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Red chert exposed in road cut on
Conzelman Road near Battery 129. |
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Geologic Trips, Marin
Headlands Hawk Hill 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. Exterior Websites Golden Gate NP Conservancy: National Park Service: Wikipedia:
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