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Interior Plains
The Interior Plains is a vast physiographic region that spreads across the Laurentian craton of central North America.
==Geology and physiography==
This area was originally formed when cratons collided and welded together 1.8–1.9 billion years ago in the Trans-Hudson orogeny during the Paleoproterozoic Era.
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.〔http://vulcan.wr.usgs.gov/LivingWith/VolcanicPast/Notes/interior_plains_region.html USGS description of the U.S. Interior Plains region〕
The Interior plains region of the United States and Canada falls within this area. The Interior Plains were often covered by shallow inland seas. Sediments from the shield and the Rocky Mountains were deposited in these seas over millions of years. Eventually the sediments were compressed by the weight of the layers above into sedimentary rock. Part of the sedimentary rock deposited in these areas consists of coral reefs that formed close to the surface of seas during the Paleozoic Era.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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