Field of Science

The fossil forests of Amethyst Mountain

The fossil forests of Specimen Ridge and Amethyst Mountain, both situated in the Yellowstone National Park, are peculiar because of many preserved trees still found as upright trunks and stumps emerging from the sediments.
The geologist, anthropologist and artist Dr. William H. Holmes was the first to study the outcrop of Amethyst Mountain and published his observations in 1878:

"As we ride up the trail that meanders the smooth river bottom [the Lamar River] we have but to turn our attention to the cliffs on the right hand to discover a multitude of
the bleached trunks of the ancient forests. In the steeper middle portion of the mountain face, rows of upright trunks stand out on the ledges like the columns of a ruined temple. On the more gentle slopes farther down, but where it is still too steep to support vegetation, save a few pines, the petrified trunks fairly cover the surface, and were at first supposed by us to be shattered remains of a recent forest."

Fig.1. Pencil drawing ca. 1879 by Holmes of the outcrop of Amethyst Mountain, National Archives.

Fig.2. Two fine specimens of fossil tree trunks of Pilyoxylon aldersoni in the fossil forest on a steep hillside of Specimen Ridge, exposed by erosion of the basic breccia in which the stumps and roots are firmly embedded. Nearby a hoodoo, showing the character of the breccia and the manner in which it has been deposited. Circa 1890. Figure 2, U.S. Geological Survey Folio 30, USGS Photo Library.

Holmes studying the outcrops realized that the silicified stumps are distributed in various levels (more than fifty are today recognized) and embedded in volcanic deposits like ash, mud flows and breccias. These deposits provided also an explanation of the remarkable well preserved structure of the wood: from the rocks the silica was solved by the percolating water and substituted the organic matter of the wood. Inside of some hollows trunks even geods with crystals of amethyst formed and the name of the mineral was adopted previous of 1870 to the entire site, unfortunately most of these specimens were removed by collectors already in the late 19th century.

The upright position and the well preserved roots are a sign that the trees became emb
edded in situ, covered by ashes, mudstone, breccias and conglomerates during a volcanic eruption. There are also signs of fluvial reworking of the deposits, many trees rooted in tuffaceous sandstones, interpretated as paleosoils, and conglomerates deposited by a braided river system.

Fig.3. Schematic profile of Amethyst Mountain as imagined by Holmes and published in "Fossil forests of the volcanic tertiary formations of the Yellowstone National Park. Bull U.S. Geological and Geographical Survey of the Territories Vol.5.(1), 1879-1880"

A modern example possibly resembling the environment of formation of the petrified Yellowstone forests was the landscape after the eruption of Mount St. Helens in 1980. Here a violent eruption razed down large areas of the forest surrounding the mountain, leaving behind stumps that were buried under pyroclastic flows and lahars.
Today, more than 30 years later the effects of the explosion are still visible, even if a new forest has begun to colonize the devastated area. Subsequent research showed that St. Helens experienced many eruptions in the Holocene, repeatedly the area was devastated and repeatedly the area was reconquered by nature.

Also in Yellowstone the buried forest was destroyed, entombed in the volcanic deposits and the new formed landscape colonized again by a new forest. The time period involved in these cycles was however at the time of Holmes unclear, however the perfectly preserved tree rings inspired geologists to try to estimate how much time was involved in the growth phase of the plants and finally in the formation of the entire sedimentary succession:

"Pine trees of the types represented in the fossil trunks require 200 or 300 years to reach maturity, and redwoods may require from 500 to 1,000 years. Twelve or more of these forest levels have been found. By multiplying this number by the minimum age of the trees (200 years) we shall have 2,400 years, and by multiplying it by the maximum age of the redwood (1,000 years) we shall have 12,000 years as the possible time during which these forests flourished. It is possible that the truth lies somewhere between these extremes."

Glacial erosion on Specimen Ridge and Amethyst Mountain showed that the volcanic activity was older than the ice age and therefore geologist inferred a tertiary age, so Knowlton in 1921 summarizes:

"After the Cretaceous period, there was a time of great volcanic activity, which appears to have lasted until perhaps the beginning of the glacial epoch."

Today the sediments are dated thanks to modern methods into the Eocene. The events at St. Helens provided also a possible time period for the formation of the fossils, well preserved wood was found in centuries old lahars of the volcano with silica beginning to substitute the organic matter, in some cases even after 36.000 years the fossilisation was not completed.

However I have to note that this scenario is only a theory and there is an alternative explanation: the forest was buried in a worldwide flood send by a magic, invisible creature living in the sky that so loved its own creation that it decided to destroy it to punish a single species of upright, naked ape, with the exception of a single family dedicated to inbreed - much more plausible than a series of boring volcanic eruptions...

Online Ressource:

COFFIN, H.G. (1976): Orientation of trees in the Yellowstone Petrified Forests. Journal of Paleontology. Vol.50(3): 539-543. (Accessed 16.05.2011)

KNOWLTON, F.H. (1921): Fossil Forests of the Yellowstone National Park. USGS Monograph 32: 651-791. (Accessed 16.05.2011)

VINEY, M. (2008): The Virtual Petrified Wood Museum. (Accessed 16.05.2011)

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