Morrison Formation

The name Morrison Formation was first applied by Whitman Cross in the 1894 USGS Pikes Peak Folio (Geologic Atlas, Folio 7) and subsequently described in detail by George H. Eldridge in USGS Monograph 27 (Geology of the Denver Basin in Colorado, Emmons, Cross & Eldridge, 1896), which included the type locality near the town of Morrison, Colorado. Eldridge's original definition encompassed upper sandstone and shale units that are now assigned to the Lower Cretaceous. The formation's stratigraphic boundaries were subsequently refined through the work of W.T. Lee (1920, 1927), Waldschmidt & LeRoy (1944), and Craig et al. (1955), among others. The age of the Morrison Formation was itself a matter of debate for decades, with some geologists assigning it to the Early Cretaceous; the USGS ultimately classified it as Late Jurassic, a determination that has been confirmed by extensive radiometric dating of volcanic ash beds and biostratigraphic correlation.

The first scientifically documented discovery of fossils from Morrison Formation rocks may have been the sauropod Dystrophaeus found in southeastern Utah in 1859, although assignment of those specific rocks to the Morrison Formation is debated. The formation's significance for palaeontology became firmly established in 1877, when major fossil discoveries were reported almost simultaneously from three localities: Garden Park, Colorado; Morrison, Colorado; and Como Bluff, Wyoming.

The Morrison Formation covers an estimated area of approximately 1.5 million km² (600,000 square miles) across the western United States. It is centred in Wyoming and Colorado, with outcrops extending into Montana, North Dakota, South Dakota, Nebraska, Kansas, the panhandles of Oklahoma and Texas, New Mexico, Arizona, Utah, and Idaho. Equivalent strata under different nomenclature are also recognised in Canada. However, only a small fraction of this area is exposed at the surface and accessible for geological study.

In the Colorado Plateau region, the formation is divided into three principal members (in ascending order): the Tidwell Member, a basal unit of red marine sandstones frequently containing chert; the Salt Wash Member, characterised by light-coloured, ledge-forming cross-bedded sandstones deposited by braided river systems; and the Brushy Basin Member, consisting of multicoloured bentonite clays (altered volcanic ash) and mudstones representing floodplain and lacustrine environments. In the San Juan Basin of the Four Corners region, an alternative subdivision includes (ascending) the Bluff Sandstone, Recapture Member, Westwater Canyon Member, and Brushy Basin Member, as established by Gregory (1938). Over much of the Western Interior, including the type area near Denver, the formation has not been formally divided into members.

Formation thickness varies substantially: it is approximately 90 m (300 ft) near the type section, but reaches up to 300 m or more in certain areas. The lower contact is generally placed at the base of terrestrial fluvial Jurassic sediments overlying the marine or marginal marine San Rafael Group or Sundance Formation. The upper contact lies beneath Lower Cretaceous units such as the Dakota Sandstone, Burro Canyon Formation, or Cedar Mountain Formation.

Radiometric dating, primarily through K-Ar and U-Pb analysis of zircon crystals in volcanic ash beds, indicates that the Morrison Formation was deposited between approximately 155 and 148 Ma (Britannica) or 154 and 145 Ma (NHM, 2024). This corresponds to the Kimmeridgian through early Tithonian stages of the Late Jurassic, with some researchers extending the age range into the latest Oxfordian.

During Morrison deposition, North America was situated farther south than its present position, and the Rocky Mountains had not yet formed. Clastic sediment was derived from western mountain ranges, including the Sierra Nevada, that were uplifted during the Late Jurassic. The depositional environment was primarily continental: rivers, floodplains, lakes, mudflats, and alluvial plains covered the western interior of the continent. Some marine or marginal marine sediments occur at the formation's base. Numerous volcanic ash beds throughout the section attest to active volcanism to the south and west.

The palaeoclimate was generally warmer and more humid than the modern climate of the region, based on oxygen isotope analysis of marine fossils and the composition of the fossil plant assemblage. However, evidence from fossil unionid clams at the Carnegie Quarry in Dinosaur National Monument indicates that the region experienced at least three severe drought events during Morrison time, followed by flooding when rains returned. This pattern of seasonal or episodic aridity is consistent with a semi-arid to sub-humid climate with strong seasonal rainfall variability.

The Morrison Formation is the most productive source of dinosaur fossils in North America. Approximately 50 or more genera of dinosaurs have been reported from the formation, making it one of the most diverse Late Jurassic terrestrial assemblages known anywhere in the world.

Sauropoda: The formation has yielded numerous large-bodied sauropods, including Diplodocus, Apatosaurus, Brontosaurus, Barosaurus, Camarasaurus, Brachiosaurus, and Haplocanthosaurus. These herbivorous giants likely consumed multiple tons of plant material daily.

Theropoda: Allosaurus is the dominant predator, accounting for an estimated 70–75% of theropod specimens. Other theropods include the horned Ceratosaurus, the large Torvosaurus, and smaller forms such as Ornitholestes and Coelurus.

Stegosauria: Stegosaurus is the iconic plated dinosaur of the formation.

Ornithopoda: Camptosaurus, Dryosaurus, and Nanosaurus represent the bipedal beaked herbivores.

Beyond dinosaurs, the formation preserves a remarkably diverse biota. Pterosaurs, crocodilians, turtles (Dinochelys, Glyptops), amphibians (Iridotriton, Rhadinosteus), lizard-like reptiles (Opisthias), and small mammals (most no larger than a modern badger, such as Glirodon) are all represented. Freshwater mussels, snails, and fishes populated the rivers and lakes. The flora consisted of ferns, cycads, ginkgoes, horsetails, and large conifers (including Brachyphyllum in the family Araucariaceae); flowering plants had not yet evolved. Some conifers are estimated to have rivalled modern coast redwoods in height.

In early 1877, significant fossil discoveries were reported almost simultaneously from three Morrison Formation localities: Garden Park, Colorado; the town of Morrison, Colorado (discovered by Arthur Lakes); and Como Bluff, Wyoming. These finds were communicated to Edward Drinker Cope and Othniel Charles Marsh, two prominent vertebrate palaeontologists already in competition. The resulting rivalry — popularly known as the Bone Wars — saw both men deploy teams of collectors across Colorado and Wyoming in a frantic race to name new species.

Among the many taxa named during this period were Allosaurus, Apatosaurus, Brontosaurus, Camarasaurus, Diplodocus, Stegosaurus, Camptosaurus, Dryosaurus, Ceratosaurus, and numerous less well-known forms. The two rivals collectively named over 130 new dinosaur species, although many were later recognised as synonyms — a consequence of the haste with which they worked. Despite its personal acrimony and methodological shortcomings, the Bone Wars brought dinosaurs to unprecedented public attention and established the foundations of American vertebrate palaeontology.

Dinosaur National Monument (Carnegie Quarry): Located in eastern Utah and western Colorado, this site was discovered by Earl Douglass in 1909. The Carnegie Quarry yielded thousands of bones and several nearly complete skeletons, including a famous Apatosaurus. The Quarry Exhibit Hall, built into the hillside, allows visitors to view fossils in their original position within the rock.

Cleveland-Lloyd Dinosaur Quarry: Situated in central Utah within the Brushy Basin Member, this site has produced one of the densest concentrations of Jurassic dinosaur fossils ever found. Allosaurus is particularly abundant, and taphonomic studies suggest that bones were transported to the site by seasonal flooding.

Como Bluff: A site in Wyoming that was one of the primary theatres of the Bone Wars, yielding numerous type specimens.

Garden Park Fossil Area: Located near Cañon City, Colorado, this was the site of excavations by both Cope's and Marsh's field crews.

All four sites have been designated as U.S. National Natural Landmarks, and Dinosaur National Monument is a unit of the National Park Service.

The Morrison Formation, particularly the Salt Wash Member, hosts the most productive uranium–vanadium deposits in the United States (Turner-Peterson, 1986). During the mid-20th century uranium boom, driven by Cold War demand for nuclear materials, extensive mining operations were conducted throughout the Moab, Utah, region and other parts of the Colorado Plateau.

A 2024 study by Susannah Maidment (Natural History Museum, London), published in the Journal of Vertebrate Palaeontology, examined how biodiversity changed through the Morrison Formation's approximately nine-million-year depositional history. The study found that many species appeared to be thriving at the very end of the Jurassic, suggesting that the subsequent faunal turnover into the Early Cretaceous was relatively sudden rather than a gradual decline. Stegosaurs became extinct in North America while sauropods became much less prominent; their ecological niches were filled by iguanodontians and ankylosaurs in the Cretaceous. Because stegosaurs persisted into the Early Cretaceous in Europe and Asia, this turnover is widely interpreted as a regional North American event rather than a global phenomenon. Uplift of the Morrison basin associated with a mountain-building event at the Jurassic–Cretaceous boundary, and concomitant climate and environmental change, is considered the most likely cause.

Maidment's research also revealed that some species were not evenly distributed across the Morrison basin. Allosaurus fragilis was predominantly found in the southern part of the formation's extent while Allosaurus jimmadseni occupied the north, with slight overlap. Similar latitudinal partitioning was observed in Camarasaurus species. No physical geographic barriers have been identified to explain this pattern, leading researchers to hypothesize that latitudinal climatic variation may have been responsible.

Despite nearly 150 years of study, the Morrison Formation continues to yield new discoveries. Recent advances include high-precision U-Pb zircon geochronology that is refining the formation's chronostratigraphic framework, CT scanning of fossils to reveal internal anatomy, stable isotope geochemistry for palaeoclimate reconstruction, and computational phylogenetic analyses of the formation's diverse fauna. The Morrison Formation is widely regarded as the best-known Late Jurassic terrestrial ecosystem on Earth (Maidment, 2024), making it an indispensable reference system for understanding Late Jurassic dinosaur diversity, palaeoecology, and palaeoclimatology. Each year, new taxa continue to be described, and re-examination of century-old museum collections using modern techniques frequently reveals previously unrecognised diversity.