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Surprise find during construction project led to massive fossil discovery in Colorado

By Piper Russell July 31, 2025 | updated 3 days ago

Photo Credit: carterdayne (iStock).

Photo Credit: carterdayne (iStock).

carterdayne

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What began as a project to deepen Ziegler Reservoir to supply the nearby growing town of Snowmass Village with water evolved into the discovery of a site containing more than 5,000 fossils.

The town of Snowmass had purchased Ziegler Reservoir, built in 1958, with plans to expand it to supply the increasing population of the Roaring Fork Valley. Plans changed, however, when in 2010, an excavator found a mammoth bone with his bulldozer. The company, Gould Construction, stopped excavation and called in the Colorado Geological Survey and the Denver Museum of Nature and Science to investigate.

Beginning on November 3, 2010, more than 65 volunteers began to excavate bones from the soil. On the second day of the dig, they uncovered Colorado’s first mastodon skull.

Over 19 days, and with the work of 67 staff and volunteers, 600 bones were discovered at the site. Winter, however, put an end to the dig, and the bones were sent to the Denver Museum of Nature and Science to be analyzed.

In 2011, a larger crew, including scientists, nine interns, and 107 trained volunteers, began another seven-week dig. The findings included a prehistoric bison skull with a horn measuring six feet and four inches across and a “Clay Mammoth” specimen.

Overall, the team found 4,056 bones and moved 4,500 yards of dirt by June 21, 2011.

According to Snowmass Ice Age Discovery, the fossils found in the area accumulated slowly over many years. Some were deposited by glaciers, others brought by mudslides, and others from animals stuck in the marsh.

Learn more about the site and how you can explore it here.

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