CHAP. XII.] ICENIAN PERIOD. 257 



Jurassic ridge, although only two of them (the Welland 

 and Nen) now retain their ancient channels, and we may 

 fairly assume that the bay of the Wash did not then exist, 

 but that the combined streams of the above-mentioned 

 rivers occupied a broad valley which passed along or just 

 outside the northern coast of Norfolk, and opened into an 

 estuary somewhere to the north-east of that county (see 

 Plate XIII.). That a river of some size did at a much later 

 date pursue such a course has been surmised from the 

 peculiar geological features presented by the northern coast 

 of Norfolk. This coast is bordered by a continuous strip 

 of alluvium, the western part of which rests on boulder- 

 clay which is banked against a steep slope of chalk. 

 Kef erring to the alluvium, Mr. H. B. "Woodward remarks, 1 

 " the physical characteristics of the area suggest that this 

 low ground was originally an old river- valley, and that the 

 heights which bordered it on the north have been destroyed 

 by the ravages of the sea." 



Passing now from these details of the Pliocene river- 

 system, let us consider what other changes may have been 

 caused by the depression which led to the formation of the 

 North Sea. The most direct and important change was 

 the submergence of the land which had hitherto from 

 the commencement of Tertiary time united Scandinavia 

 with Scotland and Iceland. Not only did this depression 

 open a communication with the Arctic Ocean, but in all 

 probability with the North Atlantic also, by the temporary 

 breaching of the isthmus between Scotland and the Faroe 

 Islands, and by the conversion of the deep valley or hollow 

 north of these islands into a strait. By these passages 

 many North Atlantic and American species of mollusca 

 gained access to the Anglo-Belgian part of the Pliocene 

 Sea, no fewer than eighteen American species occurring in 



1 " Geology of the Country around Fakenham," Geol. Surv. Mem. , 

 p. 44. 



