Old fill and made land

6 March 2017

Between the 1840s and roughly 1960, the Bay area made colossal amounts of dry land through “reclamation,” a euphemism for filling in marshland with whatever was handy. Oakland was no exception. Here’s a portion of the geologic map centered on Jack London Square, Oakland’s original harbor. Reclaimed land, or artificial fill, is shown in pink.

jlsgeomap

We were lucky in Oakland that the waterfront was largely developed by the railroads. They had the money and foresight to do the job right, by mid-1800s standards, and the fill is pretty good. An exception is the building that houses Heinold’s First and Last Chance Saloon.

heinolds

The structure was built in 1880 of ship’s timbers, and in accordance with good practice it was placed on piles, driven into the fill and the mud beneath. But it has settled, especially after the 1906 earthquake, and the land has been built up around it. You won’t notice from the outside until you approach the door, which sits below the surrounding ground.

heinoldsdoor

Go inside (as every Oaklander should at least once), and you’re in a funhouse of ancient furniture and creaky walls covered with old papers, all set off by a crazily tilted floor. Fortunately it’s merely a bar and not some place people have to live in.

For real landfill folly, you’ll want to visit San Francisco. The land south of Market Street was originally a mixed bag — the bedrock of Rincon Hill and a ridge of sand dunes to its west offered firm ground, and Sullivan Marsh to their south surrounded Mission Bay with poor ground. On the geologic map, the old marsh is mapped as “Qaf,” or artificial fill. It extends to the Civic Center.

sfmarshgeomap

The area south of Market began to be developed around 1870, and the marsh was filled rapidly and willy-nilly. Within a few decades the good ground was occupied by grand hotels, office buildings and luxury residences while the bad ground — the made land — was full of working-class boarding houses, small shops and warehouses.

The marsh contained several meters of peat on the surface and saturated sand and mud beneath. Loading this land with landfill rubbish, then placing buildings on it, left it highly vulnerable to settling and, during earthquakes, liquefaction — complete loss of strength.

When the 1906 quake struck, the buildings on firm ground did well while the former marsh area was largely ruined. But everything south of Market, rich and poor, burned down within a day. All of it was rebuilt, just like before, as rapidly as possible. And for the next century, the made land has kept on subsiding.

Main streets, like 7th Street, were regularly built up to maintain their grade. Side streets like Natoma were not.

sfslump-4

Many homes sank at the same time the street were built up. This one is typical.

sfslump-3

At the same time, many buildings suffered differential or uneven settling, like Heinold’s did. Residents, then and now, just put up with it.

sfslump-2

Today redevelopment is replacing these tired survivers. The new blocks have a hip and trendy look now, if you don’t happen to notice the occasional straggler.

sfslump-1

Soon enough, the past will vanish from the South of Market. But just wait another century, or less if they have a major quake before then, and some of these new buildings will slowly turn into antique funhouses like Heinold’s.

Oakland has been pretty good about avoiding our own versions of Sullivan Marsh, but we do have strips of fill to watch out for.

Artifacts from the history of geology

27 February 2017

I had the good fortune last week to join a gathering of the Explorers’ Club at the great David Rumsey Map Center, a famous collection housed since 2016 in the Green Library at Stanford University. Laid out for our enjoyment were precious original maps from the 18th century West Coast exploring voyages of Vancouver, La Pérouse, Cook and more; maps from the 19th century Western surveys of Fremont, King, Powell and more; and two monuments of geologic mapping: William Smith’s 1815 map of England and Wales, and an edition of William Maclure’s 1809 map of the United States. I couldn’t resist a phone pic of that one.

macluremap

But you can examine the map online as closely as if you were holding a hand lens up to it, thanks to davidrumsey.com.

It’s hard to overstate how much effort it took to make this map, crude as it is. Geology was in a crude state at the time — Maclure distinguished only four classes of rocks, using the Neptunist scheme of Werner — and the territory of America could only be surveyed laboriously by foot, horseback, boat or coach. But this is how it began.

The 1815 Smith map, widely known as “the map that changed the world,” recently had a well-celebrated bicentennial, and strata-smith.com is where to get deeply into it. It was a rare privilege to take it in with my own eyes.

Another item that impressed me was a heavy lump of dark stuff encased in a silver box, engraved with the owner’s name in 1791. It was a genuine working lodestone.

lodestone1791

A lodestone is a naturally magnetized piece of the mineral magnetite. Not every piece of magnetite actually acts like a magnet, although of course a magnet will attract it strongly. It has to have a certain amount of impurities that, in effect, lock down its magnetization in a persistent state. It also appears to need its magnetization to be “set” by the strong field of a lightning bolt.

Lodestones were the first compasses. Mariners first learned to navigate with them around 1200. Later it was learned that you could magnetize ordinary iron by stroking it with a lodestone, although this was not permanent. Iron compass needles had to touched up periodically, and that was what this lodestone was for. A ship’s captain might keep one as a backup, but they’ve always been rare and expensive.

Not until the 1700s were truly permanent compass needles made using steel, and not until we mastered electricity did we no longer need lodestones. I have this magnetite specimen, about a thumbtip’s size, that acts as a magnet. I hesitate to call it a lodestone because nowadays you could make one by just zapping it.

lodestone

I was touched by these artifacts from a time when science was closely tied to artisanship, when everything was done by hand. Today, geologizing still benefits from the human hands and senses in ways that, say, physics and chemistry have long left behind.

The Rumsey Map Center is open to the public.

Two bits of gabbro

20 February 2017

I’ve noted that while the San Leandro Gabbro has a presence in easternmost Oakland, it’s hard to find. The geologic map shows what seems like a lot of it, marked “Jgb” for Jurassic gabbro.

gabbrobitsgeomap

But if you poke around on the ground, nearly all of those sites are inaccessible due to steep woods, roads or housing tracts. But once last year and once the year before, I found some. The two spots are marked on the map with white asterisks. The northern one is at Seneca Reservoir, right next to the Hayward fault, and the southern one is in Sheffield Village at the north end of Middleton Street where it meets Marlow Drive.

The northern site, Seneca Reservoir, was once the upper pit of the old Catucci quarry. (The lower pit was repurposed as the site of Bishop O’Dowd High School.) Not much of it is accessible, but here and there you can spot pieces of the quarry waste. It looks like nothing else in town and everything like San Leandro’s namesake stone.

gabbrobit-seneca

I can’t say the same for the Middleton Street exposure, but on the positive side it’s real easy to visit. Oddly, the last time I came through here, in 2013, I paid it no mind, focusing instead on the other side of the street.

gabbrobit-middleton1

This site too is very near a strand of the Hayward fault, so it’s been rattled and squeezed for quite some time. It has a battered appearance, even a little fried.

gabbrobit-middleton2

And no matter how close you get, it doesn’t show much detail. As a whole, though, it has the typical color of the gabbro: light gray with a slight blue-green tinge. This resulted from petrochemical disruption at the time of its eruption, some 165 million years ago, when a pulse of younger magma sent up fluids that changed its black pyroxene minerals into green amphibole and some other greenish minerals — an obscure process known as uralitization.

gabbrobit-middleton3

I mentioned the Hayward fault being next to the reservoir. We probably would think twice before building that there today.