Map Courtesy of Chris Scotese, PALEOMAP Project www.scotese.com
During much of the Cretaceous Period (144 to 65 million years ago) a large portion of Montana was covered by the waters of a shallow, inland sea called the Western Interior Seaway. The sea was formed as west-central North America was subsiding to an elevation below sea level. As this sinking occurred the area filled with water from the Gulf of Mexico and the Arctic Ocean.
Source of the sediment . .
Where do I drop you off? . .
Source: Monahan, Glenn and Chanler Biggs. Montana's Wild and Scenic Upper Missouri River. Missoula, Montana: Northern Rocky Mountain Books, 1997.
Terms: subsidence, deposition
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Earth Science Teacher at Helena High School