Date of this Version
The Journal of Geology, Volume 13, Number 8, (1905), pp. 657-669
The beds now generally designated on U. S. Geological Survey maps as the Morrison formation have been a subject of interest and discussion since I877, when abundant remains of dinosaurs were found in them. The first extensive collections of the vertebrate fauna were obtained in the neighborhood of Morrison near Denver, -in Garden Park, near Canyon City, Colorado, and at Como, or Aurora, Wyoming. Since then the formation has been recognized by means of its fossils, its lithologic features, and its stratigraphic relations in the Black Hills, on the Laramie Plains, and elsewhere in Wyoming, in Montana, in western Colorado, in southeastern Colorado, and in adjacent parts of Oklahoma and New Mexico.
Of the various names that have been applied to the formation Atlantosaurus beds is, perhaps, most frequently seen in the literature, but Como stage, Beulah shales, Morrison formation, and Gunnison formation have been locally applied. In recent publications Darton has used the term "Morrison formation " in all the areas mentioned.
The formation is non-marine thrcughout, so far as known, and consists of variegated marls and shales with irregular beds of sandstone and sometimes thinner layers and lenses of siliceous limestone.
The colors of the shales and marls are greenish-gray, purplish, maroon and red, very irregularly distributed, while the sandstones are usually gray, sometimes weathering brown or with small brown spots. The limestones are gray, in some cases weathering with a reddish tinge. The general appearance of the formation is remarkably uniform over large areas, and yet the individual elements are so variable that no two detailed sections are exact duplicates of each other. The total thickness is seldom more than 200 feet, though it is possibly more than 400 feet at Canyon City.
Stratigraphically the Morrison is always rather closely associated with the Dakota formation. When the huge Morrison dinosaur bones were first discovered it was announced that they came from the Dakota, and after it was learned that they really came from a lower horizon it was generally believed for many years that there was no unconformity nor visible stratigraphic break between the two formations. Through the work of Ward, Jenney, and Darton in the area north of Colorado the Lakota and Fuson formations have been recognized between the true Dakota and the Morrison and referred to the Lower Cretaceous. I shall presently show that in southern Colorado, New Mexico, and Oklahoma the so-called Dakota should also be divided because it includes a marine Lower Cretaceous horizon. It is nevertheless true that the base of the Dakota is usually not more than 100 to 200 feet above the top of the Morrison, and it is often less than that. Darton has recognized a general unconformity at the top of the Morrison in Colorado and eastern Wyoming, but he believes that the interval represented by it is unimportant. The base of the restricted Dakota also rests on an uneven surface wherever the actual contact has been seen.