Yet during the Paleocene epoch, between about 67 and 55 million years ago, western North Dakota was home to a subtropical to temperate forest with trees up to 12 feet in diameter and over 100 feet tall. During this time, western North Dakota must have looked much like coastal Georgia or Mississippi, with meandering rivers, everglade-like swamps, and vast, forested floodplains. Petrified trees, stumps, and logs provide some of the most spectacular evidence for this ancient landscape. North Dakota's Paleocene-age rocks contain most of the petrified wood found in the state. The petrified wood occurs as entire logs, stumps that still stand upright where they once grew, and as scattered limbs and fragments.
Some localities contain so much fossil wood that they can be termed petrified forests.
Petrified wood is widespread in western North Dakota, and pieces of it can also be found in gravels in the glaciated portion of the state. It is commonly found in badlands areas, where entire logs and even upright stumps are exposed. Petrification requires rapid burial of the wood to prevent normal decay. This can happen, for example, as rivers overflow their banks, burying the forest floor under a layer of sand and silt, or when forests are covered by volcanic ash. After burial, mineralized groundwater begins to seep through the wood, coating cell walls and filling the intercellular cavities.