Crag

Crag, the East Anglian term for a shelly gravel or sand, now generally transferred by English geologists to the whole of the Pliocene system, which in the eastern counties is mainly represented by such rocks. This system represents the epoch when the continents were taking their present form and the climate of Northern Europe was gradually refrigerating to the cold of the Glacial Period (q.v.). It is only well represented in Europe in the basin of the Mediterranean, where several thousand feet of marine beds had accumulated before the first outbursts of Etna and Vesuvius. In England it is represented by some 100 to 150 feet of sandbanks deposited during a subsidence of the east coast, with patches at St. Erth in Cornwall, Lenham in Kent, and elsewhere. Of the numerous mollusks they contain at least 84 per cent. belong to living species. These beds may be subdivided as follows: -

7. Westleton and Mundesley Crag and Cromer Forest-Bed Group, 10-70 feet.
6. Chillesford Clay and Sand, each 8 feet thick.
5. Norwich, or Fluvio-marine, Crag, 5-10 feet.
4. Red Crag, 25 feet.
3. Lenham Beds.
2. St, Erth Beds.
1. White, Suffolk or "Coralline" Crag, 40-60 feet.

At the base of the series are beds of phosphatic nodules (q.v.) and fossils of cetaceans, sharks, mastodon, elephant, rhinoceros, etc., probably derived from the denudation of an earlier bed, possibly the Black Crag" or glauconitic sands of Antwerp, which has Miocene affinities. The Coralline Crag (q.v.) contains 5 per cent. of northern forms of Mollusca, the Red Crag 10 per cent., and the Norwich Crag 14 per cent. The numerous Polyzoa, Terebratula grandis and Astarte Omalii are specially characteristic of the Coralline Crag; Trophon antiquum var. contraria, the left-handed dog-whelk, of the ferruginous Red Crag; land and fresh-water shells with northern marine forms, Elephas meridionalis, E. antiquus and the giant beaver Trogontherium, of the Norwich Crag; Mya truncata and other northern marine shells, of the Chillesford Beds; and the Irish deer, Cervus megaceros, the Mammoth Elephas primigenius, with the species just named, and Machairodus, the sabre-toothed tiger, of the Forest-bed. This last series is estuarine and marine with peat and drifted fir-stumps and many plant-remains. It is only exposed under the Boulder Clay cliffs at low tide. The pre-glacial gravels at Westleton and Mundesley and elsewhere may be of the same age as the Forest-bed. The numerous mollusks of the St. Erth beds, about 60 per cent. being living species, indicate an age intermediate between the White and the Red Crags; whilst the Lenham sands filling pipes in the Chalk of the North Downs yield an assemblage perhaps even more closely related to the former, but still intermediate.