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 . These concentrations of petrified wood are known as the '' Yellowstone Petrified Forest '' . Extensive exposures on the northeast slopes of Amethyst Mountain comprise a classic study area , known as the '' Fossil Forest , '' that has been studied in detail by a number of researchers for more than 130 years . Some of the beds contain prehistoric forests buried in place ( '' in situ '' ) by volcanic lahars . Other beds are concentrations of fossilized upright stumps , flat-lying logs , and logs lying at various angles that have been transported from the higher slopes of adjacent volcanoes and buried by braided and meandering streams and volcanic lahars . Notably , virtually identical lahar and stream deposits that contain buried upright standing trunks of forests and beds of transported logs and upright stumps were created by the 1980 eruption of Mount St. Helens , and other Quaternary and Holocene eruptions of other Cascade Range volcanoes . 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
1980 eruption of Mount St. Helens
1980 eruption of Mount St. Helens
Amethyst Mountain