Research: Calaveras Dam Construction: Salvage Paleontology

By Cristina Robins, Senior Museum Scientist, University of California Museum of Paleontology

Cristina Robins

California, in addition to being a geologists’ paradise, has strong environmental laws that protect fossils found during construction on state-owned lands. These fossils must be evaluated by professionals, and, if deemed significant, collected, prepared, and curated into an official repository. This can consist of a few fossils (isolated mammoth or mastodon remains) to large-scale discoveries. Construction projects include estimates for the environmental and paleontological mitigation costs.

The excavation for the new, more seismically stable Calaveras Dam east of San Jose, CA has led to the discovery of the most significant San Francisco area fossil find in decades. The size of the find, variety of fossils, age, and, in some cases, great preservation all make it a scientifically significant fossil site. Over the course of the project, nearly 10 million cubic yards of sediment was excavated. Over 2,000 blocks containing fossils were discovered. I say “blocks” because some of the samples contain multiple fossils, but in the official count are listed as “1.”  These fossils are being prepared and curated at the University of California Museum of Paleontology, located at UC Berkeley.

This fossil site contains plant, vertebrate, and invertebrate remains. This allows for a better reconstruction of the paleoenvironment of the site. We are still finalizing a date for the site, but Miocene age, about 20-15Ma, is our current estimate. Most of the fossils we find are quite large, as is typical for a construction site – the fossils that can be seen from the cab of an excavator are more likely to be collected. Additionally, there seems to be some preferential sorting for size, as smaller fossils have been winnowed away.

The fossils show the scars of California’s active tectonic history. Located along the Calaveras Fault, a branch of the Hayward Fault, numerous fossils are fractured, offset, and bear slicken-sides, or polished areas of broken bone that have been shredded and altered by previous earthquakes. The fossils have been extensively remineralized, with large crystals of calcite filling in voids. The sediment varies, but typically contains silica, iron, and sulfur, which makes for difficult (and aromatic) preparation. Our chisels and pin vices are quickly dulled by the matrix, and the bone is usually softer than the rock, which makes preparation more painstaking than usual.

Slicken sides on bone (shiny bits, central left in photo). This whale skull has seen better days.


A baleen whale skull with the braincase exposed. The void where the brain was now contains a calcite geode. Whale is on it’s right side; spine would attach at the center left of the photo.

Underside of a baleen whale skull. The brown is bone, black is organic material (including driftwood!) and yellow is sulfur-rich sediment.
Mandible of a baleen whale. This mandible is 4 ft long (missing 3” from distal tip on right and 9” from proximal base at left). The mandible would have belonged to a whale ~20 ft long.

Thus far, we have found well over 20 whale skulls. We have four types of baleen whale, and two types of toothed whale. One of our baleen whales is a new species – luckily, it is also our best-preserved whale! The skulls can be a range of sizes, from dolphin-sized to skulls four or more feet long (whale length estimated at 20 feet). One of our skulls is too large to fit inside the preparation lab at UC Berkeley. We also have at least 6 individuals of one species, which will allow for researchers to compare interspecific variation.

Removal of the largest skull – this one is too big to fit inside the prep lab and will be prepared off-site. It weighs between 1 and 2 tons. Photo courtesy of JP Walker.

In addition to whales, we have some remains of Desmostylus, which was an elephant relative (picture a small hippo) that lived in estuarine environments. We also have seal and sea lion remains. Fish were plentiful and large, as we have some fish vertebrae that are 4 cm in diameter! We’ve also identified 13 genera of sharks, including the infamous C. megalodon.

One Desmostylus tooth. Molar cusps featured; roots in back.

Although vertebrates are what capture most people’s interest in fossils, the invertebrates and plants are what yield the most information. Many larger animals migrate and live in multiple habitats. The plants and invertebrates often stay close to home base. We have hundreds of molluscs, including scallops 20 cm across. Predatory snails (naticids) are common, but their characteristic drillhole left behind on other molluscs is very rare. The clam fauna indicate both deep and shallow water. We have numerous barnacles, but none are attached to their hosts. Sea urchins and crabs are rarely found.

Plant fossils are present on every whale prepared thus far. Mostly the fossils consist of charcoal and hash, but some plants have defined structure and are identifiable. We have pinecones large (15 cm) and small (1 cm), twigs and branches, driftwood, and large pieces (30 cm3) of pine tree and palm tree wood. The pine has been extensively burrowed into by what we interpret to be Teredo clams (Teredolites ichnofossil), also known as shipworms, or “the termite of the sea.” In some cases, the wood is solid burrows. The palm tree escaped burrowing – possibly the wood was unappetizing to the clams.

Given the presence of large sized plant remains, invertebrates from multiple depths, and a size bias against small organisms, it seems like these fossils were deposited as part of a landslide/debris flow. The bones are almost all isolated elements (one whale was found partially articulated), possibly indicating remobilization by the debris flow.

We just passed the halfway point on our two-year funding from the San Francisco Public Utilities Commission, but we are nowhere near halfway through processing the fossils! This project ended up much larger than anticipated. We received a new whale skull over spring break in March 2018, just in time for many undergraduate student employees in the prep lab to work out their final exam stress by chiseling the rock, revealing the skull beneath. Stay tuned for more updates – there’s something new every day.

To learn more (and see more of Cristina and images of fossils and fossil prep). Note: the CBS and NBC footage have ads).

Calaveras Dam Project Fossils Providing Glimpse of Prehistoric Bay Area (CBS)

Fossils 15-Million-Years-Old Found at Calaveras Dam Rebuild (NBC)

Time lapse video of the dam construction can be viewed at

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