Archive for the ‘Oakland sediment and soil’ Category

Correlates of the Lakeside Park terrace

19 July 2021

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.

Bay Farm Island

7 June 2021

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

A paean for the flats

7 December 2020

For years I’ve been working on a book manuscript about Oakland’s geology. (The first draft of the second version is nearly finished.) It’s an intense mental project in which I must strip back, like layers of wallpaper, the human overlay upon our soil, rocks and landforms to contemplate the natural setting beneath it all. The whole time, I’ve also been walking around all parts of this city snapping photos of things that strike me. Many of them have been in the flats.

Sometimes geology matters where it seems to be absent. The dazzling variety found in any walk in Oakland’s flats rests on a geological foundation that is nearly ideal for human activities. It’s a canvas ready for painting with wave after wave of civic expression.

The Oakland flats — pretty much everything between the Bay shore and Broadway and Foothill Boulevard — served the Ohlone tribes as a bountiful meadow. The flats served the Spanish and the Mexican Californios as a fertile unfenced cattle range. None of that land exists any more. We have to envision it beneath a century and a half of development, then recognize it as the product of truly geologic time.

The flats are a wide, gently sloping apron of sediment washed out from the hills by around a dozen different little streams.

In East Oakland the apron rises to about 100 feet elevation; about twice that in North Oakland. The streams aren’t strongly committed to any particular course. Over thousands of years, they strew their mud and gravel all over, like firehoses dropped loose on the ground, or groundskeepers carefully watering down a ballfield. The result is an even plain, intricate underneath but level on top, with subtle undulations made visible only by looking down our longest streets.

The streams were easy to cover and trap in tunnels. Temescal Creek in North Oakland was an early victim. Despite efforts to “daylight” it here on Telegraph Avenue, apartments now cover it with the same finality as any other spot in the flats.

But across the street the creek is commemorated, as it is in a few other spots. Awareness is dawning that our landscape was once very different and can change again.

And if the abundance of the Ohlone’s meadow in the flats is lost to living memory, here and there it is recalled in art.

To the earliest American settlers, the flats were famously productive farmland. The virgin soil could grow carrots the size of your leg. The last remnant of that farmland, off 105th Avenue at Oakland’s farthest verge, is being revived next to this lot by Planting Justice in collaboration with a land trust that will give the Ohlone people ownership of a new foothold in their native ground.

The peach groves of Fruit Vale helped make Alameda County the richest county in the state. They supported canneries that old-timers still remember as bustling enterprises. Livermore Valley wheat was milled and turned to food in Oakland, where industries of all kinds set up shop near to rail transport and the plentiful groundwater of the flats.

To the founders of early Oakland the flats were ideal for laying out the roads and rail lines and heavy industry needed to support a great city. It was good ground as well as excellent soil. When waves of residential developers filled the flats with subdivisions, they praised the locale’s sun and soil. Odd imperfections show that not every scheme worked out as Oakland grew, but still it became a great city.

The enormous freeway system of the twentieth century relies on the same firm ground, and it hasn’t suppressed the flats’ inherent fertility.

Can you see this part of our geology? Maybe now you can. Unsung and invisible, the flats are fundamental to Oakland’s character. With apologies to Walt Whitman, you will hardly know what they are or what they mean, but the flats are good health to us nevertheless, and filter and fiber our blood. The flats are full of charm and support our spirit. That is what delights me as I walk this part of town.


Chris Granillo Art

Without the flats, Oakland would not have its soul.

The Merritt sand: A little deeper

3 February 2020

My last post was about the great sand bed that underlies Alameda; now it’s time for a fresh look at the whole geologic unit of which it’s a part: the Merritt sand.

The Merritt sand is mapped in three places: in downtown Oakland, in Alameda and in Bay Farm Island. It’s labeled “Qds” (Quaternary dune sand) on this map of sediment deposits in the Bay area (Open-file Report 2006-1037), largely surrounded by artificial fill on top of Bay mud (afem).

UC Berkeley’s indefatigable Andrew Lawson named the Merritt sand in 1914 “from its occurrence on Lake Merritt, in the city of Oakland.” He considered it a marine deposit, but our understanding has advanced since then. He noted that it was 44 feet thick in a well dug at 665 16th Street (now an apartment complex) near today’s MLK Boulevard. He also mapped it in a long trench on Telegraph Avenue, noting that the top of the sand descended from the surface, just south of today’s 21st Street, to 13 feet deep just north of today’s West Grand Avenue. I’m confident that this is it on the west side of Telegraph at 20th.

The sand ends abruptly to the east of this spot, not appearing at all in the excavation for the upcoming skyscraper on the other side of Telegraph at 20th. That was historically a boggy ground that drained down 20th Street to the lake, and the sediment there is sand and gravel with a good share of clay.

Elsewhere downtown, the eastern edge of the sand is a steep slope, for instance along the lake and at Snow Park.

Since Lawson’s time, the Merritt sand has mainly been of interest to practical geologists concerned with building sites. In the late 1940s, proposals for a second trans-Bay bridge led to a concerted geological investigation of the Bay floor covering two different routes for the bridge. In a 1951 paper, UC Berkeley’s Parker Trask and Jack Rolston reported that the Merritt sand extended across the Bay, reaching a thickness of up to 60 feet. Their cross-section along the route of the Posey Tube shows it well.

Trask and Rolston noted that the sand’s texture “is remarkably uniform” and its grains were typically in the “fine” range, between 1/4 and 1/8 millimeter, although in some places it was extremely fine, forming “material with the characteristics of loess.”

Dorothy Radbruch noted in her 1957 map of the Oakland West quadrangle (USGS I-239) that the Merritt sand reached 65 feet in thickness in a boring where the Crucible sits today. A meticulous worker, she described the material as “Sand, fine-grained, silty, clayey, with lenses of sandy clay and clay. Yellowish-brown to dark yellowish-orange. Grains consist of quartz and feldspar, some magnetite, flakes of white chert from the Claremont [Shale], minor amounts of sandstone, shale, hornblende, pyroxene, biotite. Grains angular to subrounded, frosted. Well-sorted.”

The key word for my purposes is “frosted,” a textbook sign of windblown sand. By 2000, Russ Graymer of the USGS could confidently say that the sand belonged to dunes that “probably began accumulating after the last interglacial high stand of sea level began to recede about 71 ka [thousand years ago], continued to form when sea level dropped to its Wisconsin minimum about 18 ka, and probably ceased to accumulate after sea level reached its present elevation (about 6 ka).” Here “Wisconsin” refers to a formal stage of the North American ice ages.

Large areas of Merritt sand in the Bay, more than 50 feet thick, were exploited by dredgers to build (“reclaim”) land. In one area west of Bay Farm Island, as large as the island itself, some 25 million cubic meters of sand was “borrowed” from the Bay floor and used over the years, most likely, to build up Treasure Island, the Oakland Airport, Bay Farm Island itself and Alameda’s south shore. We owe a lot to this fine sand, and by fine I mean excellent. And as the sea rises, we may need more of this Ice Age resource.