52 SCIENTIFIC SURVEY OF NORWICH AND DISTRICT 
refrigeration of climate and of restriction of the marine area, which had 
originally extended as far as Kent and northern France. The southern 
shores of this sea lay eventually in the position of what is now the north 
Norfolk Coast and the Zuyder Zee. Meanwhile, the ingress from the 
south of warm-water mollusca had been gradually barred, and the southern 
migration of cold or even arctic species of mollusca from the north was 
facilitated. 
The Pliocene Deposits—East Anglia is the home of the Pliocene de- 
posits of Britain, and Norwich has appropriately given its name to one of 
the major divisions of the shelly sands and gravels generally known as 
the ‘ Crags.’ The late F. W. Harmer, of Norwich, whose contributions 
to knowledge of European Pliocene and Pleistocene geology earned for 
him an international reputation, proposed the appropriate term ‘ Icenian’ 
for the later Crag beds, including the Norwich Crag, the Chillesford Clay 
and Crag and the Weybourne Crag. 
These deposits are exposed in the valleys and sea-cliffs of eastern 
Norfolk and Suffolk. They display considerable variation in lithology, 
the lowermost or Norwich Crag consisting of yellowish or reddish brown 
sands and gravels, occasionally very fossiliferous, as at Bramerton and 
Whitlingham, near Norwich. The fauna is dominantly molluscan and is 
indicative of colder conditions than the more ancient Red Crag found in 
Suffolk. As would be expected, also, it includes a greater number of 
recent (living) species than the Red Crag, the latter being composed 
of about 70 per cent. living species, while the Norfolk Crag contains 
89 per cent. Instead of being deposited like the Red Crag in the form of 
low shell-banks (up to 30 ft. in thickness) in land-locked bays, the Norwich 
Crag seems to have been laid down in an estuary of a large northward- 
flowing river, the course of which has been traced from Aldeburgh in 
Suffolk to beyond Norwich. ‘This estuary, belonging perhaps to a fore- 
runner of the river Rhine, which reached the restricted North Sea of 
that time near where Cromer stands to-day, sank under the load of sandy 
and gravelly deposits until a thickness of about 170 ft. had been accumu- 
lated. F. W. Harmer detected sufficient change in the fauna as the de- 
posits are followed northwards to divide the Crag into two sub-zones, 
that of Mactra subtruncata below, and that of Astarte borealis above. In 
consequence of the northward recession of the Icenian sea, only the upper 
sub-zone is found in Norfolk. Among the most characteristic forms of 
the Norwich Crag are the following: Astarte borealis Chemnitz, Cardium 
edule Linné, Cardium greenlandicum Chemnitz, Corbicula fluminalis 
Miller, Corbula gibba Olivi, Cyprina islandica Linné, Leda oblongoides 
S. Wood, Lucina borealis Linné, Mactra subtruncata da Costa, Mactra 
truncata Montagu, Mya arenaria Linné, Mytilus edulis Linné, Nucula 
cobboldie J. Sowerby, Pecten opercularis Linné, Scrobicularia plana da 
Costa, Solen siliqua Linné, Tapes virgineus Linné, Tellina lata Gmelin, 
T. obliqua J. Sowerby, T. pretenuis Leathes, Buccinim undatum Linné, 
Cerithium tricinctum Brocci, Hydrobia ulve Pennant, Littorina littorea 
Linné, Melampus pyramidalis J. Sowerby, Natica catena da Costa, 
Paludina media Woodward, Purpura lapillus Linné, Scalaria grenlandica 
Chemnitz, Trophon antiquus Linné, Turritella terebra Linné. (It will be 
