Western Interior Seaway
Seawater Flooding of the Central Section of North America

Twenty-five miles south of Oakley, Kansas, on the rolling grassy plains, are several sections of high walls of chalk. Marine fossils have been found within the chalk, indicating that seawater once flooded this section of the continent for a prolonged period of time, contemporaneous with the development of the chalk deposits that form the White Cliffs of Dover in southern England. These high-standing walls of chalk in western Kansas were a familiar landmark during the western migration across North America and are known as Monument Rocks.

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The origin of these rocks—the reason for the prolonged seawater flooding—was a mystery for many years. But it is now known to have been caused by the slow movement of the Farallon tectonic plate beneath North America and a pulling downward, as a result of that movement, of the Earth's surface to form the Western Interior Seaway.

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