The Geologic Story of Colorado National Monument / Revised Edition

by Stanley William Lohman

The soft siltstone and mudstone of the Morrison Formation weather rapidly into steep or fairly steep slopes, but the harder beds of sandstone, most of which are in the lower third of the formation, known as the Salt Wash Member, are sculptured into bold ledges or low cliffs. The generally softer upper two-thirds of the formation is called the Brushy Basin Member. The Morrison is best exposed in and southeast of The Redlands, where the bare rocks are carved into badlands like the famous ones of South Dakota. Both the Fruita Canyon and No Thoroughfare Canyon approaches to the Monument pass typical badlands in the Morrison. The entire 600 feet of this formation is best seen in the high bluff on the east side of the mouth of No Thoroughfare Canyon ([fig. 21]).

MORRISON FORMATION, on east side of mouth of No Thoroughfare Canyon. Forty feet of Summerville Formation at base is concealed by slope wash, but underlying white- and salmon-colored members of Entrada Sandstone are clearly exposed at lower left. Protective caprock at upper right is lowermost sandstone of Cretaceous Burro Canyon Formation. Upper two-thirds of Morrison is typical of the Brushy Basin Member; lower one-third is not typical of the Salt Wash Member, which generally contains more and thicker lenses of sandstone, some of which are just around the corner to the right. Mesa on left skyline is above Serpents Trail in the Monument. Looking west from Little Park Road. See also figures [55] and [60]. (Fig. 21)

In parts of the Colorado Plateau southwest of the Uncompahgre Plateau, the sandstone lenses in the Salt Wash Member of the Morrison contain uranium and vanadium ore associated with carbonaceous matter, including coalified wood. No ores have been found in or near the Monument presumably because such carbonaceous matter, which helped precipitate the ores, is lacking on the northeastern side of the Uncompahgre Plateau.

Some of the beds of siltstone and mudstone in the Brushy Basin Member of the Morrison shown in [figure 21] contain bentonite, a clay derived from the decomposition of volcanic ash, which indicates the presence of active volcanos in or near the Plateau at the time these beds were deposited. Bentonite swells when wetted, so it is widely used in well-drilling muds, sealing canals, etc. Some bentonitic material has been dug from the Brushy Basin along the Little Park Road just south of the point from which the photograph in [figure 21] was taken and was used for sealing irrigation canals in the Grand Valley.

The Morrison is not well exposed in the Monument, as the formation is restricted to the higher parts where most of it is hidden by vegetation. The lower part is seen in roadcuts and outcropping ledges along a high stretch of Rim Rock Drive between Artists Point and the head of the west arm of Ute Canyon, where sandstone lenses in the Salt Wash Member are especially thick.

The climate during Morrison time was wet enough to support abundant vegetation along the many lakes and streams—at least enough to feed the hungry dinosaurs and other reptiles that roamed the area. Many bones and parts of several skeletons of dinosaurs have been found in the Morrison at several places in The Redlands not far northeast and northwest of the Monument.

The most famous dinosaur locality near the Monument is Riggs Hill where, in 1900, the late Elmer S. Riggs of the Field Columbian Museum (now Field Museum of Natural History) dug out part of the first known skeleton of a huge Brachiosaurus ([fig. 22]). This discovery made quite a splash in the scientific world, for it was the first and only type of dinosaur found whose front legs were longer than its hind legs. The fossilized thigh bone (femur) alone is 6 feet 8 inches long and weighs 549 pounds; the arm bone (humerus), though incomplete, is even longer. The ribs are 9 feet long. A bronze plaque now marks the site of the excavation ([fig. 39]).

In 1901, Riggs removed all but the forepart of a skeleton of Apatosaurus from the southeast side of a large hill of the Morrison Formation just south of the old Fruita bridge. Riggs also found remains of Diplodocus, Camarasaurus, and Morosaurus, and, in 1937, Al Look, prominent writer and amateur paleontologist of Grand Junction, and Edwin L. Holt, an instructor in Mesa College at Grand Junction, found the closely associated remains of Allosaurus, Stegosaurus, and Brachiosaurus at the western end of Riggs Hill. Dinosaurs generally are thought of as huge creatures—many were huge indeed ([fig. 23])—but they came in various sizes and some were quite small.

An interesting Late Jurassic vertebrate fossil locality in the Salt Wash Member of the Morrison Formation, about 3 miles northwest of the West Entrance of the Monument and about 3 miles southwest of Fruita, was discovered in June 1975 by George Callison, Associate Professor of Biology and Research Associate in Vertebrate Paleontology at the California State University at Long Beach. During the summers of 1975 and 1976 Dr. Callison and his assistants removed the closely associated skeletal remains of many small, primitive mammals and both small and large dinosaurs and other reptiles. Part of the results were presented in an unpublished manuscript.[28] During the summer of 1977 and later, additional mammalian fossils were removed by Callison and assistants and additional reptilian fossils were removed by Lance Erickson, paleontologist of the Historical Museum and Institute of Western Colorado ([fig. 2]). Hopefully, the work will be continued with the aid of grants from several sources.