John Day Fossil Beds -
Painted Hills Unit

The Painted Hills are composed mostly of claystone layers, which include ancient soils and lake beds. The surface of the hills have weathered and softened to clay. The claystones were formed by several events and processes. Over 30,000,000 years ago, volcanoes from ancestral Cascade Mountains, 1 00 miles to the west, slowly deposited layer after layer of cooled ash. Plants and animals churned and tilled the surface, and air oxidized the ash. Ground water flowed, leaching and redistributing minerals. Over time, the ashfall became soil, later buried under hundreds of feet of deposits. Compaction, cementation, and recrystallization processes occurred. The original ashfalls were thus changed structurally and chemically, resulting in hard and colorful stone.