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Utah’s Morrison Formation: A Fossil Treasure Chest
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The wildlife of North America during the Jurassic Period was nothing short of incredible. Long-necked dinosaurs reached 100 feet long, and Stegosaurus lumbered along under the weight of its spiked and plated body. Stalking these gentle giants were the wickedly toothy predators Allosaurus and Ceratosaurus. It was truly a land of giants. Today, we know about these forms from fossil sites in the American West, most particularly from a series of rock units called the Morrison Formation. In the late summer of 2011, the Paleontological Research Institution acquired a piece of sandstone for preparation from the Morrison Formation of Utah. On loan from the Carnegie Museum of Natural History, it has been PRI’s task to prepare to block and recover any dinosaur remains found inside.

The rocks of the Morrison Formations were deposited during the Late Jurassic Period (161-145 million years ago), from the end of the Oxfordian to the middle of the Tithonian ages. The range of the Formation covers one million square kilometers – from Arizona, New Mexico, and Oklahoma in the south, through Utah and Colorado, and north to Wyoming, South Dakota, and Montana. The thickness of the Formation ranges from 30 meters (98 feet) to 300 meters (990 feet). The Formation contains many different types of rock that vary throughout its layers and are excellent indicators of the landscape during the Late Jurassic.

Green-gray mudstone north of Moab, Utah, is very brightly green, but fades to gray in Wyoming. The drab colors are evidence of damp conditions on the floodplain. There have been a number of productive vertebrate fossil quarries in these mudstones, and the famous Apatosaurus at Yale’s Peabody Museum is from this type of rock.

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The most notable rock in the Morrison Formation is red mudstone. This can be very brightly colored, ranging from red, maroon, or reddish brown, and its banding in the outcrop is visible from a great distance. The reddish color suggests better drainage and therefore, a drier, more oxidizing condition on the floodplain. Many of the bright red mudstone deposits contain paleosols, the preserved remains of ancient soil. Paleosol deposits are common in the Brushy Basin Member of the Morrison Formation and are distinguished by abundant root stains, and red and green mottled nodules or root casts in their fine-grained sediments. Fossil quarries are less common in the red mudstones, but from these quarries have come some of the first and largest dinosaurs found in the Morrison. Excavations in 2007 uncovered an articulated sauropod skeleton in a mottled red mudstone, immediately overlain by crevasse-splay sandstone in the lower Brushy Basin Member.

Channel sandstones in the Morrison Formation represent ancient river channels. Depending on the internal characteristics of the channel, the outcrop can determine whether the grain layer represents a meandering river or a braided stream. The tan channel sandstones are commonly cross-bedded, a structure type formed by the flow of the ancient river. These sandstones can be used to determine the direction of flow. In the Morrison, these layers are often 3 meters (10 feet) or more in thickness and can be traced laterally for at least a mile. In the southwest outcrop area, the sandstones can be medium to coarse-grained or conglomeratic, and can have a range of mineralologies. In the northeastern part of the Morrison deposition, the sandstone is generally fine-grained with few other grains other than quartz. In areas farther to the north and east, the sandstones are generally thinner and more laterally restricted than in the Colorado Plateau region. There are many important quarries that are known from channel sandstone areas: the Carnegie Quarry at Dinosaur National Monument, Utah; Bone Cabin Quarry, north of Como Bluff, Wyoming; Dry Mesa Quarry, near Delta, Colorado; and Marsh-Felch Quarry at Garden Park, Colorado. These quarries contain many associated and articulated skeletons of dinosaurs.

Some areas – such as the top levels of the Morrison – contain unusual beds that are dark gray to black. These dark mudstones represent wet floodplain deposits, the dark color being caused by high carbon content. The reducing environments of the wet floodplains preserved the carbon of the plant material that fell to the ground. There are vertebrate fossils at a few localities. These locations include those at Morrison, Colorado (where the type specimen of Apatosaurus was excavated), Breakfast Bench, Wyoming (containing microvertebrate material), and Belt, Montana (with the best plant fossils in the Morrison Formation).

The Brushy Basin Member
The widespread Brushy Basin Member overlies the Salt Wash Member throughout the Colorado Plateau and consists of a large percentage of mudstone and less sandstone than the Salt Wash Member. The Brushy Basin mudstones are red, grayish purple, maroon, gray-green, green, and light to dark gray in color, and they weather to the texture of “mud popcorn” in many areas. When they are dry, walking in these areas can be like walking on ball bearings, but the mudstone becomes slick and sticky when wet. The mudstone is composed of bentonite, an altered volcanic ash with a clay mineral called montmorillonite. Montmorillonite is three-layered and can absorb large amounts of water; it is this microstructure that creates the very slick mud. There are thin gray limestones in the Member but they are rare, usually laterally restricted but in some places fairly thick. Tan channel sandstones are not uncommon and are usually medium fine to coarse-grained; some can be gravelly and conglomeratic. In the San Juan basin of New Mexico, the Brushy Basin Member can be up to 165 meters (540 feet) thick, whereas in most areas of the Colorado Plateau it is about 91 meters (300 feet) thick.

Most of the deposits of the Brushy Basin Member represent floodplain muds mixed with volcanic ash. These were the flat parts of the plain, away from or along the river channels and small lakes (source of the limestones). These muds were deposited during floods, when fine sediments settled out of the quiet waters away from the channels. Some of these muds sat in low areas of the floodplain and were kept wet by the high water table or occasional rain, forming green and gray mudstones, whereas other areas were better drained and stayed drier. The drier areas might have been away from the rivers because the water table would not have been as elevated. These muds became red mudstones. The sandstones of the Brushy Basin Member represent mostly sandy to gravelly meandering river deposits (high sinuosity).

The lateral equivalents of the Brushy Basin to the north and east are also mostly mudstone. Most quarries in the Morrison are in the Brushy Basin or similar levels. These quarries are: Dinosaur National Monument (Carnegie Quarry), Dry Mesa Quarry, Cleveland-Lloyd Quarry, and Fruita Paleontological Area (microvertebrate sites) the Cope Quarries at Garden Park, Colorado, and many quarries at Como Bluff, Wyoming (including Quarry #9).
These last two are in areas equivalent to Brushy Basin, whereas the rest are actually in the Brushy Basin Member.

A Brief History of the Carnegie Quarry
A study of Hayden’s Survey of 1871 indicates that there are exposures of Jurassic-age rock in the Uinta Range. This study inspired paleontologist Earl Douglass and Carnegie Museum Director, Dr. W. J. Holland to travel to Utah in 1908. While prospecting for dinosaurs in this area, they found the femur of a Diplodocus weathering out at the bottom of a narrow ravine. Due to its great weight they were forced to leave it in place. What it did, however, inspired them to return to the area the next year. In the summer of 1909, Douglass returned to Utah and began searching along the Duchesne River. A few weeks after his arrival, he received a message from Holland sending him east of Vernal, Utah, where he arrived on the 10th of August, 1909. For the first few days he found nothing but fragments and broken up bones. On August 17th, he moved to a new area, near Split Mountain, full of sandstones, and shortly found a series of eight large vertebrae weathering out of a sandstone layer of the Morrison Formation. When excavated, these eight vertebrae were part of the most complete Apatosaurus skeleton ever discovered, later described by Holland as Apatosaurus louisae. This was the beginning of the Carnegie Quarry in Utah.

Douglass decided that this was not going to be a seasonal operation and they would work year round. He had a road built to the site in an area where there were few roads at all. Equipment and supplies were purchased and brought to the site. Douglass bought a Sheppard’s wagon for his assistants living quarters and built a wood-framed tent for himself and his family, who arrived in September 1909. Temperatures in the winter occasionally went down to 30 degrees below zero.

Brushy
Multicolored beds of the Brushy Basin Member of the Morrison Formation near Carnegie Quarry (image by
Anky-man via Wikimedia Commons).

Douglas
Earl Douglass and the Apatosaurus vertebrae
that started the Carnegie Quarry in Utah (photo
courtesy of Willard Marriot Library, University of Utah).

In 1910, it became evident that the quarry was an incredibly large bone bed, bigger than any sites ever found before. The quarry contained the bones of many species including: Apatosaurus, Diplodocus, Allosaurus, Stegosaurus, Camarasaurus, Barosaurus, Ceratosaurus, Camptosaurus, Dryosaurus, and Torvosaurus. In addition to the dinosaurs, there were also two kinds of crocodiles, two species of turtles, a frog, a freshwater clam, and fossil plant material. By the time that the Carnegie Museum decided that it had had enough, in 1923, the local crews had shipped approximately 350 tons of material east to Pittsburgh. President Woodrow Wilson proclaimed the dinosaur beds Dinosaur National Monument in 1915. The boundaries were expanded in 1938 from the original 80 acres to over 200,000 acres in Utah and Colorado. The first Quarry Exhibit Hall at Dinosaur National Monument was built in 1957, but due to damage to the foundations making the build unsafe, it was closed in 2006. A completely renovated Quarry Exhibit Hall was reopened on October 4, 2011.

The Carnegie Slab
The Carnegie Loan 615 slab includes seven caudal vertebrae (#2-7) and neural spines, and a few chevrons — essentially the base of the dinosaur’s tail. It has been identified as from a juvenile Apatosaurus, probably Apatosaurus louisae (that’s the only species so far found in Utah). It is still undergoing preparation.

References
Colbert, Edwin H. 1984. The Great Dinosaur Hunters and Their Discoveries. Dover Publications.
Foster, John. 2007. Jurassic West — The Dinosaurs of the Morrison Formation and Their World. Indiana University Press.
Holland, W. J. 1916. A new species of Apatosaurus. Annals of Carnegie Museum, 10(1-2): 143-145.
Utah History Encyclopedia.

Michael Marano is a volunteer and expert fossil preparator at Paleontological Research Institution’s Museum of the Earth in Ithaca, New York. Michael has donated more than 400 hours of his time to preparing the Carnegie fossil in the Museum’s Prep Lab since its arrival in October 2011.
Mike

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