A polystrate fossil is a fossil of a single organism (such as a tree trunk) that extends through more than one geological stratum. The word polystrate is not a standard geological term.
This term is typically found in creationist publications.
Scientists interpret polystrate fossils as fossils buried in a geologically short time span - either by one large depositional event or by several smaller ones. Geologists see no need to invoke a global flood to explain upright fossils. This position of geologists is supported by numerous documented examples of buried upright tree-trunks that have been observed buried in the Holocene volcanic deposits of Mount St. Helens.
Polystrate fossils are not unique to any particular geological period or environment. They have been found in rocks of all ages and from all over the world. This suggests that they are not the result of any single, unique event.
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Creation apologetics is not evidence for a young earth