Rubbing rocks

13 March 2017

Rocks interact with animals of all kinds. Obviously, lizards and voles and snakes and woodchucks live on rocks and/or dig under them. Humans paint on rocks and move them around and blow them up. Today, however, I’m going to talk about animals that scratch themselves against rocks — rather, rocks that animals have rubbed for thousands of years.

Last week John Christian, a sharp-eyed and inveterate walker of our hills, showed me this outcrop next to the Little Farm in the Tilden Nature Area.

Not much to look at unless you get up close. When you do, you’ll see that it’s covered with moss and lichens, except for some oddly smooth bare spots on the outermost surfaces.

Some of these are smooth enough to gleam in the sun.

These features are well known in buffalo country, elephant country and other places around the world. Large herbivores deal with the mites and lice and other irritants in their skin by rubbing themselves against anything scritchy they can find, preferably after a nice wallow in high-quality mud or at least a good roll in the dust. These marks, in a word, are sandpapered onto the rocks.

This outcrop appears to have gotten its smooth spots from that cause. But the cover of lichen and moss shows it hasn’t been used in a very long time. Today, deer have plenty of trees to use, but historically — and prehistorically — most of coastal California was treeless because the Indians kept it that way with regular fires. However, deer aren’t tall enough to make most of these marks. Now during the ice ages, though, this was a treeless cold savanna that supported herds of elk and mammoths and ground sloths and horses and bison and camels. Could those extinct animals really have buffed these boulders?

The best case for that is on the Sonoma coast just south of Jenner. I wrote a piece about the “mammoth rocks” there, which you’ll have to pick through on the Internet Archive because the folks who paid for it threw it away. The archaeologist who discovered the site has also written it up. John and I both know that site, but I never thought to look around our own hills whereas he did. When we hiked a little farther down Wildcat Canyon and he showed me a polished boulder of the same blueschist found on the Sonoma coast, I had a shiver of recognition.

Berkeley is justly famed for the rock parks in its boulder-strewn northern hills. They, like the Little Farm outcrop, feature the Northbrae Rhyolite, a particularly tough volcanic rock that you have to climb to appreciate. Even the tiniest fingerholds are as solid as steel.

At Indian Rock, generations of climbers haven’t buffed anything smooth. Nevertheless you’ll see two kinds of smooth spots. There are slickensides, formed where rocks slide against each other.

And then there’s this wide, vertical rock face that looks like it might be very inviting to an itchy mammoth or ground sloth. And it’s polished.

A few other Berkeley rocks display the same kind of evidence, if you look closely. But I had to see if Oakland can boast it too. Thinking like a mammoth, I visited my favorite Oakland blueschist outcrop in Knowland Park to reconnoiter. It looked very mammoth-friendly, including a good site for a wallow in the headwaters of Upper Elmhurst Creek.

But no such luck. Every surface of the outcrop was rough and rugged as can be. The same with this notable serpentinite knocker farther down the stream valley.

I blame Oakland’s rocks. We don’t seem to have anything as tough as the Northbrae Rhyolite, capable of retaining a polish for tens of thousands of years. But I’ll keep my eyes open; you never know.

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.