Vol. 54.] AND THE CORALLINE CRAG. 349 



than 50 fathoms. Below that but few occur, except Isocardia cor. 

 This mud-loving form is very rare in the Coralline Crag.^ 



Mr. W. H. Wheeler, M.I.C.E., of Boston, in an interesting paper 

 read before the Institution of Civil Engineers in 1896/ shows, inter 

 alia, 'that the contour of the sea-bed, on a sandy coast, when 

 covered with a moderate depth of water, remains in a stable con- 

 dition, and that so long as the conditions remain the same, the 

 form of the banks and the depth of the channels are not altered.' 

 He points out that the channels lying between the sandbanks 

 which exist on both sides of the German Ocean at the present day, 

 as in the roadsteads of Calais, Dunkerque, and Ostend on the one 

 hand, and those of the East Anglian coast on the other, have 

 remained without noteworthy alteration for many years. Both the 

 banks and the channels are due to the action of currents, and when 

 they have been once established, and an equilibrium of forces has 

 been set up, no further change can take place until there is some 

 variation in the physiography of the area, such as an elevation or 

 depression of neighbouring land ; then a new state of things will 

 arise, and new deposits will accumulate. This, no doubt, is one 

 reason why such well-marked distinctions often exist between suc- 

 ceeding geological zones. The formation of deltas and of deep-sea 

 deposits goes on without intermission, but the sediment of shallow 

 basins affected by currents (like the North Sea at the present time, 

 or, as I believe, its western portion during the Coralline Crag 

 epoch) represents isolated rather than continuous stages in geological 

 history.^ 



The form and alignment of the area now covered by the Coralline 

 Crag are, I think, suggestive. Erom Tattingstone in the south to 

 the sunken rocks of Sizewell in the north it trends constantly from 

 S.S.W. to N.N.E., with an uniform and slightly-curved outline, 

 parallel to the line forming the north-western boundary of the Eed 

 Crag formation, which marks possibly the ancient shore-line of the 

 Crag basin. 



The form of the main mass of the Coralline Crag from Gedgrave 

 to Aldeburgh resembles strikingly that of some of the existing 

 sandbanks of the East Anglian coast. Although there has doubtless 

 been much denudation of the Crag between Tattingstone and Sutton, 

 that of the Orford district may still retain to some extent its 

 original form, and may indicate the trend of the coast during the 

 period in question. 



The absence of any deposits in East Anglia of the character of 

 the JsocarcZz'a-b eds of Antwerp, that is, of an undisturbed sea-bottom, 



^ Similar banks of dead or broken shells occur in the vicinity of Dungeness. 

 These also are due to current-action, occurring consequently with their longer 

 axes parallel to the coast-line. Other cases of the same kind might be 

 mentioned. 



2 • Littoral Drift in relation to River-outfalls & Harbour-entrances,' Proc. 

 Inst. 0. E. vol. cxxv, pt. 3. 



3 The Pliocene beds of Holland, forming part of the old delta of the Ehine, 

 represent on the contrary, I consider, a continuous sequence. 



