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. Part of a petrified stump near Medora, North Dakota. Most North Dakota petrified wood is brown or tan on weathered surfaces and dark brown where fresh, but colors range from white to gray, sometimes with streaks of black. Sometimes, cavities in the wood are encrusted with chalcedony and lined with quartz crystals.