The fossil forests of Specimen Ridge and Amethyst Mountain, both situated in the area of the Yellowstone National Park, are peculiar because of many preserved trees still standing upright.
The geologist, anthropologist and artist Dr. William H. Holmes was the first naturalist to study the outcrop of Amethyst Mountain and to publish 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 (image in public domain).
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 basal 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. U.S. Geological Survey Folio 30, USGS Photo Library.
Holmes studying the outcrops realized that the petrified stumps are distributed in various levels (more than fifty are today recognized) and embedded in volcanic ash, mudflows and breccias. These deposits provided also an explanation of the remarkable well preserved structure of the wood: from the volcanic rocks the silica was solved by the percolating groundwater and slowly substituted the organic components of the wood.
The upright position and the well preserved roots are evidence that these trees became embedded in situ in the sediments, probably covered by ashes, mudstone, breccias and conglomerates during a volcanic eruption. There is also evidence of fluvial reworking of some of the sediments, as many trees rooted in tuffaceous sandstones, interpretated as paleosoils, and conglomerates with pebbles rounded by fluvial transport.
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 forests, was the landscape as seen after the eruption of Mount St. Helens in May 1980 (as this paper by KAROWE & JEFFERSON 1987 emphasizes). The violent initial eruption destroyed large areas of the forest surrounding the mountain, leaving behind stumps that were buried under pyroclastic flows and mudflows.
Today, more than 30 years later, the effects of the eruption are still visible, even if new trees have started to colonize the devastated area. Subsequent research showed that St. Helens experienced many eruptions in the last 10.000 years. Repeatedly the forest was annihilated and repeatedly the barren landscape 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. Holmes and other geologists could only speculate how many times this happened in the past:
"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."
COFFIN, H.G. (1976): Orientation of trees in the Yellowstone Petrified Forests. Journal of Paleontology. Vol.50(3): 539-543
KNOWLTON, F.H. (1921): Fossil Forests of the Yellowstone National Park. USGS Monograph 32: 651-791