Archive for April, 2008

North knocker, Mountain View Cemetery

30 April 2008


Continuing my inventory of the knockers of Mountain View, this is on the far north end of the cemetery, along the lowest of the three roads back there. It appears to be the coarse, tough sandstone—technically a metagraywacke—that makes up the majority of the Piedmont block. I can’t always tell what a rock is at the cemetery because I can’t whack it with my hammer. Don’t you try that either.

Managing the lake

25 April 2008

lake merritt

Lake Merritt needs a lot of care and attention to perform at its top level. This view of the pergola at its east end shows one of at least three aeration fountains in the lake. Without the oxygenation provided by these fountains, the organic matter brought in by the tides and streams, and deposited by the abundant bird population, would periodically overwhelm the natural oxygen dissolved in the water and turn the lake into a stinking anaerobic pond.

Without upkeep, this site would quickly revert to the tidal marshland that it once was. That would be nice in its way, but city-dwellers would probably complain about it. Click the photo for a postcard-type view of this end of the lake taken last weekend.

A fault runs through it: Montclair

22 April 2008

montclair sag pond

The Hayward fault runs through the heart of Montclair, in the Oakland hills behind Piedmont. Montclair Park’s duck pond was constructed where the fault left a natural sag in the ground.

Only in seismologists’ equations, and perhaps deep down in the crust, are faults smooth, flat planes. In the world, on the surface, faults are as ragged and variable as any other geological feature. The Hayward fault is more of a zone, from a few to a hundred meters in width, with several fractures running through it. Where two strands overlap, a block of ground between them may slump in tension or rise in compression, depending on how the strands are oriented. At Montclair Park, two strands are mapped on either side of the sag basin. Lake Temescal is another example of a sag basin repurposed as a water feature. So, apparently, is Lake Aliso, the pond on the grounds of Mills College.

The first “great San Francisco earthquake” occurred 21 October 1868 on the Hayward fault. The epicenter appears to have been in southern San Leandro, and surface rupture extended from there all the way down to Fremont. In Montclair, the other direction along the fault, there was plenty of shaking of course, but no rupture of the ground from what we can tell. A trenching study, conducted along the third-base line of the little ballfield in Montclair Park, found no sign of recent movement along the fault there. But slow, silent motion does affect the fault in Montclair. The old fire station on Moraga Avenue has been rendered useless by aseismic creep, and some of the houses along the fault appear to show foundation disruption. But generally creep is invisible unless there is some structure that it affects, like the south curb of Medau Place, below. South of here, the fault crosses Route 13, reaching the other side at the head of Dimond Canyon.

hayward fault montclair


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