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A 20 Million Year Old Ocean at Calaveras Dam

In early 2013 during construction to build a replacement Calaveras Dam in the Sunol Valley north of Fremont, crews began to find 20 million year old marine fossils. 844 specimens are inventoried as of December 2015. There are 413 vertebrate specimens, 320 invertebrate specimens, 41 plant specimens, 70 specimens labeled as other.

A discovery site in front of the Calveras Dam project site

The “other” category includes ichnofossils (feeding tracks and burrows), and bulk samples intended for screen washing to recover very small but important fossils. Of the total specimens 30 have not yet been collected. Amongst the vertebrate fossils are:

  • Mysticeti (baleen) and Odontoceti (toothed) whales
  • Desmostylus (a hippo-like creature that is a member of the only order of marine mammal to have gone extinct)
  • Megalodon (great white shark ancestors) and Isurus (mako) sharks, and
  • boney fish.

The invertebrates include scallops (some as big as a dinner plate), clams, snails, cockles, mussels, and barnacles. The plant material includes cones, leaves, and wood.

See more fossils from the Calaveras site

See more photos from the construction site.

Remnants of coastal deposits
The specimens have been collected from formations of the Monterey Group including Temblor Sandstone and the Claremont Shale. Most of the specimens are from the Temblor Sandstone. These marine geologic units are remnants of California’s coastal deposits which were buried and lithified (turned to stone) and then uplifted, faulted, folded, and eroded to form the hills around the dam site.

The Temblor Sandstone and the Claremont Shale are both from Miocene epoch (5.3 to 23 million years ago). We currently believe the fossils from the Temblor Sandstone to be about 20 million years old.

Dental picks and excavators
Our methods of collection vary depending on the conditions across the site. We have used everything from dental picks to an excavator and tractors to collect a specimen. The paleontological program has not impacted construction or delayed work schedules.

The collected specimens are kept onsite in large metal boxes that are secured and locked. The specimens are laid out on shelving for inspection by visitors and other scientists; some of the specimens are wrapped and stored in boxes.

It is not uncommon to find fossils in the Bay Area, but the concentration of such unique and varied specimens, particularly the vertebrate specimens, is what makes this site special.
Last updated: 1/8/2016 2:15:28 PM