109 



CHAPTER IX. 

 EOCENE — continued. 



Bracklesham and Barton Beds 



Above the Lower Bagshot Beds a variable series of sands 

 and clays with lignite attains a thickness of about 700 feet. 

 There is no clear line of division between this series and the 

 underlying leaf-bearing beds, but the separation is often made 

 at the point where a pebble bed occurs, or at the lowest point 

 where marine fossils have been found. It should be remembered, 

 however, that there is no evidence of any real break, and that 

 the change is so gradual that it is very doubtful whether we have 

 really taken the boundary even approximately at the same horizon 

 at opposite ends of the Island. The difficulty of following the 

 beds inland makes it impossible to connect the sections by tracing 

 the boundaries on the Map. 



The beds now to be described are often known as the Middle 

 and Upper Bagshot series, but recent observations have shown 

 that the Upper Bagshot Beds of the London Basin are probably 

 the equivalent of the Barton Clay {i.e. of the so-called Middle 

 Bagshot of the Hampshire Basin). It has therefore been thought 

 safer to drop these names and simply to call the groups — for the 

 present at any rate, and having regard only to the Isle of Wight — 

 Headon Hill Sands, Barton Clay, and Bracklesham Beds. 



Bracklesham Beds. 



In 1847, Prof. Prestwich showed that the marine bands over- 

 lying the unfossiliferous Lower Bagshot Beds of WhitecHff Bay 

 were probably equivalent to the fossiliferous Bracklesham Beds so 

 well seen near Selsey.* Subsequently the Rev. Osmond Fisher 

 worked out the palasontology of the beds in greater detail, and 

 the following account of the sections at the two extremities of the 

 Isle of Wight is mainly taken from his paper.f 



The Bracklesham Beds are represented in Alum Bay by clays 

 and marls in the lower part, by white, yellow, and crimson sands 

 in the middle portion, and by dark sandy clays with numerous 

 impressions of fossils in the upper part. The latter alone have 

 been attributed to the Bracklesham Beds in Mr, Fisher's 

 Memoir. The lower beds are remarkable for the quantity of 

 vegetable matter contained in them, not, however, in the shape 

 of leaves, as is the case in some of the Lower Bagshot Beds, but 

 in the form of coal (lignite), constituting solid beds from fifteen 

 inches to two feet three inches thick. Four of these beds, when 



* Quart. Journ. Geol. Soc, vol. iii. p. 385. (1847.) 

 t Ibid., Yol. xviii. p. 65. (1862.) 



