How Fossil Tree Stumps Form
In-place, standing, erect fossil tree stumps are relatively common in Kentucky’s coal fields. They are found worldwide from many geologic ages. Kentucky’s coal-field stumps are the fossils of Pennsylvanian-age trees that grew in wetland forests more than 310 million years ago!
If you’ve ever been to a lake, river, or estuary and have seen tree stumps in or under the water, you’ve seen part of the process for forming a fossil tree stump. Tree stumps can be buried in many ways, but the majority of fossil tree trunks, especially in Kentucky’s coal fields, originated as trees that lived in swamps along rivers, lakes, and estuaries and were buried by flooding in those areas.
Most of Kentucky’s fossil tree stumps began as wetland trees that either were growing in standing water (like modern cypresses) or were drowned by rising water levels. In rivers and lakes, flooding can happen seasonally, or at different scales at longer intervals (for example, 50-year floods, 100-year floods, etc.). In estuaries, rising water can happen from changing tides, which also occur at different scales, and different scales at longer intervals (for example, neap tides, spring tides, etc.). Along coastlines, flooding can happen from storms (like hurricanes), rising sea level, or subsidence of coastal sediments. Trees can also be buried by shifting sand dunes.
The majority of tree stumps are not buried or fossilized, but under the right conditions, those that are can be fossilized. Rapid burial aids in preservation and fossilization. The longer stumps are exposed, the more likely that they will decompose or be eroded. Where stumps are drowned in swamps and lakes, stagnant, dysaerobic to anaerobic (low or lacking oxygen) water can also aid in limiting bacterial degradation and rotting. After initial burial, long-term burial processes work to change sediment to rock and some buried tree stumps to fossil stumps.
Two general types of fossil tree stumps are found in Kentucky’s coal fields:
- Internal casts and molds: In some cases, tree stumps were buried, but sedimentation and infilling occurred prior to complete rotting of the trees. Some trees were partially hollow in life, or were hollowed out by the initial stages of rotting. If the outer part of the tree was still intact when sediment filled the tree hollow, an internal mold could form. Once the tree decomposed, the outer surface of the sediment fill became the inside surface of the original tree, or an internal mold. The infilled hollow, which is preserved as an in-place tree stump, is an external cast. Any patterns preserved on the fossil stump and external mold represent patterns of tissues on the inside of the tree, rather than bark-like patterns on the outer surface of the original tree.
- External casts and molds: In some cases, the original tree stump was buried and rotted. A hole was left where the tree was. That hole was filled in with sediment. The outer surface of the hole is an external mold of the original tree. The infilled hollow, which is preserved as an in-place tree stump, is an external cast. Bark patterns are preserved on the fossil stump and external mold.
Most of the tree-stump fossils reported from the coal fields of Kentucky are not petrified trees. Petrification is a specific fossilization process in which mineral-rich water infiltrates the tissues and even cells of animals and plants before turning to stone (see petrified fossils). If you break open or cut and polish a slice through a petrified tree fossil, you can see growth rings and structures of the original tree. In contrast, if you cut open a sediment-filled cast of a tree, you won’t find any tree structures because the interior of the original tree was hollowed out, and the fill, which is most of the fossil, was only sediment (mud and silt). Petrified fossils are encountered in the coal fields as carbonate coal balls, but they represent parts of the peat that formed the coals, so preserve a wide array of partially degraded leaves, roots, stems and other plant parts, rather than whole stumps.
Another aspect of tree-stump fossilization is the need for relative space of burial, which in geologists’ terms is called accommodation space. In general, there has to be room (thickness) for a tree stump of a given height to be preserved and buried, and not subsequently eroded. In the short term, lakes and rivers are natural depressions, so they provide space in which burial can occur. That depression has to stay buried for long-term preservation and fossilization, however. Several different mechanisms can create accommodation space for the long-term preservation of fossil tree stumps, including sea-level rise, subsidence caused by compaction of underlying sediment, and subsidence caused by earthquakes and tectonic activity. Each of these mechanisms may have contributed to the preservation of fossil tree stumps in the coal-age rocks of Kentucky.