Paleontology and geologyMost of North Dakota was above sea level during the Tertiary. Volcanoes were erupting to the north and west, and huge quantities of volcanic ash were blown by wind or carried by rivers into the state. These ash deposits became layers of bentonite clay that were interbedded with the river and lake deposits derived from the erosion of the rising Rocky Mountains. Some of the rocks formed from these sediments are exposed in Theodore Roosevelt National Park in the western part of the state. Fossils of freshwater molluscs, titanotheres, crocodile-like champsosaurs, sequoia, bald cypress, magnolia, and ginkgo can be found in these rocks. |