THE AMERICAN NATURALIST. VOL. XVI. MARCH, 1882. — No. 3. THE TERTIARY FORMATIONS OF THE CENTRAL REGION OF THE UNITED STATES. BY E. D. COPE. THE principal Tertiary formations of the region between the Mississippi river and the Sierra Nevada are the following, as mainly determined by Dr. Hayden: The Puerco, the Wasatch, the Bridger, the Uinta, the White River, the Truckee, the Loup Fork and the Equus beds. Several of these are again distinctly subdivided, and in a few instances such divisions have been regarded by authors as of equal importance with those above mentioned; as, for instance, the Green River portion of the Wasatch. But the evidence of vertebrate palæontology is not as yet clearly favorable to further primary subdivision than is indicated by the above names. In the following pages I will briefly describe the character and distribution of these formations. The general history of the succession of the Tertiary lakes of the interior of the North American continent and their deposits has been developed by the labors of various geologists, prominent among whom must be mentioned Hayden, Newberry and King. It may be synoptically stated as follows: The Laramie Cretaceous period witnessed a great difference in the topography of the opposite sides of the Rocky Mountain range. To the east were extensive bodies of brackish and nearly fresh water, with limited ocean communication, studded with islands and bordered by forests. On the west side of the range was a broad continent, composed of mostly marine Mesozoic rocks, whose boundaries are not yet well ascertained. Towards the close of the Laramie, the bed of the great eastern sea began to emerge from the waters, and the continent of the western side of the great range descended. The relations of the two regions were reversed; the east became the continent, and the west became the sea. The latter, receiving the drainage of the surrounding lands, was a body of fresh water, whose connection with the ocean permitted the entrance of a few marine fishes only. This was the great Wasatch lake, whose deposits extend from the upper waters of the Yellowstone far south into New Mexico and Arizona, between the Rocky mountains on the east and the Wasatch range on the west. Its absence from the east side of the former range indicates the continental condition of that area at the time. The only locality where the Wasatch deposits are extensively deposited on the Laramie, is in the region intermediate between the two districts in Wyoming Territory. Here the sediments of the former are seen to have succeeded those of the latter, and to have been coincident with an entire cessation of brackish conditions. Elevations of the continent northward and southward contracted the area of the great Wasatch sea, and perhaps deepened it, for at this time were deposited the fine limestones and silico-calcareous shales of the Green River epoch. There is no evidence that these beds had a greater eastern extension than that of the parent Wasatch lake. King has given distinct names to these ancient lakes. I think it better to pursue the usual course of using for them the names already given to their deposits, as involving less strain on the memory; the more as the number of these lakes is being increased by numerous new discoveries. The only known region which it covered west of the Wasatch range, is represented to-day by the calcareous strata in Central Utah which I have called the Manti beds. The exact equivalency of these is, however, not quite certain. Further contraction reduced this area to perhaps two lake basins, whose deposits now form two isolated tracts in Southern Wyoming, and are known as the Bridger formation. Continued elevation and drainage caused the desiccation of these basins also, leaving only, so far as present knowledge extends, a body of water on the south of the Uinta mountains, in Northeastern Utah. The sediments of this lake form the Uinta formation, which is the latest member of the series now found in the region lying between the Rocky and Wasatch mountains. About the time that the elevation of the present drainage basin of the Colorado river was completed, a general subsidence of level of the great region east of the Rocky mountains com menced. Extensive lakes were formed in the depressions of the Laramie and older beds which formed the surface, which were probably connected over a tract extending from near the Missouri river to Eastern Wyoming and Colorado. Near the same time a similar body of fresh water occupied a large part of what is now Central Oregon and certain areas in Northwestern Nevada, according to King. The sediments now deposited constitute the White River formation, and the faunal distinctions which I have discovered to characterize the eastern and western basins have led me to employ for them the subdivisional names of White River beds for the former and Truckee (King) for the latter. It may have been during the early part of this period, or during the Uinta, that there existed two contemporary bodies of water, separated by a wide interval of territory. One of these extended over a considerable tract in Northern Nevada, and deposited a coal bed near Osino. A formation probably the same, has been found by Professor Condon in Central Oregon, underlying the Truckee Miocene beds. The other lake left its sediments near Florissant, in the south park of Colorado. This formation I have named the Amyzon beds,1 from a characteristic genus of fishes which is found in it. It has been referred to the Green River formation by King, but in contradiction to the present palæontological evidence, as it appears to me. The oscillations of the surface which brought the White River period to a close, are not well understood. Suffice it to say here, that after an interval of time another series of lakes was formed, which have left their deposits at intervals over a wider extent of the continent than have those of any other epoch. These constitute the beds of the Loup Fork period, which are found at many points between the Sierra Nevada and the Rocky mountains, from Oregon to New Mexico, and over parts of the Great Plains of Colorado, Kansas, and northward, and in the valleys of the Rocky mountains. King has shown that the beds of this epoch are slightly elevated to the westward, thus proving that the elevation of the Rocky mountains had not entirely ceased at that late day. A probably continuous succession of lakes has existed from this period to the present time in ever-diminishing numbers. The most important of these later lakes were in the Great basin in Oregon, in Washington and in Nebraska, and their 1 AMERICAN NATURALIST, May, 1879. deposits enclose the remains of a fauna entirely distinct from that fre d J9 g j 500 600 E а и 2 પા d 1900 2000 w, Wasatch; from Lt. Wheeler's report. ? 500 FIG. 2.-Section west of the Gallinas mountains, New Mexico, from Gallinas creek to the Eocene plateau. Letter, Jurassic; jg, Jurassic creek; d, Dakota; b, Benton; n, Niobrara; 7, lignite; fh, Fox hills, p, Puerco; 1200 ft gypsum; g, Gallinas of the Loup Fork period and of more modern character. They are known as the Equus beds. This fauna was probably contemporaneous with that which roamed through the forests of the eastern portion of the continent, whose remains are inclosed in the deposits of the caves excavated from the ancient limestones. A more detailed account of the formations is now given, with the names of a few of the characteristic fossils. THE PUERCO, This formation, having furnished numerous mammalian fossils, is known to belong to the Tertiary rather than the Post-cretaceous series. It is regarded by Dr. Endlich as a subdivision of the Wasatch, but the characteristics of its fauna are so marked as to constitute it a distinct horizon. The most southern locality at which it has been observed, the one from which I named it, and where its characters are distinctly displayed, is west of the Jemez and Nacimiento mountains, in New Mexico, at the sources of the Puerco river. At this place its outcrop is about 500 feet in thickness, and has an extent of several miles on both sides of the river. From this point the strike is northward, keeping at the distance of a few miles to the eastward of an escarpment of the Wasatch formation. It contracts in depth to Sta.40,74. Sta. 44 774. Rio Florida. Sla.51. the northward, and it extends to the south-west, beyond the overlying Wasatch beds. It is well developed in Southern Colorado, where Dr. F. M. Endlich' and William H. Holmes, of Dr. Hayden's Survey, detected it in 1876. Its mineral character is there similar to that seen in New Mexico, and its thickness is much greater. On the Animas river it is 1000 to 1200 feet; on the San Juan river, near the Great Hog Back, 700 feet. The general characters of the formation are expressed in the following description, extracted from my report to Lieut. G. M. Wheeler.3 "South of the boundary of the Wasatch, the varied green and gray marls formed the material of the country, forming bad land tracts of considerable extent and utter barrenness. They formed conical hills and flat meadows, intersected by deep arroyos, whose perpendicular walls constituted a great impediment to our progress. During the days of my examination of the region, heavy showers of rain fell, filling the arroyos with rushing torrents, and displaying a peculiar character of this marl when wet. It became slippery, resembling soap in consistence, so that the hills were climbed with difficulty, and on the levels the horses' feet sank at every step. The material is so easily transported that the drain 1Annual Report U. S. Geol. Surv. Terrs., 1875, p. 189. * Loc cit., 247. Annual Report of Chief of Engineers, 1875, p. 89. Appendix 44. FIG. 3.-Section near the borders of Colorado and New Mexico; from Dr. Hayden. |