The Interior Plains is a vast region that spreads across the stable core (craton) of North America. This area had formed when several small continents collided and welded together well over a billion years ago, during the Precambrian. 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.
List of National Parks exhibiting Interior Plains Geology
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.
USGS photo of typical prairie grassland
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 Jurassic (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.
Sediments eroding from the rising Rocky Mountains to the west washed into the sea and deposited as layered wedges of fine debris. As sand, mud, and clays accumulated, the Sundance Sea retreated northward. Preserved within the multi-hued sandstones, mudstones, and clays that made up the shoreline, are the remains of countless dinosaurs that roamed the Sundance coast.
The fossil assemblages concealed within the sedimentary layers of the Morrison Formation are among the world's richest. In some areas, bones of many dinosaurs are concentrated in a very small area, indicating that they were carried during floods, then deposited together beside a stream.
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 Mountains to the west and Appalachian and Ozark/Ouachita Mountains to the east and south throughout the most recent Era, the Cenozoic. The flatness of the Interior Plains is a reflection of the platform of mostly flat-lying marine and stream deposits layed down in the Mesozoic and Cenozoic Eras.