Published 09/2019- updated 09/2026
Imagine finding an upright fossilized tree extending vertically through several layers of sedimentary rock. If those individual layers required immense periods of time to accumulate, how could the exposed portion of the tree have remained intact long enough to be buried?
These remarkable fossils are commonly called polystrate fossils, meaning that a single fossil extends through multiple sedimentary strata. Their existence provides dramatic evidence of something geologists readily recognize: sedimentary layers can sometimes accumulate very rapidly during catastrophic events.
Mount St. Helens Showed How It Can Happen
The catastrophic eruption of Mount St. Helens on May 18, 1980 provided a modern example of processes capable of producing geological features that might otherwise be difficult to reconstruct from the rock record alone. The eruption devastated the surrounding forest and transported enormous numbers of trees. Subsequent volcanic sediment flows transported and redeposited numerous stumps and logs.[1]
Researchers documented something especially significant: some transported trees were deposited upright. A peer-reviewed study of sediment flows associated with the 1980 and 1982 Mount St. Helens eruptions found that about 4 to 13 percent of the transported stumps and logs examined were deposited upright, while most of the remainder were deposited horizontally.[1]
This demonstrates an important geological principle: An upright fossil tree does not necessarily mean that the tree grew where it was eventually buried.
Catastrophic sediment flows can transport trees and redeposit some of them vertically.
Yellowstone’s Fossil Forests
These were split and set in sediments–
not rock layers over millions of years.
Yellowstone National Park contains spectacular deposits of petrified trees, including hundreds of upright trunks and stumps preserved within volcanic sediments. For many years, scientists interpreted these deposits primarily as successive forests that grew, were buried, and were eventually replaced by new forests growing above them.
Mount St. Helens provided an important modern comparison.
More recent research indicates that Yellowstone contains a mixture. Some trees appear to have been buried where they grew, while others were transported by volcanic debris flows.[3] This matters because finding an upright fossil tree alone does not prove it grew in that location for centuries before being buried.
The National Park Service explains that the Mount St. Helens eruption showed that rapidly moving volcanic debris flows can uproot and transport trees and can even deposit them upright. The NPS specifically states that this evidence contributed to changes in the interpretation of Yellowstone’s fossil forests.[2]
A Tree Cannot Wait Millions of Years to Be Buried
This is where polystrate fossils become especially significant. Consider an upright tree extending through several sedimentary layers. Whatever timescale is assigned to the larger geological formation, the sediment immediately surrounding that individual tree had to accumulate rapidly enough to bury and preserve it before the exposed trunk was destroyed by decay, weathering, organisms, or physical erosion. The tree itself therefore places a practical constraint on how slowly those particular sediments could have accumulated.
A dead tree cannot remain partially buried indefinitely while its upper trunk waits for additional sediment. The preservation requires burial. And substantial preservation requires burial before ordinary biological and physical processes destroy the tree.
Upright Fossil Trees Are Found Elsewhere
Upright fossil trees are not unique to Yellowstone. One spectacular example occurs at the Joggins Fossil Cliffs in Nova Scotia, a UNESCO World Heritage Site famous for its Carboniferous fossil forests. UNESCO confirms that upright fossil trees occur at numerous levels throughout the cliffs.[4] Detailed geological research describes several-meter-thick units of sandstone and mudstone containing numerous “entombed erect trees.” Some preserved upright trunks reach approximately six meters, or nearly twenty feet, in height.[5] Researchers interpret many of these Joggins trees as having been buried where they stood when sediment entered the ancient wetlands.[6] Whether transported or buried in place, the preservation of such tall, upright trunks shows that substantial amounts of sediment can accumulate around trees before they are destroyed.
What Polystrate Fossils Actually Demonstrate
Polystrate fossils do not, by themselves, establish the age of the entire geological column.
But they demonstrate something extremely important: Rock layers do not necessarily represent slow accumulation over vast periods of time.
Multiple layers surrounding a fossil can accumulate rapidly.
Trees can be transported.
Trees can be deposited upright.
Standing trees can be rapidly entombed in sediment.
Volcanic eruptions, mudflows, floods, and other catastrophic processes can produce substantial geological deposits in remarkably short periods.
Mount St. Helens provided a modern demonstration of several of these processes. The lesson is therefore not that every rock layer everywhere formed rapidly. The lesson is that we cannot determine how long sediment took to accumulate merely by counting layers. The physical evidence must determine the interpretation. And when an upright tree cuts through multiple sedimentary layers, that tree provides direct evidence that the sediments enclosing that portion of the tree accumulated rapidly enough to bury and preserve it.
The Challenge
When you see a fossilized tree extending vertically through multiple rock layers, ask a simple question: How slowly could those particular layers really have formed while the tree was still there to be buried?
The fossil itself demands an answer.
Sources
1. Fritz, W. J., and Harrison, S. (1985). “Transported trees from the 1982 Mount St. Helens sediment flows: Their use as paleocurrent indicators.” Sedimentary Geology, Vol. 42, pp. 49–64. The researchers reported that 4–13% of transported stumps and logs examined were deposited upright and explicitly discussed the implications for interpreting Yellowstone’s fossil forests.
https://doi.org/10.1016/0037-0738(85)90073-9
2. National Park Service, Yellowstone National Park. “Fossils.” The NPS explains that Mount St. Helens demonstrated that volcanic debris flows can transport trees and deposit them upright, resulting in modifications to the traditional interpretation of Yellowstone’s fossil forests.
https://www.nps.gov/yell/learn/nature/fossils.htm
3. National Park Service. “The Incredible Fossil Plants of Yellowstone National Park.” Discusses evidence that Yellowstone’s deposits contain a mixture of trees buried where they grew and trees transported by volcanic mudflows.
https://www.nps.gov/articles/000/the-incredible-fossil-plants-of-yellowstone-national-park.htm
4. UNESCO World Heritage Centre. “Joggins Fossil Cliffs.” UNESCO describes upright fossil trees preserved at numerous levels throughout the Carboniferous exposures at Joggins.
https://whc.unesco.org/en/list/1285
5. Davies, S. J., and Gibling, M. R. (2003/2005). “The Pennsylvanian Joggins Formation of Nova Scotia: sedimentological log and stratigraphic framework of the historic fossil cliffs.” Atlantic Geology. The study describes several-meter-thick sandstone and mudstone units containing numerous entombed erect trees, with upright trunks preserved to heights of approximately six meters.
https://journals.lib.unb.ca/index.php/ag/article/view/182
6. Atlantic Geology, Joggins Formation research. Detailed sedimentological research concludes that major levels containing standing trees were entombed as distributary channels carried sediment into coastal wetlands.
https://journals.lib.unb.ca/index.php/ag/article/download/182/692
