Ziegler Reservoir, nearly 9000 feet up in the Colorado mountains, is dinky. When frozen over, you could walk across it in four minutes. Yet beneath its water is a repository of fossils that is unequaled worldwide. The story starts about 140,000 years ago, but humans have been involved for only the past ten. In 2010 the local water district, after acquiring the water rights from the namesake family, began deepening the reservoir in order to supply drinking water to Snowmass Village and man-made snow to the adjacent ski area. Hardly had the first bulldozer blade touched mud when old bones and teeth, gigantic ones, began surfacing. The findings were mammoth, both figuratively and literally.
The contractor summoned paleontologists from the Denver Museum of Science and Nature, who over the next several weeks worked closely with the construction crew to carefully retrieve a trove of fossilized bones, teeth, insects, and plant material, including pollen. Winter set in and work stopped until Spring. Then a large gathering of scientists and volunteers worked side by side with the heavy equipment operators to carefully remove and preserve as many specimens as possible. As agreed upon from the outset, collection continued for seven weeks, after which the construction project proceeded unimpeded. Water refilled the reservoir and continues to protect the expected vast store of remaining fossils from erosion or pilfering.
The scientists weren’t moaning, however, because they had obtained for study over 35,000 bones from roughly 50 species of animals ranging in size from mammoths, North American camels, and bison down to beavers, lizards, birds, frogs, snails, and insects. Plant material, including pollen, abounded as well. All told, the site is the most productive high-elevation Ice Age fossil site ever discovered—a reservoir indeed.
One might ask, did this accumulation occur as a result of a cataclysmic event such as a meteorite striking Earth? No, because the plants and animals became entombed in the reservoir over nearly 100 thousand years, not over a far shorter time, which would be characteristic of a sudden environmental change. The best guess is that the glacier responsible for carving the valley receded about 130,000 years ago, a lake formed, sediment accumulated gradually and turned the area first into a wetland and then into a meadow. For many millennia, animals came to drink, died naturally or became mired in mud, and were eventually silted over. What is even more remarkable, that when the next glacier which came along and maxed out about 20 thousand years ago, it skirted this valley, thus saving the entombed plants and animals from massive forces of churning and grinding.
In the sediment of this ancient lake is the layered evidence of multiple alpine ecosystems. A number of vertebrate species unearthed have never before been found at this high elevation. And although most of the site’s large animals (mastodons, mammoths, giant ground sloths, North American camels) are now extinct, most of the plants and insects still exist, although elsewhere. The varying climate, from arctic to temperate and back to arctic, can be discerned from the fossilized insects and plant material, including pollen, recovered from each layer. Whether this time capsule of climate change has current relevance is yet to be determined. Regardless, the site will always be known as …