Specimen Ridge is an approximately 8.5-mile ridge on the south rim of Yellowstone's Lamar Valley, dividing it from Mirror Plateau and extending toward Amethyst Mountain. Known before 1870 for amethyst, opal, and especially petrified wood, it became significant scientifically as part of the Yellowstone Petrified Forest. Its rocks, within the Lamar River Formation, were formed in the Middle Eocene from volcanic mudflows and river deposits eroded from the Washburn Volcano. More than 130 years of study have shown that some fossil forests were buried where they grew, while other wood was transported and redeposited. Today, a trail crosses the ridge, but fossil collecting is illegal.