§ 24. SECTION OF LEWES LEVELS. 63 



3. (b. b.) — Clay, containing fresh-water shells, with an intermixture 

 of existing marine species, as the common cockle, (cardium 

 edule,) tellina, && 



4 # ( c# Cm ) — Blue clay, inclosing marine shells, viz. cockles, mussels, 

 &c without any intermixture of fluviatile species. In this 

 deposit a skull of the Narwhal, or sea-unicorn (Monodon mono- 

 ceros), and of the Porpoise have been discovered. 



From the nature of these sediments we learn that this 

 valley was once an arm of the sea, and that the following 

 sequence of physical changes has taken place :— 



First, there was a salt-water estuary, inhabited by marine shell-fish 

 of the same species as those now existing in the British Channel, 

 into which cetacea occasionally entered. 



Secondly, the inlet grew shallow, the water brackish, and marine 

 and fresh-water shells were mingled in its blue argillaceous 

 sediment. 



Thirdly, the shoaling continued till fresh- water so much predo- 

 minated, that fluviatile shells, and aquatic insects, could alone 

 exist. 



Fourthly, a peaty swamp was formed, by the drifting of trees, and 

 plants, from the forest of Andreadswald, which formerly 

 occupied the entire Wealds of Sussex and the adjacent counties; 

 and terrestrial quadrupeds were occasionally imbedded in the 

 morass. 



Lastly, the soil being inundated by land-floods at distant intervals 

 only, became an oozy marsh, which has been gradually converted 

 into a fertile tract of country. 



Such are the natural changes which the river valley of 

 the Ouse has undergone, as shown by the character of the 

 deposits which partially fill up this depression in the chalk : 

 and historical records confirm the geological evidence.* 

 What further transmutations the plain of Lewes Levels is 

 destined to undergo cannot be foretold. It may be, that 



* See "A Day's Eamble in and about the Ancient Town of Lewes/' 

 1 vol. 8vo. 1846, by the Author. 



