non-marine material. Thus the lower Eocene has some lignite in
the eastern Gulf region, while in Texas lignite and saliferous
and gypsiferous sediments are found, though most of the system
is marine and of shallow water origin. The Eocene of the western
Gulf region is continued north as far as Arkansas. The classification
of the Eocene (and Oligocene) formations in the Gulf region,
especially east of the Mississippi, is as follows:—
| 4. Jacksonian | Upper Eocene. | |
| 3. Claibornian | Middle Eocene. | |
| 2. Chickasawan | Lower Eocene. | |
| 1. Midwayan |
The Jacksonian is sometimes regarded as Oligocene. This classification is based almost wholly on the fossils, for there seems to be little physical reason for the differentiation of the Oligocene anywhere on the continent.
On the Pacific coast the marine Eocene lies west of the Sierras, and between it and the Cretaceous there is a general, and often a great, unconformity. The system has been reported to have a thickness of more than 7000 ft. in some places, and locally (e.g. the Pescadero formation) it is highly metamorphic. The Eocene of southern California carries gypsum enough to be of commercial value. It is also the source of much oil. The system is wanting in northern California and southern Oregon, but appears again farther north, and has great development in Oregon, where its thickness has been estimated at more than 10,000 ft. As in other comparable cases, this figure does not make allowance for the oblique attitude in which the sediments were deposited, and should not be construed to mean the vertical thickness of the system.
In Washington the Eocene is represented by the Puget series of brackish water beds, with an estimated thickness exceeding that of the marine formations of Oregon. Workable coal beds are distributed through 3000 ft. of this series. The amount of the coal is very great, though the coal is soft.
Terrestrial Eocene formations—eolian, fluvial, pluvial and lacustrine—are widespread in the western part of the United States, both in and about the mountains. By means of the fossils, several more or less distinct stages of deposition have been recognized. Named in chronological order, these are:—
1. The Fort Union stage, when the deposition was widespread about the eastern base of the northern part of the Rocky Mountains, and at some points in Colorado (Telluride formation) and New Mexico (Puerco beds), where volcanic ejecta entered largely into the formation. The Fort Union stage is closely associated with the Laramie, and their separation has not been fully effected.
2. The Wasatch stage, when deposition was in progress over much of Utah and western Colorado, parts of Wyoming, and elsewhere.
3. The Bridger stage, when deposition was in progress in the Wind River basin, north of the mountain of that name, and in the basin of Green river.
4. The Uinta stage, when the region south of the mountains of that name, in Utah and Colorado, was the site of great deposition.
More or less isolated deposits of some or all of these stages are found at numerous points in the western mountain region. The present height of the deposits, in some places as much as 10,000 ft., gives some suggestion of the changes in topography which have taken place since the early Tertiary. The thickness of the system in the west is great, the formations of each of the several stages mentioned above running into thousands of feet, as thicknesses are commonly measured.
The Miocene system, generally speaking, has a distribution similar to that of the Eocene. The principal formation of the Miocene System. Atlantic coastal plain is the Chesapeake formation, largely of sand. In Florida the system contains calcium phosphate of commercial value. The Miocene of the Atlantic and Gulf regions nowhere attains great thickness. The oil of Texas and Louisiana is from the Miocene (or possibly Oligocene) dolomite. On the Pacific coast the system has greater development. It contains much volcanic material, and great bodies of siliceous shale, locally estimated at 4000 ft. thick and said to be made up largely of the secretions of organisms. Such thicknesses of such material go far to modify the former opinion that the Tertiary periods were short. The Miocene of California is oil-producing. The terrestrial Miocene formations of the western part of the country are similar in kind, and, in a general way, in distribution, to the Eocene of the same region. The amount of volcanic material, consisting of both pyroclastic material and lava flows, is great.
At the close of the Miocene, deformative movements were very widespread in the Rocky Mountains and between the principal development of the Coast ranges of California and Oregon, and mountain-making movements, new or renewed, were somewhat general in the west. At the close of the period the topography of the western part of the country must have been comparable to that of the present time. This, however, is not to be interpreted to mean that it has remained unmodified, or but slightly modified since that time. Subsequent erosion has changed the details of topography on an extensive scale, and subsequent deformative movements have renewed large topographic features where erosion had destroyed those developed by the close of the Miocene. But in spite of these great changes since the Miocene, the great outlines of the topography of the present were probably marked out by the close of that period. Volcanic activity and faulting on a large scale attended the deformation of the closing stages of the Miocene.
The Pliocene system stands in much the same stratigraphic relation to the Miocene as the Miocene does to the Eocene. The marine Pliocene System. Pliocene has but trifling development on the Atlantic coast north of Florida, and somewhat more extensive development in the Gulf region. The marine Pliocene of the continent has its greatest development in California (the Merced series, peninsula of San Francisco), where it is assigned a maximum thickness of nearly 6000 ft., and possibly as much as 13,000 ft. This wide range is open to doubt as to the correlation of some of the beds involved. Thicknesses of several thousand feet are recorded at other points in California and elsewhere along the coast farther north. Marine Pliocene beds are reported to have an altitude of as much as 5000 ft. in Alaska. The position of these beds is significant of the amount of change which has taken place in the west since the Pliocene period. The non-marine formations of the Pliocene are its most characteristic feature. They are widely distributed in the western mountains and on the Great Plains. In origin and character, and to some extent in distribution, they are comparable with the Eocene and Miocene formations of the same region, and still more closely comparable with deposits now making. In addition to these non-marine formations of the west, there is the widespread Lafayette formation, which covers much of the Atlantic and Gulf coastal plain, reaching far to the north from the western Gulf region, and having uncertain limits, so far as now worked out, in various directions. The Lafayette formation has been the occasion of much difference of opinion, but is by many held to be a non-marine formation, made up of gravels, sands and clays, accumulated on land, chiefly through the agency of rain and rivers. Its deposition seems to have followed a time of deformation which resulted in an increase of altitude in the Appalachian Mountains, and in an accentuation of the contrast between the highlands and the adjacent plains. Under these conditions sediments from the high lands were washed out and distributed widely over the plains, giving rise to a thin but widespread formation of ill-assorted sediment, without marine fossils, and, for the most part, without fossils of any kind, and resting unconformably on Cretaceous, Eocene and Miocene formations. To the seaward the non-marine phase of the formation doubtless grades into a marine phase along the shore of that time, but the position of this shore as not been defined. The marine part of the Lafayette is probably covered by sediments of later age.
In earlier literature the Lafayette formation was described under the name of Orange Sand, and was at one time thought to be the southern equivalent of the glacial drift. This, however, is now known not to be the case, as remnants of the formation, isolated by erosion, lie under the old glacial drift in Illinois, and perhaps elsewhere. It seems probable that the Lafayette formation of the Gulf coastal plain is continuous northward and westward with gravel deposits on the Great Plains, washed out from the Rocky Mountains to the west. The careful study of these fluvial formations is likely to throw much light on the history of the deformative movements and changes in topography in the United States during the late stages of geological history.
Deformative movements of the minor sort seem to have been in progress somewhat generally during the Tertiary periods, especially in the western part of the country, but those at the close of the Pliocene seem to have exceeded greatly those of the earlier stages. They resulted in increased height of land, especially in the west, and therefore in increased erosion. This epoch of relative uplift and active erosion is sometimes called the Sierran or Ozarkian epoch. The details of the topography of the western mountains are largely of post-Pliocene development. The summits of some of the high mountains, such as the Cascades, appear to be remnants of a peneplain developed in post-Miocene time. If so, the mountains themselves must be looked upon as essentially post-Pliocene. Deformative movements resulting in closing folding were not common at this time, but such movements affected some of the coast ranges of California. This epoch of great deformation and warping marks the transition from the Tertiary to the Quaternary.
Quaternary Formations.—The best-known formations of the Quaternary period are those deposited by the continental glaciers Glacial. which were the distinguishing feature of the period and by the waters derived from them. The glacial drift covers something like half of the continent, though much less than half of the United States. Besides the drift of the ice-sheets, there is much drift in the western mountains, deposited by local glaciers. Such glaciers existed in all the high mountains of the west, even down to New Mexico and Arizona.
The number of glacial epochs now recognized is five, not counting minor episodes. Four defined zones of interglacial deposits are detected, all of which are thought to represent great recessions of the ice, or perhaps its entire disappearance. The climate of some of the interglacial epochs was at least as warm as that of the present time in the same regions. The glacial epochs which have been
