Concept
The Interior Plains is a vast physiographic region that spreads across the Laurentian craton of central North America, extending along the east flank of the Rocky Mountains from the Gulf Coast region to the Arctic Beaufort Sea. In Canada, it encompasses the Canadian Prairies separating the Canadian Rockies from the Canadian Shield, as well as the Boreal Plains and Taiga Plains east of the Mackenzie and Richardson Mountains; while in the United States, it includes the Great Plains of the West/Midwest and the tallgrass prairie region to the south of the Great Lakes extending east to the Appalachian Plateau region. A series of tectonic plate collisions in the crust that formed the center of the North American continent laid the groundwork for the modern-day interior plains. Mountain building and erosion around the plains as well as flooding from inland seas provided sediments that make up the rock strata of the interior plains. Between 2.0 and 1.8 billion years ago the Hearne-Rae, Superior, and Wyoming cratons were sutured together to form the North American craton, Laurentia, in an event called the Trans-Hudson Orogeny (THO). This event was like the Indian plate colliding with the Eurasian plate, which formed the Himalayas. After initial collisions during the THO, tectonic activity at the edges of the four main cratons sparked mountain building. The interior of Laurentia remained relatively flat and became a basin for eroded sediment from mountains at the beginning of the current time period, the Phanerozoic Eon. The only remaining outcrops from this orogeny in the interior plains are in the Black Hills of South Dakota. The sediments that formed the Black Hills were granite and different types of igneous rocks, which make up the basement of bedrock in central North America. However, much of the Black Hills sediment has been metamorphosed and deformed, so it is uncertain what the conditions were like at the time of their formation. This period has a large importance in Earth’s history as it saw the Cambrian explosion and Permian extinction.
