Oakland Geology

I celebrate and explore Oakland’s low lands in this chapter of Deep Oakland — a part of town that “contributed the most, and gave up the most, in support of Oakland’s development.” This is a strong claim, but hear me out.

The shoreline of Oakland was completely rebuilt in the making of the seaport and the airport by “land reclamation.” That’s the bland name of a herculean, century-long program of digging up the sandy marsh along the San Francisco Bay to create both “made land” and “made water,” transforming an old-growth continuum into a binary state. And the coastal plain of East and North Oakland has been almost fully paved over after the soil was exhausted by farms and orchards that supported a series of major canneries, then given over to railroads, freeways, factories and broad tracts of small houses. That harbor and that fertile plain made Oakland a vigorous, ambitious and proud city.

You probably knew all that, but there’s more. Beneath the flats and shoreline, Oakland found its largest water supply in deep beds of sand and gravel. Those aquifers yielded millions of gallons per day for decades before East Bay MUD was founded, bringing us Sierra Nevada water and rescuing the East Bay from economic strangulation in the drought year of 1929 with barely a week to spare. Between the shore and the flats, our lowlands, more than anything else, made Oakland a great city — and yet their geology is almost invisible now.

That’s OK, because this chapter also goes into what we can’t see but only infer: the intricate layers of sediment that underlie it all. Over the last few million years, as dozens of ice-age cycles repeated and the basin between the East Bay hills and the San Mateo Peninsula slowly dropped, layers of gravel have alternated with layers of clay to create the rich structure of water pockets that supplied Oakland’s homes and industry.

The basin also records different versions of the San Francisco Bay that have existed during the last million years or so. Until recently there was no Golden Gate; instead the Delta drained through a river that ran across the San Mateo Peninsula through Colma, south of San Bruno Mountain. And before that there was no river at all — the Central Valley was an enormous freshwater body geologists have named Lake Clyde.

The flats don’t have the charm they had under the Ohlones, who maintained them as productive marshes and meadows and prairie. The Spanish and Mexicans were content to treat them as livestock range, and the Americans leached wave after wave of profit from them. The flats still have charm, though, as I’ve said here and as I conclude in the book:

their topography, simple as a blank wall and free of bedrock barriers, is what suits them so well for human occupation. The living environment we have built upon the flats echoes the complex geologic structure beneath it. When I take a random walk through any part of the flats, I enjoy the ways the layers of Oakland’s human history, embodied in buildings and street patterns and vegetation of all vintages, sit in companionable contact. And from everywhere below, the Oakland Hills beckon in the changing Bay light.

After introducing the Fan and its deep history, in this chapter I zoom in on a special corner of it for a closer look at a braided set of topics. Indian Gulch is the stream valley that today is better known as Trestle Glen. I single it out because it embodies several geological and social ideals:

Indian Gulch may well have been the Ohlones’ favorite neighborhood, and maybe we should be calling it Ohlone Vale. It had, and lost, chances to become a great city park. Although Indian Gulch has seen many changes and the stream in it has largely been culverted, geologically minded visitors can perceive despite all a stream valley with a soul.

I’ve posted a few times already about the valley’s geology, its lower reach and its upper reach. Briefly, it has the textbook dendritic, or treelike, pattern of a stream that has completely settled into the landscape, evident here as it curves across the bottom and up the right side of this map.

Like the ideal stream, its head is in high bedrock and its foot lies at sea level, grading from stony slopes to level beds of clay.

The Ohlones had a large village in this valley, and its traditional name recognizes how that memory lingered among the tribespeople, even generations later as they toiled for the Mexican ranchers of the Peralta family. Indian Gulch has supported a rock quarry, a brickyard, a hay farm and a stinky factory that processed eucalyptus leaves.

But from the 1890s through the 1910s, Indian Gulch was a battlefield between developers and City Beautiful visionaries. Researching this story in the old newspapers was fun, and my account is more detailed than any other I’ve seen.

Indian Gulch became “Trestle Glen” when an amusement park by that name opened in the spring of 1893, part of a real-estate scheme that extended an electric streetcar line across the valley on a tall wooden trestle.

^{Courtesy Oakland Library}

The streetcars were supposed to keep going into the Crocker Highland hills and serve a new development of fine homes, but the Panic of 1893, a deep depression that lasted four years, put the kibosh on that plan.

It was thirty years before developers moved in and built it out. During that time, four different attempts were made to preserve the picturesque valley, its grassy floodplain studded with large native trees. Proponents envisioned “a gem of a natural park” in Indian Gulch, while opponents called it “a rugged canyon of revolting appearance” and “a big ranch which we never can reasonably improve.” The bond issues needed to finance a proper park never passed, and the city leaders were divided. Instead, it was Lake Merritt that became the city’s dream park, full of diverse pleasures for the whole range of citizens.

I spend a few paragraphs indulging my own vision for what “Ohlone Vale Park” could have been after a century’s history, from gardens to rock-climbing preserves. “But stop the daydream. Oakland voters probably recognized that it takes a lot of work to civilize wild land—to improve a big ranch.” Instead, just a few outcrops peek out here and there to suggest what could have been.

The city and regional parks higher in the hills are closer to “natural parks,” a term I revisit in a later chapter.

Chapter 6, the central one of the eleven in Deep Oakland, is about a feature I would bet most Oaklanders have never given a name: the distinct arc of low foothills above Foothill Boulevard and around the head of Lake Merritt. Its hills are so hidden by homes and trees these days that my favorite photo of them is this old one from 1876, looking across Lake Merritt from 12th and Webster.

Of course, my regular blog readers have seen me write about this arc for years under the name of the Fan. It’s an intriguing set of old gravel hills that lies above Oakland’s first level (the shore and flats) and below the third level (the bedrock hills of the Piedmont block and the Millsmont-Eastmont hills), the fourth level being the high Oakland Hills east of the Hayward fault.

The simplest way to illustrate the Fan is to show it on the bare topography (digital elevation model) of Oakland with the geologic map of the same area next to it:

I call these hills the Fan because their crescent shape on the map reminds me of a tattered Japanese folded paper fan. They’re shown in a darker yellow than the flats, signifying gravel and sand and clay that’s coarser, older and more consolidated than the alluvium around them.

This ragged swath of large sediment piles presents a geological puzzle. My solution to the puzzle of the second level combines all the stories of Oakland’s geology I’ve told so far.

I’ll come back to the geology in a bit. First, some history.

In the old days the Fan was just another part of the Ohlones’ rich world; they kept it mostly tree-free with regular mild fires and harvested in season the plants and small game it supported. But it stymied the first explorers from New Spain in the early 1770s, who tried to walk around it along the Bay side. The trouble was, that route dead-ended at the slough we know today as Lake Merritt, and they had to detour past the marsh through those hills which, as Friar Pedro Font recorded in 1772, “annoyed us very much with their ascents and descents.” Four years later the Anza expedition found and took the correct Ohlone trail, the same basic route that I-580 takes today. The NL and 57 bus lines are good ways to survey the Fan in comfort.

The first Europeans to settle in Oakland territory, the Peralta family, picked a spot in the Fan for their first hacienda, and ever since it’s been a desirable place to live. To the Peraltas it was good pasture, to the Americans that followed it was good land for farms and country estates, noted for its “picturesque scenery on every side,” and in the early 1900s it became Oakland’s most desirable streetcar suburbs.

I love taking long walks in the Fan’s charming ups and downs, although the deeply dissected hills around Lake Merritt (Adams Point, Grand Lake, Cleveland Heights) are more challenging than the rest.

One thing that’s significant for me as a geologist is that the Fan wraps around the Piedmont block, which is the chunk of blue colors on the geologic map representing bedrock. I can safely assume that both the block and the Fan that embraces it were uplifted together. And what did that?

As I said in chapter 1, the Hayward fault is Oakland’s prime mover. It’s been carrying this part of town north for millions of years. And if you run the fault backward in your mind, about a million years ago the Piedmont block was down by San Leandro, where the fault was forcing it past the big body of solid rock that makes up the hills there. When the irresistible force of a fault does this, the immovable objects it brings together have nowhere to go but up, and that’s what I think happened: the Piedmont block got hip-checked. As further evidence, I cite the odd presence of deep Dimond Canyon cutting through the Piedmont block.

It makes sense if powerful San Leandro Creek was flowing over the block while it was rising, carving this textbook water gap. That hypothesis — a mix of inference, deduction and corroboration — is my contribution to science. I feel pretty confident about it.

Oakland is unusual in having Piedmont, an independent city, entirely inside its boundaries. Geology accounts for that. Little Piedmont got its start with resources from the little range of hills it sits on: water, soil, stone and prime residential land. Chapter 5 of Deep Oakland is about that set of rocky hills, the same hills that Mountain View Cemetery sits on. It’s the forested ridge in this view from the north side of Claremont Canyon.

The hills are the top of an uplifted block of the Earth’s crust in an unusual place: the west side of the Hayward fault, where almost everywhere else the west side is being gradually pushed down and buried. The rocks in it belong to the Franciscan Complex, which I introduced in chapter 4. The upper part of it is melange, which is apparent as one crosses the fault from Montclair to enter the block:

The landscape in the Piedmont block feels different from the high hills east of the fault with their roller-coaster roads and steep, straight flanks; the slopes here are gentler and more rounded. What’s different is the Franciscan bedrock underfoot.

This chapter, like some of the others, traces a route and has a direction. It leads from the pass at the top of La Salle Avenue down the full length of Blair Avenue and ends at the lower edge of the Piedmont block at Broadway and Pleasant Valley Boulevard, former site of a rock quarry. Along the way the melange gives way to hard sandstone, “rock that was once worth money.”

Blair Avenue is named for Piedmont’s first American landowner, Walter Blair, a farmer from Vermont who bought property in these hills and started a dairy farm and a rock quarry to serve the young town of Oakland below. The profits helped launch Blair’s other projects: a resort centered on the mineral springs of Bushy Dell Creek, a family-oriented nature fantasy park, selling residential lots to wealthy white people and operating a streetcar line to and from Oakland to serve Blair’s realm.

The mineral springs are now remembered in old photos and in reconstructions with interpretive signage.

The bedrock on Blair’s land was the nearest source of stone for Oakland builders, thanks to its odd position west of the Hayward fault. And Blair cashed in when the city began a major infrastructure project: paving the sandy streets of downtown Oakland with crushed stone by the macadam method. Other quarries soon started up, and after a few years two of them, the Oakland Paving Company and the Alameda Macadamizing Company, teamed up in a competition-quashing duopoly that lasted until asphalt and concrete paving succeeded macadam, around 1900.

By then the pattern that Blair began was set: wealthy residences covered the desirable hills as the quarries began to fade. The residents of Piedmont voted to incorporate in 1907, two years before Oakland annexed everything around the new city and made it an enclave. Just as Oakland had begun as a genteel refuge from raucous San Francisco, so Piedmont became a place apart from the booming city around it, as did the ring of Oakland land that adjoins it.

The quarries closed, but the old pits couldn’t hide. Today they offer access to the same rocks that compose Mountain View Cemetery’s hallowed ground.

I love quarries, the way geologists do, as scalpel incisions that expose the underworld in detail. I’m grateful for their old, slowly healing scars. The quarry stone from the Piedmont block was well suited for building roads and foundations: hard, durable and consistent. It could be predictably crushed into clean stock and sorted easily into different size grades. One could make steady money with it by keeping costs low.

There have been at least six rock quarries in the Piedmont block. The two that most people see are the one holding Zion Lutheran Church on Park Boulevard . . .

and the one holding the Rockridge Shopping Center, which for much of its eighty-odd years was the largest quarry in Alameda County.

What does the Piedmont block mean to the larger landscape around it? Its location near the Bay, on the west side of the Hayward fault, means that it collects rainfall in a broad, unified catchment. For the last million years or so, every time there’s a great rainstorm the five permanent streams on the block feed that water toward the Bay in a strong, coordinated flush. If you recall from chapter 2 that most of the time, geologically speaking, the Bay is a dry plain, you’ll see why those streams keep digging a deep ravine where Lake Merritt sits.

The two odd landforms, Lake Merritt and the hills of Piedmont, are related. Okay, thinks the geologist, now what accounts for the Piedmont block? I present more clues and a hypothesis in the next chapter.