The red and tan layers of the hills reflect the climatic cycling between warm/wet and cool/dry, which occurred between 34 and 28 million years ago. Clay type, moisture levels, and the amount of oxidized iron present in the soil determined the hills’ colour palette.
The hills’ ancient soils are mixed with past lakebeds and river deposits. Scattered about the hills are fossil leaves, remnants of the broad-leaf deciduous trees that once grew on the edge of those long-vanished lakes and streams.
The Painted Hills are part of the John Day Fossil Beds National Monument.