8,379 feet (2,554 m) is an approximately 8.5-mile (13.7 km) ridge along the south rim of the Lamar Valley in Yellowstone National Park.

Within the Specimen Mountain area, it consists predominantly of an undetermined thickness of conglomerate that is interbedded with lesser proportions of tuffaceous sandstone and siltstone.

The lahar (mudflow) deposits consist of normally massive and structureless, matrix-supported conglomerates that contain subangular, poorly sorted gravel that range in size from 1 cm (0.39 in) to 2 meters (6.6 ft) in diameter.

The fossil leaves, needles, pollen, and cones are largely found within tuffaceous sandstones and siltstones that were deposited either along the banks of either braided or meandering rivers, within their abandoned channels, or in shallow lakes of very limited extent.

At Specimen Ridge, these sediments consist of volcanic material eroded from and accumulated downslope of an adjacent Eocene stratovolcano, known the 'Washburn Volcano', in an intermountaine basin.

Based on radiometric dates and plant fossils from it, the Lamar River Formation is considered to be of Middle Eocene age.

[16][17][18][19][20] Prehistoric logs and upright trunks that are buried in Late Pleistocene lahar and stream deposits of Mount St. Helens were found to be the initial stages of being naturally petrified by silica.