The bluffs are a layer cake of late Cretaceous and early Paleocene sediments, divided by the KT boundary, a band of rock that is the remnant of the asteroid impact that ended the Mesozoic Era (the Age of Dinosaurs), 65 million years ago. The relatively soft sedimentary rocks above the boundary span 1 million years of recovery following the mass extinction and harbor a wealth of early mammal fossils coinciding with fossil evidence of vegetation recovery, from fungi to ferns to deciduous plants; during that time, the Corral Bluffs region had a climate more typical of Florida than present-day Colorado.
Mammalian fossils at the site range from small, rat-like creatures to raccoon-sized herbivores. Since the mammal evolution is closely correlated with plant recovery, one finds dental fossils that branch from those of omnivores to early herbivores and carnivores. Though mammals first appeared in the Triassic Period of the Mesozoic Era, the dominance of dinosaurs kept their evolution in check; as the fossils at Corral Bluffs demonstrate, the asteroid impact set the stage for mammalian diversification and eventual dominance. We humans, of course, are part of that ongoing process.
