Oakland Geology

When time began in this part of the East Bay, the people led a lifestyle based on the plants and animals. The Ohlones didn’t build with brick or mud and didn’t rely on pottery, so the soil below was not a concern. Their needs for good clay were modest.

When the Spanish took this land, they found the clay soil excellent for their purposes. “The habitations of these people,” wrote Joseph E. Baker in his 1914 history of Alameda County, “were fashioned of large, sun-dried bricks made of that black loam known to settlers in the golden state as adobe soil, mixed with straw, measuring about eighteen inches square and three in thickness, these being cemented with mud, plastered within with the same substance and whitewashed when finished. . . . When completed these dwellings stood the brunt and wear of many decades of years.”

Some of the adobe bricks from the original Peralta hacienda are preserved in Dimond Park.

Like every frontier settlement, Oakland bootstrapped its way to civilization by using what it had on hand. The very first Gold Rush visitors dug into the ground for resources, looking for what served them in their homelands whether it was good farm soil or something more specialized. In 1850, a French man by the name of Romby located decent clay along the shore of Lake Merritt in Adams Point, and with the help of fellow immigrant George LaFleche started a small brickworks there. The exact location is now lost, but I’ve seen the same promising material in excavations at Lakeside Park. It’s marine clay from the days when the sea level was extra high.

Other brickworks (“Brik Kilns”) sprang up in the early 1850s in the Grand Lake neighborhood, as seen in the Kellersberger map surveyed in 1853. I place this between Cheney and Wickson Avenues, behind the Grand Lake Theater.

There would have been lots of firewood handy for the kilns in Indian Gulch — one reason all of Oakland and Alameda’s oak forests disappeared within a couple of decades.

Romby soon relocated to better raw material on the west side of the lake where the Kaiser Center building sits today, then down to the Melrose area at the foot of High Street, where “Romby’s brickyard” was named as a boundary landmark when the Town of Alameda incorporated in 1854. A court case in 1885 referred to it as “the old brickyard house,” signifying to me that the business had closed shop, but by then it had been memorialized in the name of Brick Yard Slough.

The Remillard brothers (French Canadians) had brick kilns in East Oakland in the 1860s, but as their fortunes rose and the railroads made transport easier, they opened plants near Pleasanton, San Jose and Greenbrae. Oakland was not perfect territory for brickmakers, but it was good enough for small companies making serviceable products.

Premium potteries — making tile products and ornamental objects — found Oakland fertile ground, using coal from Pittsburg and high-grade clay from the Ione area of Amador County, and Oakland became a center of the architectural tile industry.

The flat ground of Lakeside Park (and also the Clinton neighborhood) is an unusual landform for Oakland and the East Bay in general: it’s a terrace made of gravelly sand and clay that was deposited around 125,000 years ago, a time when the sea level was several meters higher than today. I’ve featured it in previous posts, but today I’ll go farther afield, because the things we know in geology are based on more than just looking at what’s in front of us.

The formal description of this terrace, mapped as unit Qmt, marine terrace deposits, in the geologic map, includes this: “Similar terraces are located north of the map area on the south shore of San Pablo Bay in the extreme northwest Contra Costa County at Lone Tree Point, Wilson Point, and an unnamed outcrop in between. The oyster beds at the base of these outcrops unconformably overlie the Cierbo Sandstone of Miocene age and are in turn overlain by about 5 m of greenish-gray silty mudstone. The oysters have been dated by the uranium-thorium method and are of last interglacial age, approximately 125 ka.” So last week I made a road trip to see these terraces. I also looked into the literature. Both had mixed results.

Unless you live in the Pinole-to-Rodeo corridor, you’ve probably never heard of Wilson Point and Lone Tree Point. Here they are on the 1949 topo map, when the area was much less crowded. Hercules was still a company town, organized around the dynamite works there. (Now the area is being paved with over-the-top condo/townhome developments.)

These days the Bay Trail offers access to most of that stretch of coastline.

I’ve always been curious about these little bits of land along the coast because the Capitol Corridor trains go past them at a relatively slow speed. Wilson Point is on the other side of the tracks, which are fenced off. I must advise you not to cross them.

However, if you choose to ignore my advice, take the trail through the gap in the fence and carefully approach the point. This view shows the underlying rocks, with their tilted beds, and the small remnant of the terrace deposits on top. The difference in angles between the two layers is what “unconformably” refers to in the description.

Here’s the contact between the rock and the terrace deposits. The original source of the description I quoted above (USGS Open-File Report 93-286) clarifies that between the tilted bedrock and the shells is a layer of estuarine mud, the kind of muck you’ll find in a bayside marsh today. And behold, here’s a fine layer of consolidated mud topped by a fine layer of shells. These are the shells that were dated as 125,000 years old.

And above them is this hard gravel cemented with clayey silt. It is very much like the gravel at Lake Merritt: the particles are the same size, rounded to the same extent and similar in the variety of rock types represented in the pebbles.

So this is the evidence used to decide that Oakland’s terraces date from the last major interglacial. Now let’s look at Lone Tree Point, up in Rodeo. When you’re riding the train, it’s where you pass this wrecked boat. Note that the old pier on the left is resting on bedrock.

The train tracks punch through the waist of Lone Tree Point, and a bridge leads over the tracks. You can see those details in this photo. I advise you to take that bridge, which gets you onto the land at the south end where the marine terrace is, which is owned by the East Bay Regional Parks District.

Note the area of rock in the middle. That’s where I took the next photo.

This rock pavement is the hardened ancient Bay mud that was laid down on the surface of tilted rock beds. The terrace behind it preserves it all: bedrock, mud, shells, gravel.

Here’s a more detailed view; click to view its full 1200-pixel splendor.

The story goes like this: in cold glacial times the waves cut a wide flat surface across the tilted bedrock, the kind you’ll see active today along the Pacific coast of San Mateo County. Then the glaciers melted, even much of the Greenland and West Antarctic ice caps, and sea level quickly rose upward several meters higher than today, it was that warm. The bare rock platform collected a coat of Bay mud. Meanwhile streams got busy in the warm, wet climate, and washed huge amounts of gravel down from the hills into the bay, building thick deposits up to the waterline and perhaps above it. The shells got ripped up, tumbled around and somehow laid down first; I’m not clear on just how, but the shells clearly are not lying where they lived. Then the glaciers came back with a vengeance, the sea was drawn down more than a hundred meters, and the terraces were left high and dry and prone to erosion. Only little bits of them are left, especially now that the sea is high again (and rising) and attacking the entire California shoreline.

Now, in the Oakland terraces only the gravel part is visible. Assuming that there’s a shell layer and mud layer and a wave-cut platform beneath the gravel, those must exist a little below sea level. I don’t think we know; I’m not aware of any borehole records from Oakland’s terrace, although there must be some. But the same authors who documented these terraces on San Pablo Bay reported that shells of the same age have been found as deep as 40 meters below sea level in the South Bay.

The implication is that the whole southern part of the San Francisco Bay basin has been sinking while San Pablo Bay has pretty much stayed where it is. That’s what geologists do with a few data points: they make up a plausible story and see if any new evidence fits it. Oakland’s terrace appears to lie between those two levels, so the evidence, such as it is, fits.

I’m skeptical, frankly. The initial layer of mud is said to be 4 meters thick. That’s not true at Wilson Point, and that’s stretching it at Lone Tree Point. Also, the supposed “oyster beds” are a shell hash, not present in all outcrops, in which real oysters appear to be uncommon. Sometimes this happens when you check everything out on the ground.

The middle East Bay shoreline has three lumps in it, three bodies of ice age sand dunes that would seem more at home in San Francisco than over here. The first, the biggest, underlies downtown Oakland; as the city border signs say, it reaches an elevation of 42 feet right at City Hall. The second underlies Alameda, the former peninsula, and has a maximum elevation of about 35 feet. The third, smallest and lowest of all, barely over 10 feet, an accident of the modern sea level, is Bay Farm Island.

The Ohlone tribes came here to harvest shellfish from the tidal flats and bird eggs from the fields, although they apparently did not stay long or build shellmounds on the place they called Wind Whistle Island. The earliest maps show a small area of treeless land with marsh on three sides and a sandy bluff facing the Bay. This is Captain Beechey’s map, surveyed in the late 1820s.

The first USGS maps, from the late 1890s, accurately show the original island. Note that the marshes drained away from the Bay, a sign that the island formerly extended farther east.

Like Oakland, the land was settled by squatters in the early 1850s, but instead of real-estate speculators they were farmers who quickly spotted the advantages of clean virgin soil, a high water table and easy access to the San Francisco markets. They did so well, this isolated patch got its name almost immediately. Bay Farm Island asparagus was famous — farmers cleared $500 an acre in Gold Rush dollars — and having grown it myself I can see how that crop would thrive in this excellent fine dune sand.

In the 1870s, efforts began to drain the marshes and turn it into hayfields and “made land.” A 1878 map neatly juxtaposes the old property lines on the natural island and the new speculator lots on the reclaimable land around it. The outline of the firm ground was a miniature of the Alameda peninsula, a baby slipper next to its parent as seen in the geologic map.

^{Thompson & West map, 1878 on davidrumsey.com}

Maps from around 1900 show the island divided into large farm lots, with windmill-powered wells and long windbreaks planted against the prevailing northwesterlies. The twentieth century nearly erased all of this geography, as the former marsh was built up into the Oakland Airport, the Corica Park golf course and the Harbor Bay residential and business development. An overlay of the 1878 and current maps shows that there is no natural shoreline left.

Today Shoreline Park, at the western tip of the original island, is a manufactured shore on a high berm, armored with riprap. Inland, high residential walls and mature trees blunt the stiff wind off the Golden Gate.

But the pervasive landscaping and air traffic overhead can’t camouflage its eerie setting, a naked, remote, windswept place in the belly of the Bay.

What might Gertrude Stein, who famously bemoaned the loss of the Oakland she recalled from her youth, have said had she come instead from temporarily Bay Farm Island?

Read an excerpt from Eric Kos and Dennis Evanosky’s book on Bay Farm Island