Mammoth Fossils Found at Rio Mesa Solar Site

A tdisplay at Chicago's Field Museum showing the relative sizes of Columbian mammoths, present-day elephants and mastodons | Photo: Vic Bit/Flickr/Creative Commons License

A week after BrightSource Energy relented to pressure to survey the site of its proposed Rio Mesa solar project to determine the extent of an important fossil deposit, news has gotten out that paleontologists have found fossil fragments of Columbian mammoths on the site. The mammoths, once widespread across North America but which died out as recently as 5,800 B.C., stood up to 14 feet tall with tusks 12-14 feet long.

According to the Palm Springs Desert Sun, the continuing discovery of more fossils onsite may pose a threat to the project, which must win approval from the state by June 2013 to fulfill the terms of a power purchase agreement with Southern California Edison.

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The fossil deposit, which ReWire reported on in June, apparently underlies approximately a third of the project's nearly six-square-mile footprint. The solar project would consist of two 750-foot power towers each surrounded by tens of thousands of mirrored heliostats, and would have a total power generating capacity of 500 megawatts.

According to a paleontological resource delineation work plan submitted to the California Energy Commission (CEC) last week by BrightSource's consulting paleontologists, the majority of the fossils found on site are held in a deposit called the Palo Verde Mesa paleosol, a layer of rock at least 12 feet thick made up of sediments laid down by the Colorado River.

In addition to the mammoth fragment, remains have been found of animals ranging from sidewinders and lizards to badgers, rabbits and jackrabbits, horses, deer, and possible bighorn sheep. Most of the fossils of large animals in the deposit are smaller bone fragments: unlike the fossils of Rancho La Brea and other places where large animals were entombed whole, the fossils from this ancient arid soil were mainly entombed in animal burrows, and larger animals are represented only where rodents apparently hauled teeth and bone fragments into their dens somewhere in the neighborhood of 14,000 years ago.

Though the mammoth fragment is exciting, the most important fossil found so far is likely what appears to be a clutch of unhatched desert tortoise eggs intact in a burrow accompanied by an adult tortoise, which may be the only such fossils ever found in California.

Under BrightSource's proposed workplan, its paleontologists will dig ten 16-foot trenches on and near the site, and drill five boreholes, to get a better idea of the deposit's actual extent. The trenches would be dug to a depth of about ten feet, which is the depth BrightSource says is likely to be disturbed during construction of the solar facility. Each of the heliostats would be mounted on a pylon that would be driven into the ground by vibration, a process that would shatter any nearby fossils.

At present the CEC has made no public comment on the paleontological resources work plan.

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