Archive for February, 2011

Oakland EarthCaches

24 February 2011

When it comes to exploring Oakland’s rocks, I’m pretty self-sufficient, once I’ve consulted a map. I don’t mind a climb or a scramble, and I can find little geeky things that interest me pretty much everywhere. But for most people, rocks are unfamiliar things that don’t have obvious messages. Maybe those people are more comfortable at home in familiar surroundings. Even a nearby place like Round Top, an everyday sight, seems better admired at a distance.

round top earthcache

People need other people to show them geology. Here’s a new way to make that happen. EarthCaches are a kind of geocache that don’t require you to find a hidden box and trade a trinket; instead, they’re places to stand, specified by a precise GPS setting, where you can read a prepared lesson about the geology of that spot. Learn more about them at

There are more than ten thousand EarthCaches around the world, each one vetted by geologists through the Geological Society of America. As of today, there are seven in and around Oakland:

Round Top
Orinda Formation on Skyline Drive
Orinda and Moraga formations east of the Caldecott Tunnel
Rhyolite above Claremont Canyon
Claremont Chert (you’ll find this familiar)
Spheroidal weathering in Briones Park
Shell Ridge, Walnut Creek

Seems like there could be a lot more.


Geological Design, and Oakland’s stone industry

19 February 2011

Block by block, I’m exploring Oakland. In West Oakland there isn’t much geology, but it has a thriving (or at least widespread) stone district. This small fabricator is on Peralta Street.

geological design

This steel sign above the lovely window screen appears to say that the company name is actually “designs,” not “design.” So does Google.

geological design

Stone, like concrete, brick and aggregate, is among the first things a new settlement demands. Like their wares, stone dealers seem to hang in there for the long term. I find them charming, but of course even stone has fads and trends and every other challenge a business experiences. The extra challenge of a stone business, I suppose, is gravity.

Stone shops tend to cluster around heavy transport, and an old rail line runs past this address. Other stone yards occur in East Oakland near the tracks there. And another common site for very specialized stone businesses is near cemeteries. I often stopped and admired the monument shop on Piedmont Avenue just down from Mountain View Cemetery, a business that went back to the 1850s.


4 February 2011

Along the south shore of Chabot Reservoir, in the mudstone of the Joaquin Miller Formation, I spotted this exposure of tiny, enigmatic mineral crystals last year.

joaquin miller formation

They’re sprinkled across what looks like a bedding surface in the mudstone. This shot shows an area just a couple inches across. Because I was just taking a walk, I didn’t have my hand lens with me and I didn’t try to bring some home, so this is all we have to work with until I, or someone else, finds the spot again.

A medical maxim that’s just as useful in geology is, “where you see hoofprints, look for horses and not zebras.”

Most of the time, large grains in a mudstone are quartz. The crystals are elongated and appear to be prisms with points, just like quartz. But the honey color appears to be part of the mineral and is most unusual for quartz. Any quartz in this rock would be detrital, not authigenic—by which I only mean that quartz would not grow here, but would come in with the other sediment that turned into this rock. And if it were detrital, it wouldn’t have this clean sparry shape. Instead it would be ordinary sand grains. So I’ve ruled out quartz.

My working hypothesis is that these are calcite crystals in the typical “dogtooth spar” shape that have grown here, or very near here. I could confirm that with a quick acid test, and one little tiny piece of a small puzzle would be clarified.