The Gallatin Petrified Forest, located near Rock and Porcupine creeks in the Gallatin Range, is between 35 and 55 million years old. Geologically speaking, it extends back in time as far as the early part of the Eocene Epoch of the Tertiary Period. The Gallatin Petrified Forest is unique because many of the trees were petrified in the upright position.
Most petrified forests contain trees petrified after being transported by mud and lava flows.
This movement leaves the majority of the trees in a horizontal position. At several localities, as many as 80 superimposed laters of upright trees exist. Scientists believe that the variation in tree layers found throughout the Gallatin Petrified Forest is possibly the result of two different events. First, many of the trees were buried in place and in an upright position by ash and mud flows. Second, some trees were uprooted and deposited in bodies of water.