Welcome to the Age of Mammals

“No region in the world shows a more complete sequence of Tertiary land populations, both plant and animal than the John Day Basin.” - Dr. Ralph W. Chaney

Preserved within the assemblages of the John Day Basin is a fossil record that spans over 40 million years. If each time period recorded here is a page in a book, the National Monument holds an entire chapter of Earth’s history which tells the story of changing life and landscapes. Explore each of the fossil bearing assemblages of the John Day Basin and imagine how much life and the landscape has changed in the past 50 million years. What will it look like in another 50 million years?
Ancient Oregon used to be warmer & received much more rain than today.
Clarno Nut Beds (44 Ma)

Oregon was a hot, wet, semitropical place filled with a wide diversity of plants including palm trees and avocados.

Hancock Mammal Quarry has a large concentration of fossils.
Hancock Mammal Quarry (40 Ma)

Temperatures and rain decrease compared to the Clarno Nut Beds but remained mostly above freezing.

Lake Beds and Deciduous Trees
Bridge Creek (33 Ma)

A wide variety of plant material has been preserved in fine grain lake sediment including the Metasequoia, Oregon's state fossil.

30-25 million years ago, the forest canopy gradually opened allowing more open spaces.
Turtle Cove Assemblage (30-25 Ma)

Turtle Cove is the thickest and most productive fossil-bearing layer within the John Day Fossil Beds, yet few leaf fossils were preserved.

Grasses make their mark on the landscape and its fauna.
Upper John Day Assemblage (24-20 Ma)

The ecosystem became an open habitat with the appearance of burrowing and running animals.

As grasslands expanded some species thrived while others slowly died out.
Mascall Assemblage (15 Ma)

Long-legged hoofed animals including horses, camels, and giraffe-deer swiftly crossed open meadows to escape beardogs and cats.

Welcome to the semiarid wooded shrubland with giant sloths and saber toothed felines.
Rattlesnake Assemblage (7 Ma)

The ecosystem preserved here is more familiar to modern eyes- except for the occasional elephant and giant sloth.

Last updated: January 5, 2023

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32651 Highway 19
Kimberly, OR 97848


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