MESOZOIC AND CAINOZOIC DINOFLAGELLATE CYSTS 23 



According to the earliest authority on the Eocene of Whitecliff, Prestwich (1847), 

 the thickness of his beds 3 and 4, (which he called the Bognor Beds), is 307 feet. 

 These are equivalent to the London Clay. Bristow (1862) lists a succession but 

 attempts no estimation of thickness. In the revised Isle of Wight Memoir of 1889, 

 the section is not given in detail, but an approximate value of 320 feet for the thick- 

 ness of the London Clay is quoted by Reid. The pebble bed according to Reid is 

 255 feet above the base. The first detailed section is attributable to White (1921) 

 who measured a total of 322 feet, with the pebble bed at 285 feet above the base. 

 Since 1921 all authorities including Curry (1958a) have quoted this thickness of 322 

 feet for the London Clay of Whitecliff. In measurements carried out by the authors 

 with a tape measure a value of 300 feet was arrived at for the thickness of the 

 London Clay with the pebble bed at 255 feet. Samples were taken for 315 feet above 

 the base but the two topmost (305, 315 feet) were yellow sands barren of microplank- 

 ton. 



Lithologically, the London Clay at Whitecliff is roughly divisible into three units 

 equivalent to the Lower Silts, the Stiff Clays and the Upper Silts of the Enborne 

 Valley (Hawkins 1954). Within each of these three units alternations of clays and 

 sands with all degrees of intermingling often occur and septarian nodules, lignite 

 and iron pyrites are common throughout. Sample numbers are shown in Text-fig. 5. 



Enborne Valley (Berkshire). In the years 1947-49 a series of borings were sunk 

 in the eastern part of the Enborne Valley, penetrating beds primarily of Eocene age, 

 much of which was London Clay. Professor H. L. Hawkins has kindly allowed us 

 to take samples from cores in his possession. Two boreholes, numbers 11 and 39 

 whose geographical positions can be seen in Text-fig. 6, were examined. 



According to Hawkins (1954), the London Clay of the Enborne Valley is litho- 

 logically divisible into three sections, the Lower Silts, the Stiff Clays and the Upper 

 Silts. 



From boring number 11, six samples have been examined in detail, one from the 

 Upper Silts, four from the Stiff Clays and one from the Lower Silts. From boring 

 number 39, four samples have been studied, three from the Stiff Clays, one from the 

 Lower Silts. All the samples studied yielded microplankton usually in a good 

 state of preservation (see Text-fig. 7). 



Sheppey. The London Clay attains its maximum thickness at Sheppey where 

 Davis (1936) has estimated it to be 518 feet. Of this the upper 160 feet are exposed 

 in the Sheppey cliff section running for six miles in an east-south-east direction from 

 Scrapgate in the west to Warden Point in the east. The lithology is a uniform stiff 

 blue grey clay with frequent courses of septaria. The junction with the Bagshot 

 Beds is well exposed in the west where 10 feet of in situ Bagshot sand overlies the 

 London Clay ; elsewhere difficulty is caused by large scale slipping and the number 

 of samples collected is less than might have been desired. 



Wrigley (1924) proposed five divisions for the London Clay of the London Basin, 

 each characterized by a faunal suite but not zones in the strictest sense. The fifth 

 (uppermost) division is typically sandy with more frequent fossils than blue clay. 



