The Pliocene deposits of Britain occur in Suffolk, and are known by the name of “Crags,” this being a local term used for certain shelly sands, which are employed in agriculture.  Two of these Crags are referable to the Older Pliocene, viz., the White and Red Crags,—­and one belongs to the Newer Pliocene, viz., the Norwich Crag.

The White or Coralline Crag of Suffolk is the oldest of the Pliocene deposits of Britain, and is an exceedingly local formation, occurring in but a single small area, and having a maximum thickness of not more than 50 feet.  It consists of soft sands, with occasional intercalations of flaggy limestone.  Though of small extent and thickness, the Coralline Crag is of importance from the number of fossils which it contains.  The name “Coralline” is a misnomer; since there are few true Corals, and the so-called “Corals” of the formation are really Polyzoa, often of very singular forms.  The shells of the Coralline Crag are mostly such as inhabit the seas of temperate regions; but there occur some forms usually looked upon as indicating a warm climate.

The Upper or Red Crag of Suffolk—­like the Coralline Crag—­has a limited geographical extent and a small thickness, rarely exceeding 40 feet.  It consists of quartzose sands, usually deep red or brown in colour, and charged with numerous fossils.

Altogether more than 200 species of shells are known from the Red Crag, of which 60 per cent are referable to existing species.  The shells indicate, upon the whole, a temperate or even cold climate, decidedly less warm than that indicated by the organic remains of the Coralline Crag.  It appears, therefore, that a gradual refrigeration was going on during the Pliocene period, commencing in the Coralline Crag, becoming intensified in the Red Crag, being still more severe in the Norwich Crag, and finally culminating in the Arctic cold of the Glacial period.

Besides the Mollusca, the Red Crag contains the ear-bones of Whales, the teeth of Sharks and Rays, and remains of the Mastodon, Rhinoceros, and Tapir.

The Newer Pliocene deposits are represented in Britain by the Norwich Crag, a local formation occurring near Norwich.  It consists of incoherent sands, loams, and gravels, resting in detached patches, from 2 to 20 feet in thickness, upon an eroded surface of Chalk.  The Norwich Crag contains a mixture of marine, land, and fresh-water shells, with remains of fishes and bones of mammals; so that it must have been deposited as a local sea-deposit near the mouth of an ancient river.  It contains altogether more than 100 marine shells, of which 89 per cent belong to existing species.  Of the Mammals, the two most important are an Elephant (Elephas meridionalis), and the characteristic Pliocene Mastodon (M.  Arvernensis), which is hitherto the only Mastodon found in Britain.