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.
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.