Maroon and grey-green claystones and mudstones, and red sandstones of the Morrison Formation (on the left) and the Dakota Group (Lytle Formation and South Platte) (on the right) - Interstate 70 roadcut through Dinosaur Ridge (Dakota Hogback) at the Morrison exit west of Denver.
| Morrison Formation | Ralston Creek | Brontosaurus |
By Jack Barkstrom
By 160 million years ago, the shallow seas which created the Lykins limestone had retreated, although, not far. Eastern Colorado turned into a freshwater lagoon. For some ten million years evaporation produced layers of sandstone, shale, and gypsum in a large floodplain, known as the Ralston Creek Formation. About 150 million years ago the lagoon environment became one of freshwater lakes, fed by rivers and streams. The clay, sand, and lime sediment became the claystone, sandstone, and limestone of the Morrison Formation, seen in the photo above as the purple, red, and gray-green layers on the left. Formed between 150 and 140 million years ago, the Morrison Formation was the product of a flat land and a humid climate. The lush vegetation which resulted supported a variety of animal life, including the giant plant-eating sauropods, such as Brontosaurus.[1]
Between 1877 and 1879 quarries near Morrison, Colorado would yield the bones of a crocodile, the dinosaur Stegosaurus, and two brontosaurs, Diplodocus and Apatosaurus. These discoveries would give the formation its name - Dinosaur Ridge, otherwise known as Long Neck Meadow, for the long-necked sauropods. The dinosaur discoveries did not lead to discoveries about their diet. Few fossil plants are found on Dinosaur Ridge in the same strata as the dinosaur bones.[2]
Footnotes
(1) Andrew M. Taylor, Ph.D., "Guide to the Geology of Colorado," Cataract Lode Mining Company, (Golden, CO 1999), pp. 66-67;
Ralph Lee Hopkins and Lindy Birkel Hopkins, "Hiking Colorado's Geology," The Mountaineers, (Seattle, WA 2000), p. 86.
(2)Peter J. Modreski, "Geochemical and Mineralogical Studies of Dinosaur Bone from the Morrison Formation at Dinosaur Ridge,"
in "The Mountain Geologist," Volume 38, Number 3, Rocky Mountain Association of Geologists, (Denver, CO July 2001), p. 115;
Kirk R. Johnson and Robert G. Raynolds, "Ancient Denvers: Scenes from the Past 300 Million Years of the
Colorado Front Range," Denver Museum of Nature & Science, (Denver 2003), p. 12.
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