rights in 2010. Several dating techniques were employed in order to determine the age of the ancient lake sediments. Attempts at radiocarbon dating of lake organics, bone collagen and shell carbonate, revealed that the site was radiocarbon dead or greater than 45,000 years old. In situ cosmogenic beryllium and aluminum from a boulder that was part of the bounding moraine yielded an age of approximately 140,000 years old. Uranium-series dating of vertebrate fossils confirmed the age range of 45,000-140,000 years; however, the extremely low uranium concentrations limited the utility of that method. Optically stimulated luminescence (OSL) ages obtained from the fine-grained quartz provided reliable, replicable ages for the sediments within the established 45,000 – 140,000 time frame. The dates obtained via OSL correlate with the end of the Bull Lake glaciation and the Sangamon interglacial stage, spanning marine isotope stages 6 through 4. Vertebrate Fauna Over 36,000 bones and teeth belonging to 52 taxa of macro- and microvertebrates were uncovered during the construction of Ziegler Reservoir. These taxa represent a diverse assemblage of fauna belonging to the Rancholabrean North American Land Mammal Age. The smaller vertebrate species include: trout, frog, salamander, snake, lizard, duck, goose, pheasant, crane, finch, shrew, river otter, bear, coyote, rabbit, chipmunk, squirrel, beaver, mice and other small rodents. The most abundant species found throughout the site is the tiger salamander. Also represented in the Ziegler Reservoir are the remains of seven megafauna taxa. These taxa include: Columbian mammoth (Mammuthus columbi) – Four individuals, including the first fossil recovered from the site (a juvenile nicknamed 'Snowy') were collected. American mastodon (Mammut americanum) - The Ziegler Reserver is the largest site for mastodons in the world with at least 35 individuals present. Approximately 60% of the total number of vertebrate elements recovered belong to the American mastodon. Jefferson's ground sloth (Megalonyx jeffersonii) –Three individuals, one adult and two juveniles, were found at the site. This marks the first time this species has been found in Colorado. Giant bison (Bison latifrons) - At least 10 individuals were found making the Ziegler Reservoir one of only three sites to produce multiple individuals of B. latifrons and the highest known elevation for this species. Deer - At least 2 individuals of indeterminate species. Camelops - A single tooth belonging to the extinct camel genus, Camelops, was found in lake-center deposits. This is the highest known elevation for this genus. Horse -