But an inland sea did stretch from the Gulf of Mexico into North Dakota during the Paleocene. In fact, the interior of North America was largely covered by shallow seas during much of the distant past. After the Rocky Mountains rose during the Cretaceous Period, America's inland seas began retreating. North Dakota's Paleocene seas left a layer of rocks called the Cannonball formation, which is best exposed in the Bismarck-Mandan area and southwest of these two cities. This is the best place to find North Dakots's state fossil, teredo petrified wood.
Shipworms are actually mollusks, related to clams.
They bore holes in wood and ships even today. At the front end of a shipworm s long, worm-like body are two hard shells. Finely toothed ridges on these shells rasp wood as the shells are rocked back and forth. As the animal rasps, the disc is adjusted to slowly turn its body, resulting in a circular hole. A shipworm lines its new home with material that hardens into a protective limy tube.