Archive for October, 2007

Lake Merritt

26 October 2007

merrittfromcemetery.jpgOn good days I can’t resist taking photos of Oakland from the high hill at Mountain View Cemetery. I usually regret it because my camera isn’t good enough, but three years and three days ago, this shot of Lake Merritt came out pretty nice. Lake Merritt used to be a tidal slough, until Dr. Samuel Merritt took it upon himself to build a dam and turn it into more of a proper lake. His mansion, among those of many other of Oakland’s early swells, surrounded the lake in the late 1800s.

Nowadays the lake is regulated with a better dam, but it’s still rather artificial. In 1870 it was named America’s first wildlife refuge. Its water is brackish, with salt content that fluctuates with the rainfall. This view looks across San Francisco Bay to the high hills of the San Mateo Peninsula, with the San Andreas fault at their feet.

Piedmont Hill

16 October 2007

clarewoodchert.jpgThe town of Piedmont, and a ring of Oakland territory around it, sits on a block of bedrock of the Franciscan complex. It’s deep-sea floor material some 150 million years old that was carried across a wide ancient ocean and piled up against the edge of North America just as is happening elsewhere on Earth today. Since that time it has been sliced, shuffled, smeared and squeezed by tectonic forces, and the hill of Piedmont is one lump of a string of bedrock lumps called the Novato Quarry Terrane. This lovely piece of chert is exposed along Clarewood Drive, on the back side of Mountain View Cemetery. It displays its original deep-sea layers, somewhat jumbled and veined with silica during its later travels. More chert crops out on Clarewood Lane, which runs parallel to the Drive across a small creek.

The Novato Quarry Terrane also includes Albany Hill, Point Richmond and more pieces in Marin County. Here’s a stereopair looking from Clarewood toward Albany Hill. Click it for a big version.


Oakland’s volcano

7 October 2007

Oakland has its own volcano up in the hills. You can see it in the banner photo at the top of the page: Round Top is the hill on the skyline at the left. There is the Sibley Volcanic Preserve, a former quarry exposing the rocks of a basaltic volcano that was active about 10 million years ago. The former volcano has been shoved on its side by the same pressures across the Hayward fault that raised (and continue to raise) the hills.

When you get up there, the volcano is not easy to find, like many things in geology. It takes a lot of clambering and closely examining the rocks. Fortunately real geologists have done that work. But here and there you’ll see really clear evidence, like this boulder of lapilli tuff. An explosive eruption spraying ash in the air created these little balls, about the size of pencil erasers, which collected in layers and were eventually lithified. Nowadays the action has moved up the faults to The Geysers and the Clear Lake volcanics, but that’s outside Oakland.

My secret chert

3 October 2007

If you walk up in Mountain View Cemetery you’ll see all sorts of exotic stone. Nowadays fine stone is a worldwide product. There’s a fabulous anorthosite you’ll see sometimes that comes from Scandinavia. Rapakivi granite too. But I think the older stones include marble possibly quarried across the bay—across the San Andreas fault, actually. Some of the old granite undoubtedly comes from California, whether the Sierra Nevada or the Salinian block in Monterey County. I enjoy all those. But here and there you’ll see stonework incorporating the local stuff, chert and calc-silicate from the Piedmont hill, part of the Novato Quarry terrane in the Franciscan complex. And even more charming is the occasional outcrop, left wild, like this dignified chert.