The Blackhawk Ranch Fossil Quarry

Blackhawk Museum docent John Bingaman (right) examines an object while his co-worker uses a whisk broom to delicately uncover fossils at the bone bed at the Blackhawk Quarry in 1994-95.

By Beverly Lane

When California paleontologists speak of the Blackhawk Ranch, they are not talking about the upscale Danville housing development, but of the rich fossil quarry first studied by the University of California Museum of Paleontology in the 1930s. The quarry dates back 9 million or 10 million years.

The East Bay was then composed of low rolling hills with streams draining westward toward a southward extending ancestral San Pablo Bay. The countryside was a savanna with heavier woodlands along the courses of the streams. There was a range of hills west of what is now called the San Pablo embayment, more or less in the position of the present-day San Francisco Bay. In this part of California, an almost unimaginable change of landscape has evolved in just the past 9 million years.

The Blackhawk Ranch Fossil Quarry is in a pocket of deposition in one of those westward flowing streams. Swift waters and seasonal floods carried and jumbled up many animal bones, jaws and teeth as they were being deposited along with rock pebbles in this pocket. Much later, after the depositional pocket was consolidated, de-watered, and converted into pebbly rock, the older rocks in the core of Mount Diablo were pushed up much higher than its current elevation of 3,849 feet.

The overlaying strata, including the fossiliferous bed at the quarry, were tilted to steep angles by the up-thrust of this huge plug we call Mount Diablo. Many of the strata, even those at the quarry, have been overturned from the vertical by as much as 30 degrees. The overturning has been increased
by gravity-slumping of beds near the earth’s surface on steep hillsides.

This site is fascinating for students to examine. It demonstrates geologic faulting, mountain-making, earthquakes and tremendous surface changes in the California Coast Ranges over the past few million years.

Only a small part of the quarry has been excavated, and no one knows how deeply the fossiliferous deposits may extend under the present surface. More than 3,400 museum-numbered specimens of animals and plants have been obtained at the quarry. Most of these fossils were collected by a Works
Progress Administration crew in the mid 1930s and later. Community digs took place in 1994 and 1995 and a few UCMP-sponsored visits come to the quarry each summer.

The fossils include plants, skulls, long bones, teeth, tusks, ribs and foot bones of a great variety of animals, such as mammals, reptiles and fish. Common vertebrates found are gomphotherium simpsoni (an ancestor of later mastodons and elephants), beavers, mice, squirrels, foxes, hayaenoid dogs, cats (including a saber-toothed variety), skinks, weasels, otters, horses, camels, rhinoceros, llamas, antelopes, salmon, turtles and cranes. Plants recovered include leaves of poplar, willow, oaks, elm, sycamore, mahogany and sumac.

The quarry biota is a good representation of the larger animal and plant life in the East Bay region nine million years ago. Located in the southern foothills of Mount Diablo and owned by the University of California, the Blackhawk Ranch Fossil Quarry has put the San Ramon Valley on the map.

SOURCES: Ron and Kevin Crane, Geology of the San Ramon Valley; UC Museum of Paleontology, various materials on the Blackhawk Quarry, www.ucmp.berkeley.edu
Image of a contemporary dig c. 1995, courtesy of the Blackhawk Museum.
This article first appeared as a column called Presenting the Past in the Danville Weekly.

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Uploaded February 2007