshore Park, 120 feet across, with a tree, swings, slides, a water fountain and a nice new toilet on the sidewalk. It was dedicated in 1968, at the height of Apollo fever, and sported various space-related features including a steel “flying saucer” made from a boiler tank by the city parks department. (Read more over at the Oakwiki.) Also this concrete “moon cheese climber” that if anything has gained in charm over the years.

The other two, in skinny little Mandana Plaza Park, on Lakeshore between Mandana Boulevard and Prince Street, are much smaller.

The plaque there, placed in 1958 by the local Kiwanis Club, calls the two rings a “Creative Play Area.” The installation dates from the same period as The Thing in Lakeside Park, when progressive thinkers were reimagining children’s play as vehicles for cognitive and physical enrichment. A Tribune article from the time tells more about the scheme, which was called “Just Imagine!” and originally had three rings and a much more elaborate set of features. Does anyone have old photos of these things?

But yes, I’m here to talk about the rocks. All three circles are made of large blocks of cut and dressed sandstone. Specifically, it’s a medium to coarse grained lithic arenite (mostly clean quartz) with sparse gravel clasts. The grains are angular and subangular, suggesting a nearby source in granitic basement rocks. This block at Astro Circle displays a large mudstone clast; others feature small siliceous pebbles and holes where shale clasts apparently eroded out.

It’s decent material, not good enough for monuments but quite adequate. The blocks are expertly dressed, to judge by the tool marks, and were clearly salvaged from a demolished building.

The newspapers report in both cases that these blocks were once part of the old Oakland High School (1871-95), and were supposedly brought “round the Horn” from Indiana.

It’s plausible that they came from Oakland’s first high school, a fine old building at Market and 12th that the city was very proud of.

Bancroft Library image

The stones would have come from elements of the experior like window ledges, pediments, lintels and archways. Most of the building was probably faced with timber. I’ve found no record of its construction details in the newspapers.

It’s plausible that the parks department rescued the stones and left them in a boneyard for sixty years until the enterprising Amedee Sourdry found a new use for them. (I think his agency was behind the recycled boulders of Lakeside Park, too.)

However, I can’t vouch for the bit about Indiana sandstone coming by sea around the Horn, especially (as some say) in the form of ship’s ballast. I don’t know enough yet to say for sure, but Indiana was more of a coal-producing state than a stone-quarrying state at the time. I also can’t quite square the character of the sandstone with the geology of the state. It’s more like a California-style sandstone.

I also feel skeptical about the economics of producing this not-quite-premium material, then shipping it to New Orleans for a long, perilous sea voyage to California. The transcontinental railroad was finished in 1869, so the school’s architects more likely arranged for a load of finished stone to be fabricated to order and delivered by rail. Maybe even from Indiana. It would be bragworthy.

Just stuff to ponder as I sit and watch the kids play on the moon cheese climber.

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.”

* * *

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.