John Day Fossil Beds National Monument - Rattlesnake Assemblage (U.S. National Park Service)

Image of a map of Oregon and the area covered by the Rattlesnake Formation.

The Rattlesnake layers cover thousands of square miles.

The John Day Fossil Beds record of ancient life ends with pebbles, boulders, and sand intermingled with deep soils, and a thick welded tuff.

During the Rattlesnake, rivers chewed into the soft paleosols of the underlying Mascall.

Image of the geologic timeline of the John Day Fossil Beds strata.

The Rattlesnake layers are the youngest within the fossil beds. Click on the timeline for a larger version.

Water coursed through floodplains, leaving gravelly deposits, redistributing ancient soils, and moving the remains of some of the animals that lived there.

Seven million years ago, life during the Rattlesnake was interrupted by a dramatic volcanic event that formed the Rattlesnake Ash Flow Tuff. This massive eruption of superheated gases and ash sped overland and engulfed 13,000 square miles.

After the horrific effects of this violent event, the Rattlesnake landscape continued to be shaped by rivers that cut through newly formed floodplains, leaving conglomerates and paleosols in their wake.

Following the Rattlesnake Ash Flow Tuff the area was again dominated by shrubs and grasses growing in a semi-arid climate. Forests grew around lakes and rivers, and at the higher elevations.

An artist's rendition of the Rattlesnake Ash Flow Tuff.

A mural from the museum exhibit depicts the dramatic Rattlesnake Ash Flow Tuff event.

Grazers such as many extinct species of horses, elephants, rhinos, camels, pronghorns, and deer flourished in this prairie environment. They were joined by other common animals such as relatives of peccaries, dogs, short-faced bears, giant ground sloths, and true cats.

Image of a mural of the Rattlesnake paleoecology.

Grasslands dominated the landscape during the Rattlesnake.