Dozer Driver Makes Fossil Discovery of the Century

An accidental discovery by a bulldozer driver has led to what may be the find of the century: an ice-age burial ground that could rival the famed La Brea tar pits.

After two weeks of excavating ancient fossils at the Ziegler Reservoir near Snowmass Village, Colorado, scientists from the Denver Museum of Natural Science returned home Wednesday with their unearthed treasures in tow -- a wide array of fossils, insects and plant life that they say give a stunningly realistic view of what life was like when ancient, giant beasts lumbered across the Earth.

Since the team’s arrival in mid-October, scientists have extracted nearly 600 bones from about 20 different animals from the Pleistocene era, a period of time during the Ice Age. The remains of up to six different species have been exhumed, including five American mastodons, three Ice Age bison, a Jefferson’s ground sloth, a mule deer, a tiger salamander, and two Columbian mammoths.

The site rivals many others in terms of its diversity, as it is the only known place in Colorado -- and one of few in North America -- that contains both mammoth and mastodon fossils in the same location. And just finding an American mastodon is pretty unusual in itself.

"There are only three known records of mastodons in Colorado, and we have found at least five specimens," Miller said. "So throughout the course of 120 years of paleontology, we jumped from three mastodons to eight in a single two-week period."

And the significance of the Snowmass Excavation doesn’t stop there. Snowmass has also produced an array of insect and plant life, as well as wood that has been chewed by beavers, essentially producing what Miller calls a "window into an Ice Age ecosystem."

This question was posed in regard to the recent paleontological find of Columbian mammoths and American mastodons in Snowmass Village. The property once belonged to Everett Peirce. My father, Fred Ristine, Jr., was asked for his opinion on its investment potential in the early 1960s. The parcel was several hundred acres and contained a swamp, fed by snow melt, which supported the Boreal toad.
The Zieglers bought the property and improved it with access roads, a house, and a picture pond. Water was gravity fed to the pond from the East Snowmass Creek basin and retained with an earthen dam.
Construction of the Snowmass Ski Area followed these developments in 1967.
As a skier, I can look down upon this pond, which is a geological anomaly. The property is just below the sub-alpine elevation of 8880. There are aspens to the west and scrub oak and sage to the east. There are drops in elevation on all sides: two hundred feet to the Brush Creek drainage on the south side and three hundred feet to the Snowmass Creek drainage on the north side. The rocky land that contains the pond comes to a point like a cruise ship heading for the Elk Mountains and the Continental Divide. From the heights of the ski area, it looks like a bird bath. It is unlikely that this pool was part of a much greater body of water unless the surrounding lower elevations were filled with glacial ice.
The Snowmass elk herd travels a circuitous route through aspen groves and forests of spruce and Douglas fir of the Elk Mountains, the Snowmass/Capitol Creek Sub-watershed, and the upper Roaring Fork Valley. Similarly, the Ice Age herds probably treasured this water hole that was conveniently out of reach to most human hunters.
The Ziegler Reservoir contains a peat layer that held the Columbian mammoths. Peat takes a long time to form; the rate is about 1 to 2 millimeters per year. (26 millimeters equals approximately one inch.) The layer of peat in the reservoir measures about 1.5 meters or five feet deep. (Peat should never be sold or discarded because it is an excellent source of historical evidence.) The formation we see at Snowmass Village is probably an ancient fen, having been fed by a steady source of ground water. Bogs are rain fed.
At the foothills, in the valley below the Ziegler Reservoir, there are indications of glacial infill, a result of Ice Age flooding. Ranchers made use of these swaths of land as pastures for grazing and meadows for growing hay, until the construction of the ski area. That the top five feet of the Ziegler Reservoir consist of clay must be the result of a significant amount of water carrying finely processed shale from higher elevations. In my estimation, this happened in a relatively quick event during the last glacial recession about 10,500 years ago, during some other glacial retreat, or at the time an ice dam collapsed. The clay sealed off the peat, protecting the anoxic environment for the preservation of bones, and it allowed for the retention of water.
A lake would have an inlet, and it is probable with the introduction of glacial melt from East Snowmass Creek during an Ice Age that the site we know as Ziegler Reservoir has probably fluctuated in size from a wet meadow to a small, shallow lake, which emptied into the Brush Creek drainage.
It makes my soul glad to be associated with this place.

John H. Ristine, BSC
Aspen, Colorado

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