Firm clay


On two occasions I’ve spotted construction sites on the fringes of Haddon Hill, once at the corner of MacArthur Boulevard and Wesley Avenue and once at the west end of Brookwood Road. I asked what the ground was, and both times the owners said the same thing: “firm clay.” There is no bedrock to speak of west of the Hayward fault, outside the Piedmont block and Toler Heights, up where 98th Avenue ends. It is safe to say that any Oakland neighborhood named “Heights” or “Highlands” has some bedrock under it, and some of the “-monts” do. But the rest of the hilly places that adjoin the flats are firm clay, with maybe a little sand and gravel. If the slope isn’t too steep, this soil is good for building.

All are part of a large alluvial fan dating from late Pleistocene times. It stretches from Pill Hill to Evergreen Cemetery, and its closest approach to the Bay is here at San Antonio Park, overlooking Coast Guard Island.

san antonio

Its sediments are said to contain “extinct late Pleistocene vertebrate fossils.” I haven’t read the literature, but that could mean anything from Ice Age mice to the mammoths, horses, camels, sloths and bison known from other Bay area sites, not to mention some extinct great cats. It’s worth keeping an eye on this stuff.

One Response to “Firm clay”

  1. ho2cultcha Says:

    these kind of land fossils are much, much easier to find where they get washed frequently – like along a creek. there is enormous quantities of soft sediment which gets washed out – leaving the harder lumps in the creek beds where they can be found. i found a fossilized horse tooth and a shark’s tooth here in the hills. Does the Purissima Formation cross the hills anywhere? i’ve found some really cool Crepidula princeps fossils and a fossilized whale in that formation over near Half-Moon Bay.

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