Pliocene System. Strata belonging to this system are restricted in Britain to a limited area in Essex, Suffolk, and Norfolk; but a few isolated patches occur also in Cornwall and Kent. They consist of irregular lenticular beds of sands and shelly gravels, &c., which never occur altogether in one place. The whole series probably does not exceed 120 feet in thickness, and comprises the following groups arranged in descending order:
CROMER FOREST-BED: fresh-water or estuarine silts, clays, and sands, with layers of peat; 10 to 70 feet thick. The fossils are land and fresh-water molluscs, many land-plants, and numerous mammalian remains.
CHILLESFORD BEDS: sands and clays; 6 to 16 feet thick; contain marine shells, some two-thirds of which are existing Arctic species.
NORWICH CRAG: fluvio-marine gravel, sand, and loam; 5 to 10 feet thick. Fossils, chiefly marine molluscs; several land and fresh-water shells; and numerous mammalian remains—hence the name of 'mammaliferous crag' sometimes applied to this group. Of the shells 93 per cent. are living species—14.6 per cent. being northern forms.