The main story of the Petrified Forest is one of geology. Uplift of the Colorado Plateau and the erosional forces of water and wind have exposed rocks originally deposited during the Late Triassic Epoch. These rock layers or strata record details of millions of years of Earth’s history including changing environmental conditions through time as well as records of the plants and animals that lived in these environments—including the earliest dinosaurs.
- The Triassic aged rocks in the park are assigned to the Chinle Formation and consist mainly of sand, mud, and gravel that were deposited by a large river system from 225 to 205 million years ago.
- During this time the area that is now Petrified Forest National Park was located along the western coast of the supercontinent Pangaea and located approximately four degrees above the equator, roughly equivalent to modern-day Panama!
- These rocks preserve not only fossils but evidence of past environments including climate, which demonstrates that through time the climate of the area went from very wet and sub-tropical to more arid.
- The colors of the rocks provide evidence of the ancient environments, blues and green suggest wetter, “swampier” conditions on floodplains, tans and whites represent sandier areas such as ancient river channels, and reds and purples suggest low water tables and drier conditions.
- The large petrified log ‘forests’ are preserved mostly in the remnants of ancient river channels.
- More recent deposits include remnants of small lava flows which protected the underlying less resistant Chinle Formation from erosion forming the present Painted Desert overlooks.
The spectacular rock exposures in the park tell an amazing story of past ecosystems. Because the layers seen in the rocks represent discrete periods of time and record the environment at that time, the layers can be read in order like the pages of a book. Starting in the oldest rocks in the Tepees area, and moving upwards into younger rocks in the Flattops and Painted Desert areas, we see a story of changing ecosystems through time including marked changes in climate and a fossil record that shows how the animals and plants were affected by these changes.