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Hi all! I’m hoping to solve a mystery. I find these objects pretty regularly along the San Mateo coast of Northern California. I’m not even sure they’re fossils, but the bluffs above the beach are part of the Purisima Formation with plenty of fossils from the late Miocene to early Pleistocene so I thought it would be worth a try. They look to be made of compacted sand, like a concrete, but laid down in layers, with a more robust top with lines that have some shell-like material. Most pieces I found have this triangular shape that look like they might have formed tubes. All the pieces I’v