Answer: The scientists think that Interior Plains may have once been a large sea.
Explanation: The Interior Plains refers to a widespread physiographic region. It starts from the craton of central-North America, crossing the Gulf Coast region till the Arctic Ocean alongside the east flank of the Rocky Mountains. This area originated when billion years ago, cratons collided and welded together. Sometimes, these plains were covered by inland seas because of which the Rocky Mountains and sediments from the shield were deposited over millions of years.
