In an old painting, a paleontologist contemplates [fossil] bones found near Morrison. The date is 1877. The bones are those of the 70-foot dinosaur Apatosaurus, more commonly known as Brontosaurus, shown below in reconstruction.
Some of the dinosaurs known from the Morrison Formation reached 80 feet in length. Both plant-eating and meat-eating types are known. In addition to the bones themselves, gastroliths or gizzard stones can frequently be found; these highly polished stones were as essential to dinosaur digestion as gravel is to a chicken or a caged canary.
Along with the dinosaur [fossils] are found abundant remains of water plants called charophytes. These plants formed tiny spiralled balls of calcite as part of their reproductive activities; both the little balls and the stalks of the plants themselves occur in many parts of the state. In western Colorado, near Grand Junction, silicified shells of freshwater snails can also be found in the Morrison.
Early in the 1900s vanadium, radium, and uranium were discovered in Jurassic sandstones and mudstones of western Colorado. Extensive mining in this area has revealed that these elements often become concentrated by groundwater in organic material such as [fossil] plant stems or dinosaur bones. The search for radioactive minerals has thus brought to light many ancient fossil accumulations.
Cretaceous Period
(70-135 million years ago)
Early in Cretaceous time, marine conditions once more prevailed in Colorado. This is indicated by a marked change in rock types from beach and near-shore deposits to true marine sediments.
Between the Front Range and the Plains the Cretaceous Dakota Formation forms a [hogback] ridge which can be traced for 200 miles or more. The well-cemented sandstone resists erosion, and so remains as a ridge when softer layers are stripped away. (Jack Rathbone photo)
The sandstones derived from beach sands sometimes include coarse pebbles of chert which can be traced to sources in Permian rocks of Utah and Nevada. Occasionally the beach and near-shore deposits include marine shells like oysters, indicating that there were brackish and salt water lagoons and marshes along the shore. The Dakota Formation represents the beach of the transgressive or advancing sea. This formation contains oil in eastern Colorado, Nebraska, and Wyoming; the oil itself may have been derived from decay of organic materials in swamps behind the beaches and bars.