The geologic formations within the 20 square mile area of the John Day Fossil Beds contain a remarkable, diverse fossil record covering roughly 40 million years of earth's history. This record provides a view of a large portion of the Tertiary Period, a time when mammals and flowering plants were undergoing dramatic changes. The quality of the preservation of many of the fossil specimens is also exceedingly good.
The Painted Hills Unit is reknowned for the varied hues of red, orange, black and tan found in the exposed volcanic layers. This area also is home to a diverse assembledge of leaf fossils from a temperate forest.
The Clarno Unit features the commanding Palisades, a cliff formed by a series of ancient volcanic mudflows (lahars) that now is high above the surrounding landscape. Glimpses of fossilized plants from millions of years ago can be seen from the Trail of Fossils.
The Sheep Rock Unit was named for the prominent Sheep Rock peak, which towers 1,100 feet above the John Day River valley. The exposed green claystone layers seen in this area back 30 million years and new vertebrate fossils continue to be exposed through natural erosion.