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Fossil Reef Park

Park Details

  • Fossil Reef Park is 17 million years old - only preserved portion of Fossil Reef in Orange County.
  • Tropical shell reef associated with fossil rich beach sand.
  • 48 species of marine fossil vertebrates including shark teeth and Desmostylus.

Pecten Reef

Extending for 6 miles north-south across the Saddleback Valley is an unusual limestone deposit called the "Pecten Reef ". The actual extent of this limestone is suggested to be as great as 20 square miles in Orange County and also found on the Palos Verdes Peninsula, San Clemente Island, Santa Catalina Island, and in Santa Barbara and San Luis Obispo Counties. Its maximum thickness appears to be 300 feet.

Reef Exposure

When Lake Forest was graded in 1972, a very large limestone formation was uncovered. Many Pecten shells were found at the site and thus the name "Pecten Reef" was given by local paleontologists. The exposures represent the ocean floor, as it existed 17 million years ago. The site was destroyed by a housing project after only a limited time of research. A 2nd exposure of reef was discovered in Laguna Hills in 1973. The Natural History Foundation of Orange County worked with the Orange County Environment Management Agency planning staff and the Affordable Communities land owner to preserve a portion of the reef. The exposed fossil ridge was recognized as a County prehistoric site by the Orange County Historical Commission which dedicated the 1 acre Fossil Reef Park in Laguna Hills in 1982.

Reef Length

Most of the fossils in the reef are the molds and internal casts of large scallops (Pectens), clams, and snails. The large size of the scallops suggests a tropical environment. The reef is unique, as it is part of the ancient sea floor that has been exposed by tectonic uplift and weathering processes. The uplift took place during the last million years and has formed the Santa Ana Mountains and the San Joaquin Hills. The rigid limestone did not bend during uplift but broke along local faults. A fault extends east-west across the northwest edge of the park and appears to extend in a general direction under the scoreboard on the high school baseball field.

Marine Muds

The marine muds, that later covered the limestone, contain plankton fossils, shark teeth, fish bones, marine mammal bones, and seaweed imprints. A large baleen whale was collected between the park and the high school in 1981.

Displaying the Fossils

Fossils collected from the reef have been curated by the city and placed on display at the Laguna Hills Community Center.

Fossil Reef Sign

"Before you are the white limestone remains of an 18,000,000-year old tropical shell reef. Formed in a shallow bay, it contains fossils of scallops, clams, and tube worms. Mudstones of the same age, found nearby, hold fossil whales and shark teeth. Later, as the Santa Ana Mountains rose, the rigid limestone buckled and broke along small faults. We can now see evidence that tropical seas once covered the spot where you stand."

Historical Site No. 28
Orange County Board of Supervisors
Orange County Historical Commission
Donated by AFCOM Development
Placed 1952


Fossil Reef Park is located on Via Lomas between Moulton Parkway and Alicia Parkway.