Clues exposed at the surface help the nearby hills tell their story. Most were formed from abundant volcanic ash-falls and floods of lava over many millions of years. About five million years ago the land-building slowed and erosion cut down into the previous layers resulting in the landscape we see today.
Now only hilly remnants to the west, the ancestral Cascade Range of volcanoes once erupted cloud after cloud of ash that landed here. These beds of ash-fall make up the John Day formations - The Big Basin, Turtle Cove, and Haystack Valley strata, each revealing ancient changes in regional geology and life forms.
Further clues reveal that the John Day formations were later subjected to massive forces that tilted the layers downward to the east. Then, floods of lava poured out across the surface, forming flat molten lakes. This series of basalt floods hardened and protected much of the softer layers underneath from the forces of erosion.
Picture Gorge Basalts - 16 Million Years Ago
Over 60 lava flows issued from nearby cracks in the earth, covering over 2,000 square miles, about one flow every 8,000 years. These dark basalt layers form much of the rimrock of the John Day country.
Haystack Valley - 20 to 22 Million Years Ago
Episodic, heavy ash-falls from volcanoes of a mature ancestral Cascade Range created these sediments. Fossilized mammal bones are commonly found in this soft yellowish rock.
Picture Gorge Ignimbrite - 28 Million Years Ago
From an explosive eruption to the west, a fiery tidal wave of volcanic ash and debris raced across the land. It fused together to create this hard layer. Ignimbrite was much too hot to preserve plant and animal remains.
Turtle Cove - 22 to 28 Million Years Ago
Named after a cove landscape where many turtle fossils have been found, the Turtle Cove layers were formed from abundant volcanic ash-fall from a growing ancestral Cascade Range. Portions of these layers are rich in fossils.
Big Basin - 28 to 39 Million Years Ago
Vivid red and gold layers make up the Painted Hills, part of the Big Basin strata. Sporadic ash-falls from a young ancestral Cascade Range did not bury organic remains deep enough to fossilize well. Nearby pond deposits of this age are rich in leaf fossils.