Archive for January, 2009

Sibley to Siesta

29 January 2009

siesta valley

I showed a view looking down from the head of Siesta Valley last June. Here’s another view, taken from the north end of Sibley Volcanic Reserve in May 2005. Click it for a 1000×750 version.

One of the many things I love about the Bay area hills is the way the vegetation so sensitively reflects the sun, soil and moisture—the terroir, if you will—of every locality. Here stands of oaks mark the rhythmic declivities where a little more shade and a little more water give them a foothold.

I hope you are all taking advantage of the fine weather to tramp these hills; alas, making a living has kept me indoors and off this blog lately.

Huckleberry slickenside

19 January 2009

slickenside

Up in Huckleberry Botanic Reserve my eye was caught by a stone in the path—a clast of Claremont chert, to be exact—bearing this fine example of a slickenside. Slickensides are the polished, striated surfaces on a rock that result from rubbing together, usually by movement along a fault or, in this case, by internal deformation when the rock was being squeezed and uplifted into the Oakland Hills. Little ones are more common than you might think, but ones the size and quality of the one in San Francisco’s Corona Heights are exceedingly rare. I shot this one because there aren’t a lot in the Claremont chert. Usually deformation is accommodated in the shaly beds between the cherty layers, so there’s little chance to get a good rubbing going. The bluish color is reflected skylight, not the chert itself.


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