Black Mesa’s rock layers form a record of geologic history, with the basalt cap on top less than 5 million years old and deeper layers growing progressively older. From top to bottom are Raton Basalt; sands and gravels of the Pliocene Ogallala group, 2 to 5 million years old; cross-bedded sandstone of the Cretaceous Dakota Group and gray shale and sandstone of the Cretaceous Purgatoire Formation, 92 to 106 million years old; the Jurassic Morrison Formation, 134 to 150 million years old; marl and shale of the Jurassic Exeter Sandstone, 152 to 157 million years old; and red and buff shale, marl, and conglomerate of the Triassic Dockum Group, 215 to 231 million years old. About 150 million years ago, this area was a tropical swamp inhabited by dinosaurs such as Allosaurus. The Morrison Formation revealed dinosaur life including Apatosaurus, Stegosaurus, and Allosaurus, and about 6,000 dinosaur bones were excavated from these 134 to 150 million-year-old Jurassic rocks during projects led by Dr. John Stovall, a University of Oklahoma geology professor and museum director who devoted much of his career from 1930 until his death in 1953 to the geology and paleontology of the Cimarron Valley.