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Ice Age Fossils in Fremont

During construction of a seismic project to retrofit two large water transmission pipelines in Fremont, crews made an unexpected discovery: more than 50 specimens of ice age fossils. Found in what seems to be two separate deposits, or geologic layers, one grouping dates back to between 11,000 to 240,000 years before present during the Rancholabrean North American Land Mammal Age and the other possibly belonging to the slightly older Irvingtonian North American Land Mammal Age – 240,000 to 1.8 million years ago. These fossils have found a new permanent home not far from where they were found – the Children’s Natural History Museum of Fremont.

As part of our $4.8 billion Water System Improvement Program, we upgraded two large water transmission lines where they cross over three traces of the Hayward Fault near Highway 680 and Mission Boulevard in Fremont. Construction on the retrofit of the pipes as they cross the fault began in fall of 2012, and used the latest in cutting edge seismic technology. A new 305-foot-long articulated concrete vault, which includes the pipeline with one-of-a-kind ball joints and a slip joint, was constructed under Mission Boulevard. The pipeline can absorb 6.5 feet of horizontal offset and 9 feet of compression during an earthquake on the Hayward Fault.

Crews excavating the site by hand

Crews Find Specimens
During excavation of the articulated concrete vault in August of 2013, construction crews found the humerus of a bison at a depth of approximately 25 to 30 feet. Crews would later find more than 50 specimens during construction. Upon discovery of significant paleontological artifacts, the project doubled their paleontological staff from one ½ time to one full time staff person in order to properly excavate and catalogue the discoveries.

Seismic Upgrade Fossils

What Ice Age Fossils did they find?
Both groupings of fossils belong to the Pleistocene Epoch, which spans 2.6 million to 11,700 years ago. This was the time of the last ice age, when large glaciers covered portions of North America, Europe, and Asia.

Remains of at least 2 bison, horse, elk, camel, deer, brush rabbit, deer mice, and pocket gophers were found in the soil unit that can be dated to the Rancholabrean North American Land Mammal Age of 11,000 to 240,000 years before present. This area of Fremont could have looked like the Serengeti of today, with grasslands accented by brush and trees. These fossils were deposited in what appears to be a fast moving stream bed.

The layer that possibly dates to the Irvingtonian North American Land Mammal Age, which dates back 240,000 to 1.8 million years ago, show indications of a freshwater lake bed during this period. The fossil record includes the presence of freshwater snails, fish, mussels, and crayfish as well as reptiles and amphibians.

This portion of the East Bay hosted many animals that are familiar to us today: deer, rabbits, pocket gophers, and coyote, for example. Animals that later became extinct in North America also roamed this area, such as horses and camels. Camels actually originated in North America and migrated to Asia and beyond via the “Land Bridge” between North American and Asia. Modern horses in America today descend from horses brought by Europeans; they are not the ancestors of the North American horses. Bison at this time were about 20% larger than their modern relatives. Columbian Mammoths, huge ground sloths, sabertoothed tigers, and short faced bear also lived in this area during the Rancholabrean period. We know this because their remains have been found in other fossil deposits.


The name Rancholabrean stems from the La Brea Tar Pits in Southern California, where the first ‘index’ specimens from this time period were found. The La Brea Tar Pits continue to be the world’s premier location for Pleistocene animal fossils.

The earlier Irvingtonian North American Land Mammal Age takes its name from the Irvington District in Fremont, and can be directly attributed to the activities of a group of self-named ‘boy paleontologists’ and their mentor Wes Gordon starting in the 1940s. For more than 10 years, Wes and his students excavated tens of thousands of Pleistocene fossil finds from a gravel quarry in Fremont. As with the Rancholabrean, the index species for the Irvingtonian Land Mammal Age were discovered here in the Bay Area by the work of Wes Gordon and his young paleontologists.

Children’s Natural History Museum in Fremont
Although the majority of the fossils from the ‘boy paleontologists’ went to the University of California Berkeley, some of the collection went to the San Lorenzo school district, where Wes worked. In 2004, the Gordon family approached the Math Science Nucleus in Fremont to take on this collection, which not only included Irvingtonian fossils, but also books, and an extensive rock collection. Six truck loads later, the Math Science Nucleus was in possession of 30 display cases, 150 boxes of fossils, posters, a 400-pound mammoth skull, and one mounted moose head. The Math Science Nucleus began to plan to continue Wes Gordon’s legacy of educating children about the earth and the natural history of their hometown. The museum has been growing ever since.

Children's Natural History Museum in FremontThe museum, located at 4074 Eggers Drive, hosts dozens of student field trips each year, and is the only museum in the Bay Area to regularly display fossils from the Bay Area. The San Francisco Public Utilities Commission was approached by Dr. Joyce Blueford, Paleontologist, from the museum following the news of a fossil discovery from another SFPUC construction site, the Calaveras Dam Replacement Project. It became immediately apparent to project staff that the Children’s Natural History Museum would be an excellent partner and future location of the ice age fossils found at the Seismic Upgrade Project in Fremont.

Last updated: 7/26/2018 10:42:45 PM