The Pliocene is not a conspicuous formation in this country, and only of comparatively late years has it been recognized at all on the Atlantic coast. The movements which closed the Miocene gave to the Atlantic and Gulf shores nearly their present outlines, but some differences may be noted. Much of southern Florida was still under water, and a gulf invaded northern Florida, covering a narrow strip of Georgia and South Carolina. Isolated patches of Pliocene rocks in North Carolina and Virginia may be remnants of a continuous band. The Gay Head Sands on the island of Martha's Vineyard have marine fossils and lie unconformably on the Miocene, forming the most northerly known exposure of marine Pliocene on the Atlantic coast. All of these marine formations in the eastern United States are very thin and in notable contrast to the Pacific coast. Florida also has some fresh-water Pliocene. A small part of eastern Mexico, much of Yucatan, and some of Central America were still submerged.
On the Pacific coast the post-Miocene upheaval had laid bare the western foothills of the Sierra and greatly disturbed the Miocene strata of the Coast Range. The latter range sank again early in the Pliocene, whose strata lie unconformably upon the Miocene, and extend over upon older beds. The transgression of the sea was limited, and Pliocene rocks form only a narrow band along the coast in California, Oregon, and Washington. The San Francisco peninsula was an area of subsidence and maximum deposition, for here no less than 5800 feet of sandstone (the Merced series) were formed, and quite lately Professor Lawson has described a series of beds, containing much volcanic material, 7000 feet thick, lying below the Merced and above the Monterey Miocene. This would make the Pliocene near San Francisco have a thickness of nearly 13,000 feet, by far the thickest mass of Pliocene in North America. On the other hand, a deduction, perhaps a very considerable one, should probably be made from this thickness, for the upper part of the Merced appears to be Quaternary. (Dall, Arnold.) The mountains of British Columbia are believed to have been at a higher level than now, an elevation which probably connected Vancouver's and the Queen Charlotte Islands with the mainland.
Marine Pliocene also occurs in southern Alaska. The marine Pliocene faunas of California and Japan became closely similar, due to a migration along the shore around the North Pacific, where the climate was temperate, no Indian species joining in the migration of the Japanese forms. In the Upper Pliocene the waters of the California coast appear to have been somewhat colder than they are now.
In the interior region a few areas of Pliocene, resembling the Upper Miocene in physical character and in mode of formation, have been described. The oldest of these, the Republican River stage, overlies and is intimately associated with the upper Loup Fork, in northwestern Kansas, northern Nebraska, and eastern Oregon. The Blanco stage is typically displayed in the Staked Plains of northwestern Texas, where it contains South American mammals, and is also found in Nebraska and Oregon. The Upper Pliocene is not definitely known to be represented in the interior, but its presence is suspected in Texas and elsewhere.
Some isolated areas of Pliocene which cannot yet be correlated with the stages mentioned, are found in southern Idaho, eastern Washington, etc., and no doubt much of the surface deposits of the Great Basin and other regions is Pliocene, but lack of fossils prevents their determination.
The volcanic activity in the Rocky Mountain and Pacific coast regions, which had begun in the Cretaceous, continued through the Pliocene. The great outflow of rhyolite which built up the Yellowstone Park plateau is referred to the Pliocene. Some of the enormous fissure eruptions, which flooded northern California and Nevada, southern Idaho, eastern Oregon, and Washington, with thick sheets of basalt, obliterating the valleys and revolutionizing the system of drainage, are probably Pliocene, as some are demonstrably Miocene.
A problematical formation is the Lafayette, whose geological position and mode of origin are still debated. The Lafayette is a belt of sands and gravels which runs through Maryland, Virginia, the Carolinas, and the Gulf States, around the southern end of the Appalachians, up to southern Illinois, whence it turns south-westward to Texas. As in the typical exposures the Lafayette rests unconformably upon the Miocene and is uncomformably overlaid by the Pleistocene, many authorities refer it to the Pliocene. The mode of formation is somewhat obscured by the absence of fossils, but this very lack and the physical characters of the beds make a marine origin improbable. It is more likely that the deposition was subaerial, "resulting from a comparatively rapid Pliocene uplift in the Appalachian region." (Dall.) According to another view, the Lafayette is due to a depression of the coastal plain while the Piedmont region was elevated, "and streams gorged with detritus from the decayed, uplifted Piedmont above rushed down to the sea and poured their contents into the ocean.
Either the waves were weak or the sea advanced rapidly or this decayed material was discharged in enormous quantities, for the sea was unable to cope with it and deposited it unsorted on the bottom." (Shattuck).
At or near the close of the Pliocene, extensive upheavals took place in several different parts of the continent, especially on the Pacific slope. The rise of the Rocky Mountains continued, raising the western part of the Miocene beds 3000 feet higher than the eastern. The height of the Sierra was greatly increased by the rise of the mountains along the eastern fault-plane and the tilting of the whole block westward. The new valleys cut through the late basalt sheets of the Sierras are much deeper than the older valleys excavated in Cretaceous and Tertiary times, which is due to the greater height of the mountains and consequent greater fall of the streams. The fault-blocks which form the Basin Ranges were still further displaced, increasing their height. The Wasatch Mountains and the High Plateaus of Utah and Arizona were again upraised. The great mountain range of the St. Elias Alps, in southeastern Alaska, was upheaved at this time, or even later, and the mountains of British Columbia were probably raised still higher. In Washington and Oregon the uplift was small, but became much greater in southern California, reaching 2500 feet in the Monte Diablo range. On the eastern side of the continent the uplift was on a much more restricted scale, not generally exceeding 100 feet.
The Florida anticline underwent renewed compression, which increased its height; in Georgia, the continuation of this fold rose to 400 feet. The same movement extended the coast of Mexico and Central America and brought the continent to nearly its present outlines.
It is not necessary to suppose that all these movements were contemporaneous; merely that they occurred, now in one place, now in another, at or near the end of the Pliocene epoch.