At one time in Earth's history this massive formation of sands, silts, and clays completely covered over a half million square miles of area that included eight of our western states and a portion of Canada. A slope-former where it crops out, the Morrison Formation is a product of 8 million years of erosion and deposition that began during the Jurassic Period, some 155 million years ago. This deposit of multicolored siltstones, mudstones, and sandstones stretches from Arizona and New Mexico northward to Idaho, Montana, and Canada, but only a small part of it is exposed. A combination of uplift and subsequent erosion has stripped the Morrison Formation from the crest of the rising Rocky Mountains, leaving only remnants of this important rock layer on either side of the mountain range. The rest is still buried under the prairies of the western Great Plains.
The Morrison Formation is mostly mudstone but thin layers of coarser siltstone and sandstone and even the occasional limestone are found within the formation. These sediments and the fossils found within them all indicate a non-marine origin for the Morrison, although the presence of thin micritic limestones may be evidence for minor oceanic transgressions in some areas. Crossbedding is common in the formation as are flood deposits, which occur in scoured-out channels within the surrounding mudstone. These lag deposits of flood debris sometimes contain the flotsam and jetsam of ancient life. The Morrison Formation is not a particularly thick bed of sediments. Averaging only 200 feet in thickness, it reaches its greatest dimensions to the southwest and generally thins eastward. It is more than 400 feet thick in many sections of its range.
The Morrison Formation has provided more information about the ancient Jurassic world than any other formation in North America. It is a time capsule of Jurassic life and has yielded a treasure trove of fossils. The remains of fish, turtles, crocodiles, and mammals have all been recovered
from the Morrison, while as many as 250 species of plants have been identified in the deposit. As a source of dinosaur fossils, it is unrivaled. Indeed, it is from this humble mudstone that most of the Jurassic dinosaur fossils in North America have come!
Dinosaur remains are rare in the Morrison Formation, but occasionally the odd bone will turn up in the flood deposits that crisscross the ancient mudstone surface. On extremely rare occasions, the fossilized skeletons of multiple individuals will be found; their disarticulated bones concentrated and jumbled up by the forces of nature. But once in a blue moon, fully-articulated dinosaur skeletons are found. Only a few of these have been discovered in the last century and a half and all are important to science. But none are more important than the fossils found by a paleontologist named Earl Douglass one hot August day in 1909 near the Colorado/Utah border. Douglass's discovery would lead to the formation of Dinosaur National Monument, one of America's great national monuments and parks.
The fossil discoveries made by Earl Douglass in the Echo Canyon area during the summer of 1909 were not the first dinosaur finds in the now famous Morrison Formation. In 1871, the Hayden Survey team noted the presence of Jurassic-age sediments in eastern Utah and northwestern Colorado. These beds were probably the same bone-bearing formations described by John Wesley Powell as he explored the Green River that same year. Powell recorded a number of fossilized reptile bones weathering out of the steep river bank as he passed by.