FORMATION OF THE “SNOWMASTODON” SITE - A DEATH TRAP FOR LATE PLEISTOCENE ANIMALS, SNOWMASS VILLAGE, COLORADO

The informally named Snowmastodon site is above Snowmass Village, Colorado, at an elevation of about 2,720 m. In October 2010, during the process of enlarging and deepening a small reservoir, bones of a young female Columbian mammoth were exposed. Subsequent work has recovered more than 5,000 bones from extinct species of late Pleistocene fauna including mammoths, mastodons, ground sloths, bison, camels, horses, and a variety of smaller animals.

During the Bull Lake glaciation, approximately 150 ka, a glacier formed in the headwater region of the Snowmass Creek drainage in the Elk Mountains. During its maximum extent this glacier flowed down valley about 26 km, was more than 250 m thick, and terminated at an elevation of 2,315 m. As it neared its maximum extent the glacier overtopped a low point in a ridge on its eastern valley wall, about 8 km up valley from its terminal area, and sent a small lobe into the head of the adjacent Brush Creek valley that extended about 0.5 km. When the Snowmass glacier retreated this small lobe left an encircling, broad-crested moraine that is today approximately 35 m in height. Till comprising the moraine consists primarily of unstratified, unsorted, matrix- to clast-supported, pebbly cobble gravel in a dense, reddish brown (2.5YR 5/4), clayey sand matrix. Clasts consist of mainly subangular to subrounded sandstone and conglomerate pebbles, cobbles, and boulders derived from the nearby Maroon Formation. Larger clasts are usually about 1 m in diameter. The till also contains some highly weathered granitic pebbles and cobbles. This moraine then impounded the local drainage and formed a small lake (4.4 ha) in which numerous late Pleistocene animal remains are preserved.