Red chert exposed in road cut on Conzelman Road near Battery 129.

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The layered red chert exposed in this road cut near Battery 129 is part of the Franciscan Formation. The chert was formed as a silica gel on the floor of the Farallon Plate during Jurassic time, and is often associated with pillow basalt, also formed on the ocean floor. The chert also has numerous Radiolaria—one-celled floating animals that live in seawater and have a silica shell. We know that the chert was formed far from land because sediments from nearby land areas would have drowned out the chert. The red shale between the chert layers is fine mineral dust from the atmosphere that fell into the ocean and accumulated in the silica gel on the sea floor. The thin layers of red shale segregated from the silica as the silica hardened into chert.