The Morrison Formation is one of the most famous dinosaur-bearing rock units in North America. It was deposited during the Late Jurassic Period, about 156 to 146 million years ago, across parts of what are now the western United States. Its colorful layers of sandstone, siltstone, mudstone, and limestone preserve evidence of rivers, floodplains, lakes, and seasonal wetlands.
Studying this formation helps scientists connect fossils to the environments where ancient organisms lived and died.
Paleontologists use the Morrison Formation like a layered archive, reading rock types, fossil positions, and sediment structures to reconstruct past ecosystems. Famous dinosaurs such as Allosaurus, Stegosaurus, Apatosaurus, Diplodocus, and Camarasaurus are known from these rocks. Fossils are not scattered randomly, because bones are moved, buried, exposed, and preserved through specific physical and chemical processes.
By combining stratigraphy, radiometric dating, fossil comparison, and sedimentology, scientists build a timeline of changing Jurassic landscapes.
Key Facts
- Morrison Formation age: about 156 to 146 million years ago.
- Geologic time span = older layer age minus younger layer age, so 156 Ma - 146 Ma = 10 million years.
- Principle of superposition: in an undisturbed sedimentary sequence, lower layers are older than upper layers.
- Typical Morrison rocks include sandstone, mudstone, siltstone, limestone, and volcanic ash beds.
- Fossil preservation is most likely when remains are buried quickly by sediment and protected from weathering and scavengers.
- Common Morrison dinosaurs include Allosaurus, Stegosaurus, Apatosaurus, Diplodocus, and Camarasaurus.
Vocabulary
- Morrison Formation
- A Late Jurassic sedimentary rock unit in western North America that is famous for abundant dinosaur fossils.
- Stratigraphy
- The study of rock layers and their order, age relationships, and meanings.
- Sedimentary rock
- Rock formed from particles, minerals, or organic material that were deposited, buried, compacted, and cemented.
- Fossil
- Preserved evidence of ancient life, such as bones, shells, tracks, teeth, or plant material.
- Floodplain
- A flat area beside a river where sediment is deposited during floods.
Common Mistakes to Avoid
- Assuming every dinosaur fossil in the Morrison Formation lived at the same time is wrong because the formation spans about 10 million years of deposition.
- Calling all rock layers fossil beds is wrong because many sedimentary layers contain few or no fossils and may record ordinary mud, sand, or lake deposits.
- Thinking deeper always means much older without checking deformation is wrong because faults, folding, erosion, or tilted beds can disturb the original order of layers.
- Treating a fossil site as a complete ecosystem snapshot is wrong because preservation is selective and may favor large bones, rapid burial zones, or animals transported by water.
Practice Questions
- 1 The Morrison Formation ranges from about 156 Ma to 146 Ma. How many million years of geologic time does this interval represent?
- 2 A measured Morrison outcrop has 18 m of sandstone, 27 m of mudstone, and 5 m of limestone. What is the total thickness of the exposed section?
- 3 A paleontologist finds a dinosaur bone in a mudstone layer above a sandstone channel deposit. Explain what the mudstone suggests about the ancient environment and why the bone may have been preserved there.