The Coteau des Prairies: blue arrows indicate paths of the two lobes of the glacier around either side of the formation.This excerpt from the Lewis and Clark map of 1814 shows the rivers of western Iowa and eastern South Dakota. The Coteau des Prairies is seen near the upper center of the map, "High land covered with wood called mountain of the prairie."
The flatiron-shaped plateau was named by early French explorers from New France (Quebec), coteau meaning "hill" in French; the general term coteau has since been used in English to describe any upland dividing ridge.[1]
The plateau is composed of thick glacial deposits, the remnants of many repeated glaciations, reaching a composite thickness of approximately 900 feet (275 m). They are underlain by a small ridge of resistant Cretaceousshale. During the last (Pleistocene) Ice Age, two lobes of the Laurentide glacier, the James lobe on the west and the Des Moines lobe on the east, appear to have parted around the pre-existing plateau and further deepened the lowlands flanking the plateau.