9,609 feet (2,929 m)[1] 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.

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.

In the Amethyst Mountain area, the sediments comprising the Lamar River Formation consist of volcanic material eroded from and deposited downslope of surrounding stratovolcanoes.

Dark colored pyroxene andesite lava flows, volcaniclastic rocks, and basalts of the Sunlight Thorofare Creek Group overlie the Lamar River Formation.

[15] A noteworthy feature of Amethyst Mountain are the multiple horizons of well-preserved petrified forests and concentrations of silicified wood found within the Lamar River Formation that is exposed on it slopes.

[20][21][22][23] The Late Pleistocene lahar and stream deposits of Mount St. Helens contain buried prehistoric logs and in place (in situ) upright tree trunks that are in the initial stages of being naturally petrified by silica.

[16][17][18][19] As noted above, it is now known that the petrified wood found within the Lamar River Formation is a mixture of material that were either buried in place or transported downslope by lahars and streams.

In addition, more recent research has found that the lateral extent of the beds containing either transported wood and stumps or an in place fossil forest is, contrary to assumptions made by earlier studies, quite limited.