According to the National Park Service:

Nearly 150 species of fossil plants from Yellowstone have been found, spanning 500 million years, from the Cambrian to the Holocene. Most petrified wood and other plant fossils come from Eocene deposits about 50 million years old, which occur in many northern parts of the park. Best known are the fossil forests of Specimen Ridge, where the remains of hundreds of these 50-million-year old trees stand exposed on a steep hillside, with trunks up to eight feet in diameter and some more than 20 feet tall. The specimens include sequoia, fir, and numerous deciduous species.

You can hike to this "stone forest" using the Specimen Ridge Day Hike Trail.