GEOLOGY 



lent beds in west Sussex and the Isle of Wight, it appears to the 

 writer that part of the sediments classed as Weald Clay in one district 

 may be equivalent to strata classed as Hastings Beds in another part 

 of the Weald. This point is mentioned because of its practical con- 

 sequence in cases where it is intended to penetrate the Weald Clay 

 by borings for water-supply or other purposes. 



The Weald Clay is interstratified at intervals with thin bands of 

 sand and silt, and with layers of limestone made up almost entirely of 

 a freshwater shell of the genus Paludina. This limestone, often known 

 as ' Bethersden marble,' from a locality where it was extensively dug, 

 was formerly much used, like the ' Sussex marble ' of similar origin, in 

 ecclesiastical architecture, both as a polished stone and unpolished, as 

 for example in the church towers of Headcorn, Smarden, Biddenden 

 and Tenterden, and in the polished altar stairs of Canterbury Cathedral. 

 The fossils of the Weald Clay resemble those of the Hastings Beds, being 

 chiefly freshwater shells and cyprids, with the teeth and scales of fish 

 and the remains of land plants.^ Where exposed at the surface the clay 

 forms a heavy tenacious soil, expensive and difficult to cultivate. But 

 from its low-lying position much of its outcrop is overspread by alluvium 

 and other superficial deposits, and the tracts thus modified are very fertile 

 and embrace some of the principal hop gardens of the county. The 

 clay itself is dug in many places for brickmaking. Deep borings have 

 shown that this division undergoes the same rapid diminution in thick- 

 ness in its northward underground extension as the Hastings Beds, and 

 that it thins out entirely before reaching the north-eastern border of 

 the county.^ 



LOWER GREENSAND 



The invasion of the sea, of which, as already noted, there are slight 

 preliminary indications in the brackish water fauna towards the top of 

 the Weald Clay, appears to have become suddenly accelerated at the close 

 of the Wealden period, so that the long prevalent freshwater conditions 

 were abruptly terminated and the whole district submerged beneath the 

 tides of an encroaching ocean. The marine conditions thus established 

 were thenceforward persistent throughout the remainder of the Lower 

 Cretaceous and the whole of the Upper Cretaceous times. During the 

 first stages of this great period of submergence the Atherfield Clay and 

 Lower Greensand were deposited ; afterwards the Gault Clay and Upper 

 Greensand; and finally the thick white mass of the Chalk. Minor oscil- 

 lations of level during this long submergence were frequent, rendering 

 the sea now deeper and now shallower, and the coast-line sometimes near 

 and sometimes more remote ; and thereby causing modification or change 

 of character in the sediments. Indeed it is probable that during the 

 earlier stages the shore at times approached within the northern limits 



1 For description of the scanty vertebrate remains of the Kentish Wealden, see subsequent article 

 'Palaeontology,' p. 31. 



2 See subsequent records of deep boring-sections, pp. 25-8. 



I q 2 



