The John Day Formation
Oregon ash beds capture 20 million years of continuous change in one fauna and flora.
- When
- —
- Duration
- 11 million years
- Known to
- ± 1 million years
- Era
- Oligocene, Cenozoic
The John Day beds accumulate as a nearly unbroken sequence of ash falls and volcanic soils, preserving both plants and animals across a span from the late Eocene into the Miocene. That continuity is the point: rather than a snapshot, it gives a running record of how a single region's ecosystem responded to cooling, drying and the spread of open habitats — leaf floras shifting from subtropical to temperate, and mammals shifting from browsers toward grazers, in the same rocks.