The Cretaceous time period lasted from 146 to 65 million years ago, a time span of 81 million years. The Cretaceous Period is the last of the Mesozoic Era. The term Cretaceous was first used in the 1820s to describe the "Terrain Cretace," the chalk deposits of the Paris Basin and the English Channel. These Cretaceous rocks were defined based on a particular type of fossil assemblage contained within them. They are located above the rocks of the Jurassic System and below the rocks of the Tertiary System.