INTRODUCTION. 



493 



risen more than 600 feet, the elevatory movement liaving- been o-radually less 

 eastward, in the direction of Belgium, the present heights of these deposits 

 above the level of the sea being at Lenham GOO, at Cassel 515, at Grammont 375, 

 at Brussels 235, and at Diest 205 feet (see Fig. 1). On the other hand subsidence 

 has taken place in Holland, regularly increasing nortlnvards towards Amsterdam, 

 where probably it may have amounted to 1500 feet (Fig. 3). 



Since the Lower Pliocene period the Anglo-Belgian basin has moved as on a 

 pivot, rising in the south and sinking in the north. The edge of this earth- 

 movement of northerly depression may be traced also in East Anglia, giving us 

 a clue to the conditions under which o\ir newer Pliocene deposits may have 

 originated. In the earliest of our Red Crag beds, those of Walton, Beaumont 

 and Oakley, which in 1899 I grouped as Waltonian,^ correlating them with the 

 Scaldisien and Poederlien of Belgium, we find, as before stated, many of the more 



CASSEL 



Asschien - — . 



OSTEND 



Heyst 



Goes 



GORKUM 



Utrecht 



Amsterdam 



CyrenaUbeds 

 Ypreiien 



'Ojfeet below Ordnance -datum 



Fig. 3. — Section from Cassel to Amsterdam. — F. W. H. (Reproduced by permission of the Council of the 



Geological Society of London). 



abundant mollusca of the Coralline Crag. As the result of the upheaval of the 

 Lenham-Diestien area, communication between the Crag sea and the Atlantic was 

 interrupted, while by the northerly subsidence, connection with northern seas was 

 established and many boreal and some arctic species were introduced, apparently 

 rather suddenly, to the Crag basin. Hence the facies of the molluscan fauna of 

 the region in question was gradually and steadily changed, its general character 

 eventually becoming strongly northern. As long as communication with southern 

 seas remained open the southern mollusca held their own, but when it was inter- 

 rupted and the latter came under the influence of colder currents from the north, 

 they gradually disappeared. At first, during the Waltonian, it was boreal species, 

 such as the sinistral Neptunea coiitraria, that established themselves, while after- 

 wards, at the Oakley stage, before the southern forms had died out, characteristically 



1 " A New Classification of the Pliocene Deposits of the East of England," Rep. Brit. Assoc. 

 (Dover), p. 751, 1899. 



65 



