The Rattlesnake Formation, exposed along Oregon’s John Day River Valley, preserves a long terrestrial record from the late Miocene into the Pliocene. It includes thick fanglomerate and other land-laid sediments, plus a prominent ignimbrite layer whose welded ash-flow tuff structure made it especially useful for dating. Potassium-argon dating in 1964 placed that volcanic unit at 6.4 million years old, helping date the surrounding beds. Fossils from below the ignimbrite include a rich mammal fauna such as Amebelodon, Teleoceras fossiger, Machairodus, and the earliest record of modern-day beavers, as well as Pekania occulta, identified as the earliest known fisher.