Via National Parks Traveler:

Hidden within the sagebrush covered hills and badlands of eastern Oregon is evidence of past worlds covered by ancient forests and inhabited by strange looking beasts.

Although
paleontologists have been studying the area for nearly 150 years, the
forces of erosion still uncover previously unknown plants and animals
buried within the rocks. Many of the most dynamic finds from the John
Day Fossil Beds are on display at the Thomas Condon Paleontology Center
at the Sheep Rock Unit of the monument, where visitors can peer inside
the fossil laboratory as paleontologists work on new finds.

John Day Fossil Beds National Monument
preserves over 40 million years in a fossil record representing nearly
two-thirds of the Cenozoic Era, the Age of Mammals. It is not just the
breadth of time covered that makes this area unique, but also the
ability to radiometrically date the rock layers, and thereby the
fossils, with a high degree of accuracy.