Archive for May, 2010

Fields of goldfields

31 May 2010

serpentine prairie

The efforts to improve Serpentine Prairie are paying off. Last year I mentioned that the heart of the prairie would be fenced in for a few years. They have also removed the non-native pines that were inappropriately planted. This photo, taken March 28, shows the prairie awash in goldfields. Early summer is the best time to see the rare Presidio clarkia in bloom up there.

Goldfields (Lasthenia spp.) is a very hardy genus that often carpets serpentine soils. Naturally it deserves its name for that alone, but it also has a deeper association with gold. The Mother Lode, running along the western flank of the Sierra Nevada, is a major fault zone, long inactive, that once marked a tectonic suture. Hot gold-bearing fluids rose along the fault and infused the country rock. Serpentinite came up the same way, in the same places. Roadcuts along state route 49 expose lots of it. So where the goldfields blooms, the odds of finding gold are significantly better than chance. Oakland has a great variety of rocks, but no gold that I know of.

Making land

18 May 2010

middle harbor point

Oakland began with a marshy waterfront; sure it was full of fish and oysters, but you couldn’t do big-time commerce there to match the transcontinental railroad depot. So the marshes were filled and the shore extended into deeper water and passages dredged until Oakland had a splendid harbor, and it still does today. Thankfully, we’ve even had the drive and funding to recreate a small example of a working shoreline here at the new Middle Harbor Shoreline Park . . . we hope. If this kind of work is done with a geomorphologist’s guidance, there’s a chance that it could age gracefully instead of sinking or slumping or washing away. With that hope, I dub this piece of land Middle Harbor Point.

Franciscan rocks of Dimond Canyon

13 May 2010

The last piece of bedrock I want to show from the Sausal Creek watershed is the Franciscan Complex. It crops out on the west side of the Hayward fault, unlike all the other ones I’ve been showing, but it’s roughly the same age as the Shephard Creek–Redwood Canyon package, 80 to 70 million years old. This is a sandstone outcrop just uphill from the Zion Lutheran School on Park Boulevard. It’s part of the Novato Quarry terrane.

franciscan outcrop

The sandstone is hard and gray, composed of fairly well sorted sand. The other week a geologist chided me (and the rest of my field-trip group) for our habit of calling this graywacke. It’s not graywacke unless it has at least 10 percent clay, he said. So OK, this is sandstone. And the bluish color is reflected skylight; sorry about that too.

sandstone

As you come in the school driveway, you’re greeted (if you’re paying attention) by this fine outcrop of mélange.

melange

I didn’t linger the day I took these photos because the yard was full of kids and two patrol cars were sitting there. I have a plan for any time that police want to check me out: I’ll start talking all about geology and showing them rocks and stuff, so they’ll know I’m a harmless nut. I tried it once, up in the Sierra, but it was a time of terrible wildfires and the sheriff just went back to his car and waited until I moved on.

Serpentinite of Visionary ridge, Joaquin Miller Park

6 May 2010

The ridge behind Woodminster Theater, in Joaquin Miller Park, is mapped as serpentinite. You start to notice this around the Moses pyramid, but as you explore you’ll find a lot more of it plus various associated metamorphic rocks.

joaquin miller serpentinite

For instance, on the Siwalik Loop Trail I spotted a fine chunk of actinolite schist, detailed below.

actinolite

The ridge doesn’t have a name on the map, so I’ll give it one. It has the Moses pyramid, the Elizabeth Barrett Browning monument, the fireplace that Joaquin Miller built for his own cremation, and Lookout Point where Miller loved to contemplate Oakland and the bay. So by my decree, this is Visionary ridge.

At Lookout Point, the large boulders surrounding the viewing patio caught my eye. High-grade blocks!

blueschist

A closer look showed me the garnet-muscovite-glaucophane association that marks blueschist. There was also some green (chromian) mica. Bring your magnifier, not your hammer, if you come this way.

blueschist


Follow

Get every new post delivered to your Inbox.

Join 4,040 other followers