The marine Miocene rocks, which have an enormous development on the Pacific coast, are rather scantily displayed along the Atlantic and Gulf borders. The eastern coast, which had emerged during the Oligocene, was slightly depressed, and the Miocene beds were deposited unconformably upon the Eocene, in some places overlapping the latter landward, and it may be that the narrow belt of coastal Eocene has all been exposed by the denudation of the overlying Miocene. In any event, the Miocene coast-line was nearly the same as that of the Eocene had been, save for the reduction of the Mississippi embayment and the presence of the Florida island. Miocene beds occur in the island of Martha's Vineyard, apparently are concealed beneath the sea along the New England coast, and, from New Jersey southward, are almost continuous. In New Jersey their thickness is only 700 feet, thinning to 400 feet in Maryland, but reaching 1500 feet in eastern Texas, where they are concealed under later deposits, but their presence is revealed by deep borings. In the North the strata are unconsolidated sands and clays with local accumulations of diatom ooze, as at Richmond,Va. (seep. 314), but in Florida they are compact limestones, and in Georgia, limestones and conglomerates.

Owing to the nearly complete closing of the Mississippi embayment, Miocene strata do not extend into Tennessee and Arkansas.

The Oligocene of the Atlantic coast had been a time of warm waters, but in the Miocene a cool current flowed southward along the shore and through the straits between the Florida island and the mainland into the Gulf of Mexico. "The change by which the Oligocene was brought to a close and the typical Miocene inaugurated, caused . . . the most remarkable faunal break in the geological history of the United States after the Cretaceous.,, (Dall).

On the Pacific coast the Miocene rocks, though reaching the enormous thickness of 5000 to 7000 feet, form only a narrow belt,, and lie unconformably upon the Eocene. The Coast Range formed a chain of reefs and islands in the Miocene sea. Volcanoes were very active and showered great quantities of ash into the sea, where it was extensively mingled with diatoms, which largely compose the Monterey series, though sandstones and bituminous shales also occur. The sea did not extend into the northern part of the Sacramento valley, which is filled with continental sediments, fluviatile and subaerial and perhaps partly lacustrine. Orogenic disturbances took place in California, for the older part of the series in the Santa Cruz Mountains near San Francisco is folded and metamorphosed and the newer part there rests unconformably upon it.

The foothills of the Sierras had been worn down to a peneplain, which was elevated, perhaps early in the Miocene, and carved into valleys and ridges, and in the lower stream courses the "deep Auriferous Gravels" were laid down. In the Upper Miocene came a depression and very thick masses of the "bench Auriferous Gravels" accumulated in the valleys. Then followed a time of great volcanic activity in the Sierras, at first forming lava-flows and tuffs of rhyolite, then, after an interval, andesite tuffs and breccias, which poured down the valleys as great torrents of mud.

The coast of Washington and Oregon was covered by the sea, which extended up the valley of the Columbia and its tributary the Willamette, but the beds are far thinner than in California and, in places, lie upon folded and eroded Eocene. The sea also extended over parts of British Columbia. Early in the Miocene, Alaska was depressed, especially to the north, and the valley of the Yukon invaded by the sea and much of western Alaska was submerged, yet in the Middle and Upper Miocene, at least, some land connection with the Old World must have existed.

The Miocene fauna of California was largely indigenous and shows no evidence of communication with Asia, which would indicate that Bering Strait was open; if so, the undoubted connection of America with Eurasia must have been by some other route, perhaps by way of Greenland and the north Atlantic. Atlantic Miocene species are not known in the Pacific fauna, whence it may be inferred that the upheaval of Central America and the Isthmus of Panama, joining South and North America and separating the two oceans, took place at the close of the Oligo-cene, though there are some difficulties in accepting this view.

In the interior region Miocene continental deposits, mostly fluviatile, cover a vast area, though to no very great depth. The Arikaree, or Rosebud, stage, which is in part transitional from the uppermost Oligocene, is found overlying the White River beds in South Dakota, western Nebraska, and eastern Wyoming, with small areas in northeastern Colorado and Montana. The middle Miocene Deep River stage, occurs in widely scattered areas of restricted extent, in central Montana, central Wyoming, northeastern Colorado, northwestern Texas, and eastern Oregon. In this stage the migration of land mammals from the Old World, which ceased at the close of the White River Oligocene, was resumed, bringing in several new types, particularly the primitive elephants, which migrated from Africa to Asia and reached Europe and North America at nearly the same time. In this stage also appear the first forerunners of the migration from South America for which the junction of the two Americas opened the way. The Loup Fork stage covers much of the Great Plains region with a thin sheet of fine sands and marls, in successive disconnected areas from South Dakota far into Mexico, with outlying areas in Montana and New Mexico.

In addition to these comparatively well-known and well-defined stages of the Miocene, there are several others which are referred to the Miocene, although for no very convincing reasons. In southwestern Nevada is an immense thickness (14,000 feet) .of supposably Miocene beds, described as being lacustrine, but containing some coal and sulphur. Several other areas are found in Nevada, Washington, and British Columbia. A small area of probably Miocene rocks occurs in the South Park of Colorado, the Florissant beds, which have usually been called Oligocene, but which recent and more extended studies have shown to be probably Miocene. The deposits are thin, papery shales, composed of fine volcanic ash showered into a small body of water, and have preserved countless insects and plants, many fish and a few birds, but no mammals.

Fig. 307. - Idealized section of the great Bad Lands of South Dakota. (Osborn).

The Miocene was a time of great volcanic activity in the Pacific mountain ranges and along the principal range of the Rocky Mountains; the great volcanoes of the Cascades and of Mexico are believed to date from this epoch, and in the Yellowstone Park were immense eruptions of andesites and basalts, both lavas and tuffs. The great Columbia River basaltic flows are of early Miocene date, for they lie upon the slightly disturbed and eroded John Day, while Middle Miocene beds were deposited upon them. Vulcanism was also displayed in the West Indies, the Andes, and Patagonia.

As we have seen, orogenic movements went on in California between the Lower and Upper Miocene. Later in the epoch and at its close, these movements grew very important and widely extended, affecting the mountains of all the Pacific States and causing the principal upheaval of the Coast Range in California and Oregon. The great fault bounding the Sierras on the east was made and the block mountains of the Great Basin raised by an extensive system of faults. The high plateaus of southern Utah and northern Arizona were raised, beginning the great erosion-cycle of the Colorado River. In the East the West Indian islands were raised and the Florida island was joined to the mainland.