Fossil Reef Park
Extending for six 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 twenty 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.
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 second 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 one acre Fossil Reef Park in Laguna Hills in 1982.
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.
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