502 PLIOCENE MOLLUSCA. 



Weybourne Hoeizon. * 



The deposition of tlie Chillesford Ijeds, whicli tliiis indicates a slight upheaval of 

 the western part of the Crag area, was followed by an inconsiderable depression 

 which enabled the sea to re-invade the northern portion of the county of Norfolk, 

 encroaching, probably in the form of a small inlet, as far to the south as Belaugh 

 in the valley of the Bure, and to Crostwick, Spixworth and Rackheath to the south 

 of that river. Unfortunately none of these sections are now accessible. 



At Weyl^ourne and elsewhere on the Cromer coast there are some fossiliferous 

 deposits containing, together with those of the localities named above, a meagre 

 molluscan fauna of about 50 species, of which 34, or 68 per cent., are common and 

 al)undant British forms, 21 are arctic, 38 Scandinavian, 32 west European and 

 23 Mediterranean. Of the non-British species 4 are arctic and 7 Scandinavian, 

 while 6 are not recorded living. 



The most distinctive feature of the Weybourne beds, however, is the occurrence 

 in them, as stated above, of TdVma (Macoma) haWiica, a species unknown from the 

 Norwich or Chillesford horizons, which made its first appearance in the Crag at this 

 stage in great abundance, specimens of this shell outnumbering those of all the 

 other Weybourne species taken together. The increasingly impoverished character 

 of the Crag fauna is strongly marked at tliis horizon. 



A list of the Weybournian mollusca was published ])y me in 1905 in the Bull. 

 Soc. Beige de Geol., vol. xix, p. 326. 



There are some laminated beds at Walton-on-Naze of somewhat similar though 

 not identical character to those of Chillesford, which were considered by Prestwich 

 and afterwards by the younger Wood and myself to be Chillesford Clay ; I now 

 think this was a mistake, and confine that term to such deposits as occur in the 

 region lying between Chillesford and Burgh, as shown in Fig. 5, where they always 

 maintain their typical and highly micaceous appearance — an indication I think of 

 their southern origin. Other laminated clays are found in places on the Cromer 

 coast which the Rev. J. Gunn and CI. Reid regarded as Chillesfordian, but they are 

 associated with beds of typical TeUina haltJiica (Weybournian) Crag, a deposit I 

 consider to have been introduced from the north, TeUina halthica being a Scandi- 

 navian shell and a northern immigrant which reached this district suddenly and in 

 great abundance at that stage of the Pliocene history. The Chillesfordian 

 and southern estuary must represent a slight elevation of the Icenian area or 

 a shrinking of the Icenian basin, the Weybournian beds a limited re-invasion of 

 Norfolk by the northern sea. If these views are correct, the presence or absence 

 of this mollusc from the fossiliferous l)eds of north-east Norfolk may give us a 

 reliable test of their comparative age. 



