Thousands of years ago, during a slightly warmer part of the Pleistocene Epoch, there was a spruce forest where Peffer Park now stands. Mammoths, saber-toothed cats, ground sloths, and other now-extinct beasts roamed the forest. As the climate cooled, an ice sheet advanced and plowed the forest beneath it, burying the spruce trees in the clay-rich glacial till at its base. Sealed off from oxygen in the dense glacial till, the wood from these trees was preserved well enough to allow geologists to date it using naturally radioactive carbon-14. Radiocarbon dating of wood from the till at the Bluffs has shown that the wood is about 24,000 years old. In some instances, fossil wood may be petrified, where all the organic materials are replaced with minerals. The study of fossil wood is called palaeoxylology.

Find more information at these sites:
National Park Service: A Link to the Past
Fossil Wood at Wikipedia.org