Polystrate fossils are single organism fossils that are encased in multiple layers of rock, such as a tree trunk. "Poly" means more than one, and "strata" means rock. These fossil types are caused by rapidly-moving, sediment-laden fluids that create an abundance of layers, such as volcanic eruptions.
Polystrate fossils can be found at Yellowstone National Park, the Mississippi River Delta, Mount St. Helens and Mount Pinatubo in the Philippines. They are often used in arguments between creationists and evolutionists; creationists believe a catastrophic event such as Noah's flood must have caused them. However, polystrate fossils can be easily recreated in lab experiments.