In Late Jurassic time the Colorado area, which had been predominantly desert since Permian time, appears finally to have been submerged once more. Fine calcareous muds of the Curtis Formation, containing ammonites, belemnites, and other marine shellfish, show us that a shallow sea transgressed from the west over the wind-blown sands. This sea was, geologically speaking, of short duration—only a few million years. Bounded on almost all sides by desert, it seems to have dried up, depositing the gypsum that is now present in a thin layer along the Front Range between Denver and Canon City in the Ralston Formation. At about this time, however, the climate underwent a major change. Deposits above the Ralston indicate an increasingly moist environment, the environment in which the Morrison Formation was deposited over most of Colorado and parts of the adjacent states of Kansas, Arizona, Utah, and Wyoming. The Morrison Formation is exposed in many places, and is characteristically composed of layers of fine, limy mud, brightly colored in streaks of red, brown, green, and blue. In most areas it is so soft that it becomes soil-covered; it is well exposed only in roadcuts or where it is protected from erosion by a “caprock” of harder sediments or lava. Spectacular outcrops can be seen in new roadcuts along U. S. Interstate highway 70 just west of Denver. [Illustration: In this roadcut along U. S. Interstate 70 west of Denver, Jurassic and Cretaceous rocks are unusually well exposed in the Dakota hogback. Green and purple shales represent the dinosaur-bearing Morrison Formation. The Cretaceous Dakota Group forms the eastern, higher half of the cut. Black layers are carbon-rich clays of the South Platte Formation, frequently quarried locally for ceramic uses. (John Chronic photo)] Fossil dinosaur bones occur in great numbers in the Morrison Formation near the towns of Morrison and Canon City and at several other places in Colorado. Those at Canon City have been quarried extensively, and are now mounted in a number of museums in the United States. At Dinosaur National Monument, in eastern Utah and northwestern Colorado, many excellent remains have been found; those in Utah can be seen in place in the rock in a striking exhibit at the National Monument. [Illustration: In an old painting, a paleontologist contemplates fossil bones found near Morrison. The date is 1877. The bones are those of the 70-foot dinosaur _Apatosaurus_, more commonly known as _Brontosaurus_, shown below in reconstruction.] [Illustration: Apatosaurus] Some of the dinosaurs known from the Morrison Formation reached 80 feet in length. Both plant-eating and meat-eating types are known. In addition to the bones themselves, gastroliths or gizzard stones can frequently be found; these highly polished stones were as essential to dinosaur digestion as gravel is to a chicken or a caged canary.