Archive for July, 2008

Cactus Rock

18 July 2008

cactus rock

Walking along Acacia Avenue, you may feast your eyes on the homes and grounds of the core street of the Upper Rockridge neighborhood, but one exception stands out at 6240 Acacia: this rock peeking over the scene. (click it for full size) This appears to be Cactus Rock, attested to in old postcards about the ongoing development of this streetcar suburb (still served by the improbable bus route 59A). I can’t get a good look at it, but it appears to be a standard Franciscan knocker that is much smaller than the mysterious Rockridge Rock. To judge from the prospect at Alpine Terrace, the next street uphill from here, the views from the rock are fantastic.

Adams Point alluvium

15 July 2008

adams point

Adams Point isn’t really named Adams Point—the name refers to the neighborhood overlooking Lake Merritt from the north. (And it’s named for Edson Adams, who once owned it all.) But I like to think of the low peninsula of Lakeside Park, between the lake’s two arms, as being Adams Point proper. One day in early 2003 I strolled along the water at its base, looking for outcrops. It appears to be the only spot on the entire Oakland shoreline that is nearly in its original condition.

The rocks in this view are not original; they’re pieces of the old wall that lines the rest of the lake. And the large boulder is a decorative one that fell or was pushed down here from the park lawn up above. What truly belongs here is the sand and gravel, which makes up the lake shore and the hills around it, Haddon Hill on the east and Adams Point hill on the north. Both are part of a large alluvial fan of Pleistocene age.

Under the roots of a tree, I found the original sediments exposed:

adams point

This being a city park, I left the material untouched, but the pebbles of local red chert and bluish basalt are unmistakable. These particles eroded from the hills and were carried here by vigorous streamflow, which also rounded their sharp corners somewhat. Outside of building excavations, exposures of this material are rare in Oakland.

The labyrinths of Sibley

11 July 2008

sibley maze

Doing urban geology in a place like Oakland adds a new question to the mental checklist that cannot be bypassed: “Is this truly a natural feature?” A boulder may be imported. A terrace may be an old railroad bed. Sibley Volcanic Reserve is a former quarry, therefore it’s safe to assume that this huge pit is not natural and that the labyrinth, one of several in the park, is of even later vintage. But park staff and other visitors have told me that some people insist, against all persuasion, that the labyrinths were made by cosmic visitors.

There is something about human beings, isn’t there? I used to trouble myself over our ability to believe nonsense, but now I realize that banging my head against that wall just hurts my head, and the wall likes it. The fact is, the general run of people love to be amazed. The trouble is, they aren’t particular about what amazes them.

Don’t get me wrong about labyrinths—they are good to experience, they do things to your head, they help pull you out of tedium. That’s cool. I think that crediting them to space aliens is a failure of imagination and a poor reflection on human ingenuity. What amazes me about labyrinths is that we invented them.

But what amazes me more satisfyingly is that people could examine this ground and figure out that it used to be the insides of a small basaltic volcano, now tilted onto its side. It takes imagination first, then the perseverance to test your imagination against the rocks again and again until every question you can think of has been met with a reasonable answer. I haven’t done that at Sibley, but having been to geology school I know how to do it if I set my mind to it. The people who did do that amaze me. They used directed imagination and rigorous skepticism instead of listening for voices and watching for signs, unhuman as that is.


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