03 February 2008 - 09:40, Tinman said
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Going through older photos to assemble records of interesting events prior to the existence of this web site, we found this view of the nearly-vertical southern face of Corona Heights just above the Randall Museum parking lot. This is what it looked like in January 2004:

This view is significant because of the appearance of this face right now, January 2008:

This major slide was not precipitated by a quake, though it occurred during a period of unusually-heavy rainfall. Still, it clearly demonstrates that while chert is a particularly hard type of rock — and thus constitutes a substantial portion of the hills of San Francisco that haven’t yet worn down to flatness — it is also a rock whose sedimentary origins produce innumerable fracture planes and a tendency to shatter into gravel.

Think about that next time you’re considering buying a house perched on a steep chert slope!

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