The Lower Pliocene of the Mediterranean basin differs from the Miocene in the disappearance of numerous species and the ap pearance of many new ones. One finds most of the molluscs now living in the Mediterranean as well as others now only found in the marine waters of the west African coasts.
The Upper Pliocene of the Mediterranean basin has a fauna almost identical with that of the Mediterranean of to-day, but in cludes a few northern forms such as Cyprina islandica, now living in the cold seas of the north of Europe, and hence it is believed that the temperature of the Mediterranean sea in late Pliocene times was much lower than it is to-day. This marine type of Upper Pliocene, well developed in Italy, is known as Calabrian or Sicilian. More often the Upper Pliocene is represented by continental formations known as the Villafranchian (from Villa franca d'Asti in Piedmont). The clearest classification of the continental types of Pliocene is by means of the mammalian remains : Upper Pliocene or Villafranchian—marked by appearance of true elephants (E. meridionalis), and true horses (E. stenonis), and true oxen (Bonus etruscus), and by the last mastodons (M.
arvernensis). Another mammal is Rhinoceros etruscus. The mastodons are absent from the higher Villafranchian, which is sometimes, therefore, known as Saint-Prestian. Famous localities include Villafranca d'Asti, Val d'Arno and St. Prest near Chartres.
Lower Pliocene—with Mastodon arvernensis, M. borsoni, Rhi noceros leptorhinus and large antelopes. Famous localities include Montpellier and Bresse.
The Pliocene rocks of Britain now occupy principally a small area in Norfolk, Suffolk, and part of Essex. Small outlying patches in Cornwall (St. Erth and St. Agnes) and elsewhere in the west of England supply evidence that the Pliocene sea was responsible for the planation of much of the present surface of the peninsula of Devon and Cornwall. Small patches of ferrugin ous sands and gravels on the North downs of Kent and Surrey, on the chalk hills north of London and on the South downs of Sussex have also been shown to be of Pliocene age. In early Plio cene times a large hay of the North sea occupied what is now the London basin; the shore lines of this bay have been traced, as well as the plane of marine denudation for which it is responsible.
Later in the period the sea retreated and covered only parts of East Anglia whilst a great spread of fluvio-marine gravels was deposited in the London basin at an elevation of, roughly, 40o ft. above present sea-level. Still later in the period the sea left even East Anglia, and the highest Pliocene there were probably laid down in a distributary of the Rhine-Thames system. The Pliocene of East Anglia has been classified as follows:— The Pliocene deposits of Belgium and Holland are closely re lated to those of Britain but are much thicker and more extensive. The sea retreated northwards as it did in England. In Germany the retreat of the Oligocene sea left vast lakes in which swamp forests gave rise to lignites.
In Brittany and Normandy there are patches of marine sands comparable with those of Cornwall; in Central France no marine beds are found, but many interesting and in some cases highly fossiliferous deposits occur in volcanic rocks.
In North America the marine Pliocene—marls, clays and lime stones—are well developed in Florida and can be traced into the Carolinas and Virginia ; they have been classed as the Lafayette group (with lignites), the Florida group, and the Calooshatchis stage. On the Pacific coast the marine beds have attained great thicknesses, notably in the Merced series of San Francisco. In the San Luis Obispo region the non-marine Paso Robles beds, said to be i,000 ft. thick, belong to this period. Other local formations of marine origin in California are those of San Diego and Wild Cat. In the Rocky mountains are large lacustrine formations of considerable thickness, and certain conglomerates in Wyoming and Bishop mountain are assigned to this age.