ORINDA — Fossil hunter Joshua Wyatt sifted and chipped his way through excavated dirt and rocks by the Caldecott Tunnel in search of the remains of camels, mastodons, rhinoceroses and bone-crunching wild dogs that once roamed the wilds of the East Bay.
A few days before, he was excited to have found the imprint of an avocado tree leaf — about 10 million years old — preserved in smooth rock like leaves flattened between layers of wax paper.
"This job is like Christmas," he said while gently cracking open rock near the east portal of the tunnel on Highway 24. "You never know when you open up something, what you're going to find."
The $420 million excavation of a fourth bore of the Caldecott Tunnel between Orinda and Oakland has opened a door for paleontologists to search for fossils expected to give clues to old life-forms and climate change in the Bay Area.
Private paleontologists hired by Caltrans already have found a tooth — likely a remnant of a camel — and dozens of remains of fish scales, plants and other bone bits in dirt and rock dug up, shoved around and shored up in early construction work outside the new bore site.
The best finds may be yet to come. Fossils not exposed to light or air for eons may be unearthed when a giant boring machine called a roadheader starts the heavy muscle work of hacking, slashing and clawing a tunnel 3,389 feet long and 41 feet wide.
The scientists will examine and sift through dirt and rock at the site and bring some back to a lab to be dampened, screened and searched for tiny bone parts. Inside the tunnel is a potential treasure to scientists: a series of earth formations tipped on its side by geological pressures, creating a snapshot of the ages.
One area considered prime for fossil finds is the Orinda Formation, a jumble of fractured layers of old stream beds and flood plains. The silt and sediment there is ideal for covering up and preserving fossils from creatures that roamed the East Bay 9 million to 10 million years ago in the Miocene period.
Camels, three-toed horses, small mastodons with four tusks, rhinoceroses, dogs with jaws like a hyena, and small antelope-like creatures carved out a niche in the East Bay.
The environment was much different then. It was a warm, wet and flat savanna. Mount Diablo and the Oakland-Berkeley hills had not emerged. Powerful geological pressures from faults later pushed up earth layers in Orinda and Berkeley and tilted them, so that the fourth-bore drilling project will carve through many ages.
"This is a window through time," said Lanny Fisk, principal paleontologist for PaleoResources Consultants, the Auburn-based company doing the fossil surveys and collection at the Caldecott Tunnel fourth bore. "It's rare to tunnel through a sequence like this. It's like laying time on its side."
Fisk compares the tunneling project to viewing all the cards in a deck at once, rather than a few on top.
This is the first time that Caltrans has called in paleontologists to a Bay Area freeway project at its beginning to monitor for fossils.
In previous excavations of the first three bores in the 1930s and 1960s, construction workers found whale, camel, horse, rhino and baby mastodon fossils now kept at UC Berkeley's Museum of Paleontology, according to a Caltrans report. That fossil collection was not guided by professional paleontologists.
Caltrans said it is paying about $35,000 a month for the paleontological work. The duration of the collection will be shaped by how long the crews keep finding fossils.
Fisk said the expense is justified because the plant and animal fossils should yield clues to understanding several natural climate changes.
"If you look at the fossil records, we see numerous times we had global warming," he said. "If you look at the past and those episodes of global warming, which had nothing to do with fossil fuel, then maybe we can better understand the current global warming trend."
Analysis of pollen and tiny plants can yield clues about the swings in climate and the different ice ages and warming periods that caused land to be dried and submerged. The Bay, for example, dried up several times.
The mammal fossils are likely to be small, partial and incomplete, as water and earth movement likely split them up.
But their presence is enough to fire the imaginations of fossil hunters who know that the places where people walk their Labradoodles were once home to small mastodons rooting about for food.
"It's exciting to know that camels lived in Orinda and Berkeley," said Pat Holroyd, a paleontologist at the UC Berkeley museum, where the Caldecott fossils will end up. "Think of horses running around then. There were no horses here when the Spaniards arrived and introduced them."
Fossils from the Miocene period were unearthed by UC scientists for several decades at a site near Blackhawk that featured stream beds and plains with sediment and gravel to cover remains.
The mastodons and camels at the Blackhawk and the Caldecott sites thrived and disappeared some 55 million years after the age of the dinosaurs, but long before the first humans arrived in California about 12,000 years ago.
"We benefit from preserving these fossils at development sites," Holroyd said. "They are a treasure of knowledge. If we lose them, they're gone forever."
Contact Denis Cuff at 925-943-8267. Read the Capricious Commuter blog at www.ibabuzz.com/transportation.