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

 

Featured below are a few of the fossil shells that I came across earlier this year.

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.

The first shell photo shows a nice, whole oyster fossil resting on the Del Mar Formation. The second shell photo shows the underside of an oyster fossil along with some embedded, coiled snail shells. The 4th shell photo is a closer view of another coiled snail shell. These three fossil photos were taken at the Torrey Pines State Beach, just north of San Diego.

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Photo Courtesy Dave E. Matson

The 3rd photo is from San Onofre Beach, a short walk south of the nuclear reactors. These fossils are "only" about 20 million year old and are found in the Monterey shale.

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Photo Courtesy Dave E. Matson

Geology Photo #1: Torrey Pines. At the very top, not much in evidence here, is the Bay Point Formation (~120,000 years) which consists of loosely consolidated, brown sediments washed down from the hills. The thick, sculpted strata is the Torrey Pines sandstone (~45 million years). It is probably the remains of sand from off shore, barrier islands. The Del Mar Formation (45-50 million years) which consists of layers of mudstone, shale and white/gray sandstone lying on a greenish shale, came from lagoonal muds occasionally flooded with sand from the barrier islands. The scene is similar to the coast of Texas today, which is flat and has barrier islands and lagoons.

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.

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Shell Photos #1 - Whole oyster fossil resting on the Del Mar Formation.
Photo Courtesy Dave E. Matson

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Shell Photo #2 - Underside of an oyster fossil along with some embedded, coiled snail shells
Photo Courtesy Dave E. Matson

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Photo Courtesy Dave E. Matson

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Photo Courtesy Dave E. Matson

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Del Mar Formation
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Article last updated October 22, 2005
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