Of the three major rock groups in geology, only sedimentary rocks occur within the city limits of Fullerton. Igneous and metamorphic rocks are not found in the Fullerton area. However, these sedimentary rocks tell a remarkable story of how the landscape that is now Fullerton has changed from beach to present-day dry land. Sedimentary rocks are formed from the breakdown of pre-existing rock into sediments that are then compressed and cemented into a hard rock in a process known as lithification. This process is one of preservation and not destruction, which allows plants and animals that were entombed in the rocks to be preserved as fossils.
Fullerton can be divided into two distinct regions: 1) the flat, coastal plain where downtown Fullerton is located; and 2) the elevated Los Coyotes Hills. Loose sand and gravel derived from erosion of the nearby hills and mountains underlie the coastal plain. The Los Coyotes Hills consist of folded layers of sandstone, shale, and conglomerate that are over a thousand feet in thickness. A good spot to examine these rocks more carefully is at Ralph B. Clark Regional Park located at 8800 Rosecrans Avenue. There is a nominal fee to enter the Park grounds. Rocks and fossils found within the Park are on display in the Interpretive Center, which is an onsite museum. To visit the Center, contact Clark Park directly for hours of operation: (714) 973-3170. Prior to becoming a park, this area was the site of sand and gravel excavations that exposed the rock layers that led to the discovery of fossils. To preserve the rich fossil record, the park was established in 1974.
Three rock units are exposed within the park. The oldest unit is the San Pedro Formation that consists of fine to medium-grained sandstone. This sand size is typical of sand found at a beach. Fossils discovered within this unit include pectin shells, snails, sand dollar, and crab, allowing paleontologists to determine an age of 1.4 million years for the San Pedro Formation. Even a few shark teeth have been recovered. Overlying the San Pedro Formation is the Los Coyotes Formation consisting of coarse-grained sandstone and pebbly conglomerate layers. The rounded pebbles and coarseness of the sand indicate deposition by a high-energy river. Few fossils have been found within this unit, but paleontologists give the Los Coyotes Formation an age of about 700,000 years.
The youngest unit, ranging in age from about 400,000 to 10,000 years, is the La Habra Formation, consisting of fine-grained sandstone, claystone, and some coal lenses. These finer-grained sediments show the La Habra Formation to be deposited by a slower moving river system. Fossils of horse, American lion, camel, bison, deer, saber-toothed cat (our state fossil), and the imperial mammoth have been found within this unit and are on display in the Interpretive Center. Besides the large mammalian bones, fossils from smaller organisms such as rodents, snakes and plants provide additional clues to fine tune the depositional environment of the La Habra Formation.
By reading the story within the rock units and fossils, the geologic history of Fullerton can be interpreted. The marine fossils and beach sand of the San Pedro Formation indicate that Fullerton was basically at a seashore setting about 1 million years ago. Beach front property would be nice for most citizens of Fullerton today! With uplift of the seashore by earthquake activity, the area became dry land as the ocean slowly retreated toward its present position. The deposits of the Los Coyotes Formation show a high-energy river flowing through the area, which brought in coarser sediments perhaps from the uplifting San Gabriel Mountains. The variety and size of the mammalian fossils in the La Habra Formation show a landscape that was more like the African savanna of today. The high-energy river changed to a lazier flowing river allowing the organisms in the La Habra Formation to be better preserved as fossils. This was also a time when the southern California climate was cooler and received more rainfall to support such large organisms. This cooler climate was due to the world being in a global Ice Age.
A large wall mural in the interpretive Center at Clark Park shows what a wetter Fullerton may have looked like with mammoths and saber-toothed cats roaming the area.
Climate warmed and became dryer over the last 10,000 years causing these organisms to either become extinct or migrate to wetter regions. Over the last few thousand years, sediments have been filling under the coastal plain by repeated flooding by the Santa Ana River and its tributaries. Ocean levels are rising today due to melting of world-wide glaciers and the polar ice caps. If ocean levels continue to increase by over 100 feet in the thousands of years to come, Fullerton could again become a coastal community with our property values soaring.