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At the very heart of North America, between the Rocky Mountains and east of the Mississippi River, we find one of the continent's largest regional landforms, The Great Plains. Its most striking feature is its flatness and wide open spaces. The Great Plains encompass all or part of 15 of the United States and three Canadian provinces. In Canada, the Great Plains is known as "The Prairies." One might wonderĄhow did the Great Plains become so flat? The answer goes back about 250 million years. Back then, the Great Plains were not plains or prairie at all. In fact, they were not even dry land. Scientists have determined that it was once covered by a vast inland sea known as the Western Interior Waterway. This waterway ran from the Arctic Ocean in the north to the Gulf of Mexico in the south.
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