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Precambrian metamorphic and igneous rocks now form the basement of the Interior Plains and make up the stable nucleus of North America. With the exception of the Black Hills of South Dakota, the entire region has low relief, reflecting more than 500 million years of relative tectonic stability.
Throughout the Paleozoic and Mesozoic eras the mostly low-lying Interior Plains region remained relatively unaffected by the mountain-building tectonic collisions suffered by the western and eastern margins of the continent. During much of the Mesozoic Era, the North American continental interior was mostly well above sea level, with two notable exceptions. During part of the JurassicThe Jurassic period is a major unit of the geologic timescale that extends from about 135 million years BP at the end of the Triassic to 195 million years BP at the beginning of the Cretaceous. As with other geologic periods, the rock beds that define the (208-144 million years ago), rising seas flooded the low-lying areas of the continent. Much of the Interior Plains eventually lay submerged beneath shallow Sundance sea .
Once again, during the Cretaceous Period (144-65 million years ago), record high sea levels flooded the continental interior with shallow seas.
The Interior Plains continued to receive deposits from the eroding Rocky MountainsThe Rocky Mountains often called the Rockies, are a broad mountain range in western North America. The Rocky Mountains stretch more than 3000 miles (4800 km) from Mexico, through the continental United States, into Canada and Alaska. The highest peak is M to the west and AppalachianThe Appalachian Mountains are a system of North American mountains running from Newfoundland and Labrador, Canada to Alabama in the United States, although the northernmost mainland portion ends at the Gaspe Peninsula of Quebec. The system is divided into and Ozark/ Ouachita MountainsThe Ouachita Mountains are a mountain range located in Arkansas, Oklahoma and central Texas. The Ouachita (pronounced "wah-shi-tah") Mountains are fold mountains like the Appalachian Mountains to the east. During the Pennsylvanian part of the Carboniferou to the east and south throughout the most recent era, the CenozoicThe Cenozoic or Cainozoic Era (sometimes Caenozoic Era is the most recent of the four classic geological eras. It covers the 64 million years since the extinction event at the end of the Cretaceous that marked the demise of the last dinosaurs and the end. The flatness of the Interior Plains is a reflection of the platform of mostly flat-lying marine and stream deposits laid down in the Mesozoic and Cenozoic eras.