This name is very confusing because some wood was bored by other shipworm species. Also, the bored wood is not always petrified, or fossilized. In some cases, the wood has been replaced by mudstone or sandstone, leaving a rock full of holes. North Dakota s state fossil was promoted by members of mineral and gem clubs. Many rock collectors polish the unusual wood, sometimes making it into bookends and other useful items.
Cretaceous rocks cover most of the state between the Red River Valley and Missouri River.
These contain fossils of both land and sea creatures from the Age of Dinosaurs, including Tyrannosaurus. The Missouri River forms a rough border between the Central Lowlands that cover so much of the Midwest and the Great Plains that stretch south into Texas. Rocks west of the Missouri River represent mostly the Cenozoic Era, or Age of Mammals. Fossils of Ice Age creatures lie strewn across the entire state. From Frozen Sea to Grassland Sea North Dakota remains as flat as it probably was when it was still covered with ancient seas.