Cretaceous   Time: 144 to 66 million years ago Duration: 78 million years   The amount of Cretaceous coal in North America is greater than that of any other geologic period. Half of it is in Canada with most of the rest in Montana, Wyoming, Colorado, Utah and New Mexico. The coal-forming swamps of the Cretaceous Period were for the most part located between the rising Rocky Mountains and the repeatedly advancing and retreating shoreline of a vast interior sea that at times extended from the Arctic Ocean to the Gulf of Mexico. Volcanic activity in the Rocky Mountains introduced layers of volcanic ash into the swamps. These weathered to produce layers of bentonite clay. The coals of this period are primarily subituminous. They are mined extensively in Utah, New Mexico and western Wyoming.

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