The John Day Formation in north-central Oregon is a major sequence of rock layers exposed around the John Day River basin, the Warm Springs district, and the Ochoco Mountains. Resting above the Clarno Formation and below the Columbia River basalts, it preserves a record from about 39 million to 18 million years ago. Formed mainly from volcanic ashfalls and related deposits linked to calderas now associated with the Yellowstone hotspot, it is especially important for the fossils rapidly buried and preserved there. More than 100 mammal species and many plants have been found, making it one of Oregon’s most important records of Oligocene and early Miocene life and environmental change.