Yellowstone Petrified Forest
| Amethyst Mountain | |
|---|---|
| Highest point | |
| Elevation | 9,609 ft (2,929 m) |
| Coordinates | 44°49′43″N 110°15′15″W / 44.82861°N 110.25417°WCoordinates: 44°49′43″N 110°15′15″W / 44.82861°N 110.25417°W |
| Geography | |
| Location | Yellowstone National Park, Park County, Wyoming, U.S. |
| Parent range | Washburn Range |
| Topo map | Amethyst Mountain |
Amethyst Mountain, el. 9,609 feet (2,929 m) is the highest peak and central part of a northwest – southeast trending ridge that lies between the Lamar River to the northeast and Deep Creek to the southwest within Park County, Wyoming. From northwest to southeast, this ridge consists of Specimen Ridge, Amethyst Mountain, and the Mirror Plateau in Yellowstone National Park. The nearest town is Silver Gate, Montana, which is 19.2 miles away.
In 1872, it was named Amethyst Mountain by the United States Geological Survey for amethysts found on its summit. This mountain is known for its abundance of amethyst, opal and exposures of well known and visited petrified forests. The summit of Amethyst Mountain is traversed by Specimen Ridge Trail, which also traverses the south side of Specimen Ridge between Tower Junction and Soda Butte Creek and through the Yellowstone Petrified Forest.
Amethyst Mountain consists of a geological formation known as the Lamar River Formation. Within the Amethyst Mountain area, it is over 440 feet (130 m) thick and consists predominantly of conglomerate and lesser proportions of tuffaceous sandstone and siltstone. Volcanic breccia is absent and only a very few thin airfall volcanic ash beds have been identified in the exposures at Amethyst Mountain. The conglomerates consist of a variety of mudflow deposits (lahars) and braided and meandering stream deposits. The lahar (mudflow) deposits consist of conglomerates that occur in massive and structureless beds. These deposits typically consist of matrix-supported, subangular, poorly sorted gravel that ranges in size from 1 cm (0.39 in) to 2 meters (6.6 ft) in diameter. Fluvial conglomerates are normally well-bedded and cross-bedded. They typically consist of grain-supported, subrounded, and moderately well-sorted gravel that typically ranges in size from 1 cm (0.39 in) to 30 cm (12 in). Most of the petrified wood and buried tree trunks occur within the conglomerates. Almost all of the fossil leaves, needles, pollen, and cones are found in the tuffaceous sandstones and siltstones. These tuffaceous sandstones and siltstones accumulated either along the banks of either braided or meandering rivers or within their abandoned channels. In the Amethyst Mountain area, the sediments comprising the Lamar River Formation consist of volcanic material eroded from and deposited downslope of surrounding stratovolcanoes. Locally, the sediments of the Lamar River Formation accumulated in a basin lying between two belts of Eocene volcanoes and at the base of the northern edge of Eocene 'Washburn Volcano'. The Lamar River Formation is part of the Washburn Group.
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