Geology of the Quarry

Stump Formation

(160 million years before present)

Redwater Member: What was it like in this area before the dinosaurs lived here? Around 160 million years before present, a mountain range extended north-south along what is today the western border of Utah. While the southern half of the state was a terrestrial environment, an arm of the ancestral Pacific Ocean had advanced from the north, inundating Wyoming and parts of adjoining states including the Uintah Basin, and deposited sand, shale and lime mud that was later transformed into the Stump Formation. This ocean supported a diverse fauna that included belemnites, clams, snails, and ammonites.

Toward the end of deposition of the Stump Formation, the area we now call Dinosaur National Monument was covered by the ocean. As these waters began to retreat, a new period of deposition was ushered in that would later become the Morrison Formation. It was during this time that dinosaurs roamed the region. The Morrison Formation has four members in the monument, each member representing a gradual progression of changes in the environment.

Morrison Formation

(155 to 147 million years before present)

Windy Hill Member: Makes contact with the Stump Formation and is the oldest rock of the Morrison (155 million years before present). The Windy Hill is marine, deposited in an ocean with a regressive shoreline.

Tidwell Member: Overlies the Windy Hill and represents broad coastal mud flats. As time passed the deposits became less marine as the influence of fresh water from rivers increased and river deposits increasingly became more dominant than marine deposits. The Tidwell has been dated at 155 million years before present.

Salt Wash Member: Overlies the Tidwell and was a fully terrestrial environment. Braided rivers flowed down from moderately high mountains situated along today's Nevada/Utah border. This Morrison mountain range to the west, known as the Elko Highlands, was probably similar to the Sierra Nevada range in length, height, width, and effect. The mountains were high enough to cause a rain shadow over the Morrison landscape, which stretched out as a flat plain to the east. Some of the drainage was north toward present-day Wyoming, and some was east toward the present-day Mississippi River Valley. Salt Wash rivers shifted their courses often and reworked the deposits of sand, gravel, and mud transported by the rivers (The new species of Allosaurus jimmadsoni was found in river channel sands from this member of the Morrison.)

The climate of this landscape east of the mountains was semi-arid. Prevailing winds, blowing from the west, picked up sand from river channels on the alluvial plain, and deposited it in dunes at and near the present-day Four Corners area. Occasionally the rivers flowed strong enough to reach the eastern plain creating seasonal mud flats. Breaking through this monotonous flat land were hills or low mountains, the ancestral beginnings of the Rocky Mountain range we know today.

Brushy Basin Member: Overlies the Salt Wash and features two environmental characteristics that are different from the previous deposit. One of these was the rivers being confined to fairly straight, shallow valleys not present during the Salt Wash period. The mud embankments that formed the sides of the river valleys kept the rivers confined to straight or moderately sinuous courses running from the Elko Highlands to the west and the Mogollon Highlands to the southwest. Overbank flooding deposited mud rather than any considerable quantity of sand and gravel. The gradient of the rivers probably was less than during Salt Wash time. This entire region consisted of a gently sloping alluvial plain; the main topographic features being the gradual gradient to the east, the slight depressions of the river valleys, and the low ancestral Rocky Mountains to the east. The semi-arid climate persisted. Discharge waters from the rivers collected in the vast, landlocked Lake T'oo'dichi'. Here the evaporation rate was high, gradually concentrating salts, washed down by the rivers, into the lake. A second environmental difference to this landscape were volcanoes that arose in the mountain ranges to the west and began to eject ash into the atmosphere that eventually settled in this area. Each ash fall probably resulted in an accumulation of several inches, and, occasionally, several feet. Some of these falls may have had deadly effects on the plant and animals as well as the aquatic life in the rivers.

The Dinosaur Quarry is in the Brushy Basin Member of the Morrison Formation. Because of the rain shadow caused by the mountains, the climate was semiarid, probably much like it is today, but without the cold winters. An example of how dry nonriparian communities could become is found at Deerlodge Park, where the Salt Wash Member of the Morrison outcrops. Here there is evidence of wind-blown sand dunes, suggesting a dry environment. The source of sand for the dunes was from river sandbars and beaches situated toward the southeast. The prevailing wind blew from the southwest to the northeast. The climate was also warm-warm enough that dinosaurs had no problems regulating their body temperature-but exactly how warm is not known.

Typical river residents were aquatic insects, freshwater clams, crayfish, fish, turtles, crocodiles, and probably frogs and salamanders. In the shallow river valley, equiseturn, ferns, cycads, tree ferns, and conifers grew in abundance as riparian vegetation. On the land situated slightly above the river valley, plants of unknown species, probably used the same strategies to survive in the semi-arid environment as plants do today. They were probably widely dispersed with broad lateral root systems to gather up any rain that fell. Other xeric plants probably thrust roots deep into the soil to reach the water table, which was fairly close to the surface. Pollen and spore studies suggest there was a good diversity of plants considering the semiarid environment. This diversity was much lower on the arid lands than in the riparian community.

Perennial and ephemeral ponds dotted the landscape. Perennial ponds were fed by the high water table. The origin of the high water table may have been water that percolated through the alluvial deposits that sloped down from the mountains. Ephemeral ponds were the result of overbank flooding from the river valleys into depressions. Since the climate was semiarid, the evaporation rate was fairly high and the humidity was probably low, resulting in a short life span for ephemeral ponds. Much like the creatures that inhabit today's slickrock potholes, ephemeral pond creatures had to hatch, mature, and lay eggs quickly. Ostracods and concostracons meet these requirements. In perennial ponds, frogs, salamanders, turtles, and crocodiles were typical residents.

In its broader aspects, the Morrison landscape was varied. The area to the north in present-day Montana was close to the coast of the Late Jurassic ocean that extended into Canada. Here the Morrison has coal beds, representing a wetter environment with swamps and abundant vegetation. To the south of our region, beginning around present-day Grand Junction, Colorado and extending to near Albuquerque, New Mexico, there was a huge alkaline/ saline lake, a result of the arid climate during the latter part of Morrison deposition.

What strikes me about the Morrison is that plants and animals faced a fairly harsh environment. The scarcity of water was the limiting environmental factor, like today. Natural events such as drought, drifting sand dunes, shifting riverbeds, floods, and volcanic ash falls could and did destroy parts of the ecosystem's communities, setting back plant succession and repopulation by wildlife. After an ash fall and subsequent to rains, the rivers probably flowed like thick soup, choked with silicious ash, suffocating and burying river residents.

Herbivorous dinosaurs probably concentrated in the riparian community, although some species probably were adapted to the arid plains between the rivers. On the arid lands, bones of dinosaurs probably were not covered over and protected quickly enough to berome fossilized. So we do not know if one or more of the herbivorous species we have collected from the river channel sandstone in the Quarry would have foraged in these areas, or if some species, as yet undiscovered, lived on the dry plains. Was there enough plant material for the dinosaurs, or did they have to migrate to survive? From eggshell fragments and a Camptosaurus embryo we have found, we know that some of the dinosaurs nested here. That's an indication that the local environment met most of their habitat needs. Museum of the Rockies Paleontologist Jack Horner offered some of the following thoughts on this question. It is not logical to assume dinosaurs consumed food at the same rate as such contemporary large mammals as elephants. If you observe reptiles today, such as snakes, they eat several mice and won't eat again for a week or two. So the amount of plant material needed to sustain herbivorous dinosaurs was probably lower than we might assume at first. Also, why assume the dinosaurs were racing around burning up their energy, or that their digestive efficiency was as poor as mammal herbivores? Horner believes the dinosaurs lived here year-round, and didn't migrate, and that there was plenty of vegetation to sustain them. This may have been true for Montana, where coal swamps existed, but it doesn't seem to apply farther south on the Colorado Plateau, where coal is lacking and there are extensive deposits that indicate a dry or fairly dry environment. We also need to keep in mind that dinosaur populations probably rose and fell with the abundance of vegetation. It may be that the number of dinosaurs living in this area year round was small, given the harsh environmental conditions and the probable relatively low density of vegetation overall.

The Morrison depositional period ended about 147 million years before present. The formation is overlain by the Lower Cretaceous Cedar Mountain Formation. The boundary between the Morrison and Cedar Mountain is marked by an unconformity and there is one or more layers of limestone called calcrete at or near this contact. The unconformity represents a 15 to 20-million-year gap in the rock record due to the erosion that took place during this period. Calcrete is a replacement limestone that forms below the surface of the ground from the action of highly calcareous ground water.

To further illustrate that each rock formation represents an extinct ecosystem, and that the environments and their ecosystems continually change throughout time, I will describe.

Cedar Mountain Formation

(About 110 to 130 million years before present)

Far to the west, in present-day California and Nevada, high mountains rose shedding their sediments eastward. Since Precambrian times, oceans had encroached from the west, but no more. Oceans would now have to invade from the south and east because of the new highlands to the west. Sediment washed from the western mountains accumulated on top of the Morrison and later became the Cedar Mountain Formation. The Cedar Mountain environment was probably much like the Morrison. Dinosaurs roamed this environment, but the types of dinosaurs had changed. Deinonychus, for example, comes out of the Cedar Mountain Formation.

Dakota Sandstone

(About 100 million years before present)

From the east, the Dakota ocean expanded into this area, inundating the Cedar Mountain Formation. In this area the shoreline fluctuated east, then west, laying down sand beach deposits. Ripple marks and mud cracks found in the Dakota Sandstone indicate intermittent wetting and drying on mud flats. Coal-bearing strata in some portions of the Dakota indicate accumulations of dead plant material from coastal swamps.

Mowry Shale (Member of the Mancos Shale)

The Dakota ocean floor began to subside, deepening the sea. From the northwest, ash from volcanoes sifted into the sea, depositing a silica-rich mud that became the porcelain-like Mowry Shale. Perhaps the fish that lived in the Mowry ocean were killed by the ash falls. Their scales are found preserved in the shale. Along with fish, the Mowry ocean contained oysters and large ammonites.

Frontier Sandstone Member (Member of the Mancos Shale)

The Mowry ocean retreated eastward temporarily, exposing the ocean floor. A little west of present-day Salt Lake City, stood high, newly-formed mountains. Rivers washed sediment down the mountains and piedmont to coastal plains, forming gravelly alluvial fans. Ocean waves and littoral currents constructed sandy beaches, offshore sand bars and dense jungle-like forests grew down to the coastal lagoons behind barrier beaches. As the ocean depth fluctuated, these ecosystems moved east and west over present-day Wyoming, Utah, and Colorado. The coastal waters teamed with shellfish.

Mancos Shale (Main body)

The high mountains that arose during Frontier time perhaps depressed the crust in front of them, causing the Frontier ocean to deepen. The Frontier deposits thinned eastward giving way to deep-water clay and mud that later became the Mancos Shale.

The Mancos seaway gradually withdrew to the northeast (85 million years before present) with many temporary readvances. Beaches and coastal swamps marked this retreat and are preserved in the Mesaverde Formation (About 70 to 80 million years before present) west and south of Vernal, Utah.

Sixty-five million years before present, the old seaway that dominated the west gave way to mountain-building forces and the present-day Rocky Mountains, including our Uinta Mountains, began to rise. Formations were stripped off the Uinta anticline, leaving hogbacks as evidence of the former uplift. Thus, the scene we see today was created by mountain uplift and erosion. The ecosystem we live in today is created from the soils derived from the weathered formations, the variety in elevation and aspect created by uplift and erosion, and the life-giving gift of water provided by the Yampa and Green rivers and the many creeks, springs, and seeps.

By David Whitman, with review by Pete Peterson, United States Geological Service.

References: Hansen, Wallace. The Geologic Story of the Uinta Mountains. Geological Survey Bulletin 1291, U.S. Government Printing Office, 1975.

Hansen, Wallace; Peter Rowley; Paul Carrara. Geologic Map of Dinosaur National Monument and Vicinity, Utah and Colorado. U.S. Geological Survey, 1983.

Maione, Steven J., Stratigraphy of the Frontier Sandstone Member of the Mancos Shale on the South Flank of the Eastern Uinta Mountains, Utah and Colorado. Colorado School of Mines, 1971.

Interviews with Pete Peterson and Christine Turner, U.S. Geological Survey geologists, Denver, CO.