While smaller animals died first, it took larger animals, like rhinos, up to five weeks to perish. Drifting ash quickly covered their bodies.
The skeletons of these animals are preserved in their death positions, complete with evidence of their last meals in their mouths and stomachs.
The scene is reminiscent of Pompeii, the Italian village destroyed by a volcanic eruption in A.D. 79 where people were smothered in ash where they lay.
"I think many of us enjoy going to the museum, but it's an entirely different experience to actually see the fossils just the way Mother Nature left them," Voorhies said.
"These are fossils that don't need to be artificially enhanced, we don't have to label them and tell people what they are. Even after 12 million years, they still look like animals."
Among the animals are five species of horses, three species of camels, barrel-bodied rhinos, saber-toothed deer, several species of dogs, and two species of turtles.
"Many of these animals are subtropical or tropical, which shows how different our environment was in this area 12 million years ago," said Rick Otto, the superintendent of the park.
"When you look at the fossil bed, you not only have an idea of what these prehistoric animals looked like, but the fact that they're buried in this pure volcanic ash gives you an idea of the catastrophe that resulted in their death," Otto said.
John Francis, who heads the Research, Conservation, and Exploration Group at the National Geographic Society, says visiting the fossil site is like stepping back in time.
"You get a sense of the mass and the diversity and the strangeness of our plains area 12 million years ago," he said.
"It's a very evocative, physical portrayal, both of the process of fossilization and the richness of life a long time ago."
Francis, who headed the U.S. National Park Service advisory board that recommended the designation of Ashfall Fossil Beds as a national natural landmark, says the NNL program can help protect unique natural sites.
"There's a lot to celebrate in terms of the natural wonder in our country," Francis said.
"This is an innovative program that increases attention to the diversity of natural places in our country, yet [it] doesn't overburden the taxpayer, but rather leaves it to local landholders and interests to celebrate what can be a nationally recognized feature."
Natural Landmarks Program Revived
The NNL program, which was established in 1962, is a sister program to the more well known National Historic Landmarks Program.
The goal of the NNL program is to highlight U.S. natural history and encourage the conservation of geological and biological sites.
There are close to 600 NNL sites in the country today, from the Tijuana River Estuary in southern California to Mount Katahdin in Maine.
NNL sites can be publicly or privately owned. The U.S. National Park Service administers the program and assists NNL landowners and managers with the conservation of the sites if requested.
Political entanglements caused the program to be suspended from 1989 to 1999. Since then, the regulations that govern it have been revised to ensure that participation by NNL sites remains entirely voluntary.
Now the Park Service has a sizable backlog of sites being considered for NNL status.
"We've had to go in and dust off those evaluations and update them to the NNL criteria that apply today," said Brooks, the NNL program manager.
She says her office has finished 30 evaluations, which are set to pass through a peer review process before the sites can be designated as national natural landmarks.
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