-CHAP. XII.] ICENIAN PERIOD. 255 



dined to believe that all the great gaps which exist in this 

 escarpment, those of the Thames Valley, the Wash, and 

 the Humber, date from Pliocene times, and are, there- 

 fore, pre-G-lacial. Let us see how the evidence of the 

 newer Crags and their contents fits in with this theory. 



Professor Prestwich has shown that the Red Crag was 

 accumulated in a shallow sea studded with reefs of the 

 Coralline Crag, round which set strong and shifting cur- 

 rents. " The direction of the currents, judging from the 

 nature of the foreign materials found in the Red Crag, 

 seems to have been from the west to the south-east." l He 

 thinks the occurrence of unrolled flints and of large blocks 

 of Coralline Crag indicate transportal by shore-ice, but this, 

 I think, is hardly likely ; the blocks of Crag are only de- 

 scribed as occurring near the cliffs from which they were 

 quarried, and ordinary wave-action is sufficient to account 

 for such limited dispersal. Neither does the presence of un- 

 rolled flints really necessitate the existence of shore- ice ; 

 they may have been carried by uprooted trees borne to sea 

 by rivers in time of flood, or by ground-ice formed in the 

 same rivers during severe winters. 



Speaking of the Norwich Crag Mr. H. B. Woodward 

 remarks, " A study of the subject in conjunction with the 

 Suffolk Crag leads to the suggestion that the area was 

 gradually being depressed, and that the land to the nortli 

 was being slowly submerged, so that the Chalk cliffs which 

 then formed the coast-line were gradually destroyed north- 

 wards and westwards, and the neighbourhood of Norwich 

 came within the influence of the Crag sea before that of the 

 Bure valley. This gradual submergence perhaps led to 

 the introduction of Tellina balthica by the opening up of a 

 connection with some previously distinct area." 2 



1 " Quart. Journ. Geol. Soc.," vol. xxvii. p. 354. 



2 "Geology of the Country around Norwich," Geol. Surv. Mem., 

 p. 37. 



