Geological History of the Area:
These oysters were found on the Huntington trail about 1.0 miles from the Thacher School. They are part of the Topatopa strata, which is dominantly marine in origin. The Topatopa Range is composed of Oligocene-Eocene age sandstones of the Juncal, Matilija, Cozy Dell, and Coldwater Formations. The uplifting of the Topatopa Mountains was chiefly caused by the San Cayetano fault, which was initiated at 1.9 Ma. The fossils were found in slightly consolidated, matrix- and clast- supported, pebble-boulder conglomerate ground from the Topatopa Mountains.
Paleontology of the Fossil:
The Cenozoic bivalves were likely to have become extinct in the Pliocene mollusc extinction in California and particularly in the Etchegoin Group that occurred due to falling sea levels and the subsequent cooling of California waters. The mussels are dark blue and have a black sheen when wet. The specimens were hypothesized to be composed of calcium carbonate, which was reinforced by a Hydrochloric Acid test which dissolved it. The fossil have the appearance of regular oysters, and are shaped by concave contours.They were found in clusters, each facing random directions. There appears to be no pattern in their orientation or size.
Other Fossil Sites Nearby:
Coal Point: link
Rincon Point: link
Sespe Wilderness: link
Surfer’s Knoll: link