The Morrison Formation is a distinctive sequence of Upper Jurassic sedimentary rock exposed across the western United States, dating to roughly 156.3 to 146.8 million years ago, spanning the earliest Kimmeridgian through early Tithonian stages. It is composed mainly of mudstone along with sandstone, siltstone and limestone, colored light gray, greenish gray or red, and reaches up to 200 meters thick where fully preserved. The formation is centered in Wyoming and Colorado, with outcrops extending into Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah and Idaho, covering an area of some 1.5 million square kilometers even though only a small fraction is exposed at the surface. It represents the rivers and floodplains of the Jurassic period and has produced more dinosaur fossils than any other rock unit in North America, including hundreds of specimens of sauropods such as Diplodocus, Camarasaurus and Brachiosaurus alongside numerous theropods and stegosaurs. This entity documents the Morrison Formation specifically as a geological formation, a fluvial and floodplain sedimentary deposit rather than any one fossil site within it.
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