Old Earth Ministries Online Earth History Curriculum

Presented by Old Earth Ministries (We Believe in an Old Earth...and God!)

This curriculum is presented free of charge for use by homeschooling families.

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Chapter 10 - The Jurassic Period

Lesson 50: The Morrison Formation

 

      The Morrison Formation is a distinctive sequence of Late Jurassic sedimentary rock that is found in the western United States, which has been the most fertile source of dinosaur fossils in North America. It is composed of mudstone, sandstone, siltstone and limestone and is light grey, greenish gray, or red. Most of the fossils occur in the green siltstone beds and lower sandstones, relics of the rivers and floodplains of the Jurassic period.
    It is centered in Wyoming and Colorado, with outcrops in Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho. Equivalent rocks under different names are found in Canada. It covers an area of 1.5 million square km (600,000 square miles), although only a tiny fraction is exposed and accessible to geologists and paleontologists. Over 75% is still buried under the prairie to the east and much of the rest was destroyed by erosion as the Rocky Mountains rose to the west.

Chapter 10 - The Jurassic Period

 

 Lesson 49 - Jurassic Overview

 Lesson 50 - Morrison Formation

 Lesson 51 - Plesiosaurs

 Lesson 52 - Species In-Depth - Archaeopteryx

 Test

Morrison Formation

The distictive banding of the Morrison Formation, a group of rock layers that occur throughout Dinosaur National Monument. The formation originated as muds and sands laid down by ancient rivers, and some of its outcrops have been found to contain 150-million-year-old dinosaur fossils like those found at the monument's Dinosaur Quarry.  (Picture Source)

 

     It was named after Morrison, Colorado, where the first fossils were discovered by Arthur Lakes in 1877. That same year, it became the center of the Bone Wars, a fossil-collecting rivalry between early paleontologists Othniel Charles Marsh and Edward Drinker Cope.

     According to radiometric dating, the Morrison Formation dates from 156.3  2 million years old (Ma) at its base, to 146.8  1 million years old at the top, which places it in the latest Oxfordian, Kimmeridgian, and early Tithonian stages of the late Jurassic. This is similar in age to the Solnhofen Limestone Formation in Germany and the Tendaguru Formation in Tanzania. Throughout the western USA, it variously overlies the Middle Jurassic Summerville, Sundance, Bell Ranch, Wanakah, and Stump Formations.

 

Deposition

 

     At the time, the supercontinent of Laurasia had recently split into the continents of North America and Eurasia, although they were still connected by land bridges. North America moved north and was passing through the subtropical regions.

     The Morrison Basin, which stretched from New Mexico in the south to Saskatchewan in the north, was formed when the precursors to the Front Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins, carried by streams and rivers from the Elko Highlands (along the borders of present-day Nevada and Utah) and deposited in swampy lowlands, lakes, river channels and floodplains, became the Morrison Formation.

     In the north, the Sundance Sea, an extension of the Arctic Ocean, stretched through Canada down to the United States. Coal is found in the Morrison Formation of Montana, which means that the northern part of the formation, along the shores of the sea, was wet and swampy, with more vegetation. Eolian, or wind-deposited sandstones are found in the southwestern part, which indicates it was much more arid a desert, with sand dunes.

     In the Colorado Plateau region, the Morrison Formation is further broken into four sub-divisions, or members. From the oldest to the most recent, they are:

 

  1. Windy Hill Member: The oldest member. At the time, the Morrison basin was characterized by shallow marine and tidal flat deposition along the southern shore of the Sundance Sea.

  2. Tidwell Member: The Sundance Sea receded to Wyoming during this member and was replaced by lakes and mudflats.

  3. Salt Wash Member: The first purely terrestrial member. The basin was a semi-arid alluvial plain, with seasonal mudflats.

  4. Brushy Basin Member: Much finer-grained than the Salt Wash Member, the Brushy Basin Member is dominated by mudstone rich in volcanic ash. Rivers flowed from the west into a basin that contained a giant, saline alkaline lake called Lake T'oo'dichi' and extensive wetlands that were located just west of the modern Uncompahgre Plateau.

 

     Deposition in the Morrison Formation ended about 147 Ma. The latest Morrison strata
Brushy Basin Member, Morrison Formation
Brushy Basin Member of the Morrison Formation, west of Green River, Utah (Picture Source)
 are followed by a thirty-million year gap in the geologic record. The overlying units are the Lower Cretaceous Cedar Mountain, Burro Canyon, Lytle, and Cloverly Formations.

 

Fossils

 

     Though many of the Morrison Formation fossils are fragmentary, they are sufficient to provide a good picture of the flora and fauna in the Morrison Basin during the Kimmeridgian. Overall, the climate was dry, similar to a savanna but, since there were no angiosperms (grasses, flowers, and some trees), the flora was quite different. Conifers, the dominant plants of the time, were to be found with ginkgos, cycads, tree ferns, and horsetail rushes. Much of the fossilized vegetation was riparian, living along the river flood plains. Insects were very similar to modern species, with termites building 30 m (100 ft.) tall nests. Along the rivers, there were fish, frogs, salamanders, lizards, crocodiles, turtles, pterosaurs, crayfish, clams, and monotremes (prototherian mammals, the largest of which was about the size of a rat).

     The dinosaurs were most likely riparian as well (dwelled by rivers). Hundreds of dinosaur fossils have been discovered, such as Allosaurus, Camptosaurus, Ornitholestes, several stegosaurs comprising at least two species of Stegosaurus and the slightly older Hesperosaurus, and the early ankylosaurs, Mymoorapelta and Gargoyleosaurus, most notably a very broad range of sauropods (the giants of the Mesozoic era). Since at least some of species are known to have nested in the area (Camptosaurus embryoes have been discovered), there are indications that it was a good environment for dinosaurs and not just home to migratory, seasonal populations.

     Sauropods that have been discovered include the Diplodocus (most famously, the first nearly-complete specimen of D. carnegiei, which is now exhibited at the Carnegie Museum of Natural History, in Pittsburgh, Pennsylvania), Camarasaurus (the most commonly found sauropod), Brachiosaurus, Apatosaurus (also wrongly known as Brontosaurus), Barosaurus, the uncommon Haplocanthosaurus and the Seismosaurus. The very diversity of the sauropods has raised some questions about how they could all co-exist. While their body shapes are very similar (long neck, long tail, huge elephant-like body), they are assumed to have had very different feeding strategies, in order for all to have existed in the same time frame and similar environment.

     There are numerous excavation sites within the Morrison, but the most famous is Como
Como Bluff
Multicolored (variegated) beds of the Morrison Formation at Como Bluff, Wyoming. Many historical dinosaur sites are located along the flanks of the bluff. The Sundance Formation is visible as the reddish beds at the base of the bluff. (Picture Source)
 Bluff.  Como Bluff is a long ridge extending east-west, located between the towns of Rock River and Medicine Bow, Wyoming. The ridge is an anticline, formed as a result of compressional geological folding. Three geological formations, the Sundance, the Morrison, and the Cloverly Formations, containing fossil remains from the Late Jurassic of the Mesozoic Era, are exposed at the bluff. Nineteenth century paleontologists discovered many well-preserved specimens of dinosaurs, as well as mammals, turtles, crocodilians, and fish from the Morrison Formation. Because of this, Como Bluff is considered to be one of the major sites for the early discovery of dinosaur remains. Among the species discovered is the only known specimen of Coelurus. Significant discoveries were made in 22 different areas scattered along the entire length of the ridge. It is included on the National Register of Historic Places as well as the National Natural Landmark list.

     Dinosaur National Monument in Utah is also a quarry site of the Morrison.  For a list of quarry sites, see Morrison Formation on Wikipedia.

 

Dinosaur National Monument

 

     Dinosaur National Monument is a National Monument located on the southeast flank of
Dinosaur National Monument

Above: Douglas Quarry.  The dinosaur bones now exposed on the Quarry cliff were buried in an ancient river about 150 million years ago. Over time the sediments turned to rock, protecting the bones in a stone time capsule. This photo shows a "bone jam," made up of the fossilized bones of large sauropods. Huge leg and shoulder bones of sauropods such as Apatosaurus and Diplodocus dominate this section of the quarry face.

Dinosaur Dig

Paleontological field work at Dinosaur NM. Fossil excavation often uses small tools, either pneumatic or manual, to carefully remove rock from delicate fossils.

 the Uinta Mountains on the border between Colorado and Utah at the confluence of the Green and Yampa Rivers. Although most of the monument area is in Moffat County, Colorado, the Dinosaur Quarry is located in Utah just to the north of the town of Jensen, Utah. This park has fossils of dinosaurs including Allosaurus, Abydosaurus, and various long-neck, long-tail sauropods.

     The rock layer enclosing the fossils is a sandstone and conglomerate bed of alluvial or river bed origin. The dinosaurs and other ancient animals were washed into the area and buried presumably during flooding events. The pile of sediments were later buried and lithified into solid rock. The layers of rock were later uplifted and tilted to their present angle by the mountain building forces that formed the Uintas. The relentless forces of erosion exposed the layers at the surface to be found by paleontologists.

      When dinosaurs are found in a mass graveyard, with many fossil specimens in one location such as Dinosaur National Monument, the remains are deposited in that location due to water, typically from a flood event, or several flood events.  Young earth creationists often point to these mass graveyards as proof of the worldwide flood of Noah, and they claim it is proof that the earth is young.  However, there are many reasons why this is not the case. 

  1. The geologic record is full of examples of many individual flood events, separated by rock layers that are non-flood rocks, such as desert sandstones.

  2. Dinosaurs in mass graveyards typically display evidence that they were scavanged by other dinosaurs, such as bite marks and teeth left behind by feeding dinosaurs.  Dinosaurs rapidly buried by a great single flood would not be scavanging other carcasses, as they themselves would have been buried.

  3. The rock layers containing dinosaurs, such as the Morrison, are located on top of thousands of feet of rock layers that are also considered Flood deposits.  They could not have survived during the deposition of these rock layers.

  4. If all the dinosaurs we see in the rock record existed at the same time as Noah's Flood, then we would expect to see mass graveyards containing hundreds of species of dinosaurs.  Most mass graveyards contain only a handful of species.

    

     On the other hand, old earth creationism has no problems with accepting the theories of scientists, who view these graveyards as created by individual floods, or by a group of floods.  (For additional reading - As an example of a young earth claim about a dinosaur graveyard, consider this young earth claim.)

 

End of Reading

 

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Source Pages:  Jurassic; Morrison Formation