SAN ONOFRE BEACH News & Recreation

Geology at San Onofre
Contributed by Dave E. Matson, Oak Hill Free Press

Featured below is a photo of some fossils shells that were discovered earlier this year at San Onofre Beach by Dave E. Matson. The were found just a short walk south of the nuclear reactors. These fossils are “only” about 20 million year old and are found in the Monterey shale.

About 45 to 50 million years ago the West Coast resembled the East Coast in that there were barrier islands with quiet, mud-filled lagoons. The climate was such that large beds of oysters flourished along with other creatures, such as small, coiled sea snails. Eons later, that mud later got converted into somewhat greenish-gray shale–the Del Mar Formation.

Geology Photo #2: San Onofre. A secondary fault may be seen as a thin diagonal in the San Mateo sandstone (4-5 million years) truncated by a later layer of marine boulders of about 125,000 years of age. The brown strata on top is less than 120,000 years old, being washed down from the hills. The white, San Mateo sandstone is underlain by Monterey shale (15-20 million years). To the right, (out of sight) of the secondary fault is the main fault, the Christianitos Fault, which has been inactive for at least 125,000 years as indicated by the undisturbed layer of marine boulders above it.