Archive for the ‘sausal creek watershed’ Category

Sandstone concretion, Joaquin Miller Park

11 January 2012

This odd tumorous-looking thing, on a sandstone boulder in the Oakland Conglomerate in Joaquin Miller Park, is a concretion.

concretion

I’ve documented concretions in Oakland before, in rocks of the Great Valley Sequence and in the nameless unit of Eocene mudstone above Shephard Canyon. This concretion is unlike the other two in (I assume) not having a siliceous matrix like the first and not being finely layered like the second. I assume that this is a typical featureless ball of extra-strong mineralization that formed slightly before the rest of the rock lithified. (And on KQED Quest Science Blogs this week, I talk about other concretions in the Bay area and California.)

By the way, I visited the lower end of Joaquin Miller Park the other day, below the Woodminster area where the Miller cottage is, and finally saw my sign about the rocks of the park. I hope that people have gotten some benefit from it.

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.


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