The prairies are, in brief, a contribution of the glacial period; they consist for the most part of glacial drift, deposited unconformably on an underlying rock surface of moderate or small relief.
Cambrian System.The lower part of the Cambrian system, characterized by the Olenellus fauna, is restricted to the borders of the continent, where it rests on the older rocks unconformably in most places.
Pennsylvanian SystemThe Pennsylvanian or Upper Carboniferous system overlics the Mississippian unconformably over a large part of the United States.
Widespread changes at the end of the period exposed the areas where deposition has been in progress during the period to erosion, and the (Upper) Cretaceous formations rest upon the Comanchean unconformably in most parts of the country.
Under these conditions sediments from the high lands were washed out and distributed widely over the plains, giving rise to a thin but widespread formation of ill-assorted sediment, without marine fossils, and, for the most part, without fossils of any kind, and resting unconformably on Cretaceous, Eocene and Miocene formations.