58 



THE GEOLOGIST. 



1. The Thanet Beds. — This formation, almost a purely Kentish one, 

 being but little shown in any other county, thins out a little west of 

 London. 



Under London it is from 30 to 40 feet thick, and consists of fine, soft, 

 light- coloured sand, without fossils. It continues the same through West 

 Kent, but with a thickness increasing to 60 feet or more. In East Kent 

 a gradual change sets in, the beds get more clayey, and fossils occur in 

 parts ; until the Isle of Thanet, and to the south, there are little else than 

 marls, mostly hard, and clays — with a little sand at top however — often 

 full of fossils, and from 80 to 100 feet thick. At the bottom, everywhere 

 there is a bed of clayey greensand, with green-coated flints lying at once 

 on the Chalk. 



The number of kinds of fossils in these Thanet beds is small. They 

 are all marine, and seem to be such as would have lived in the sea of a 

 more or less temperate climate ; the chief genera are Cyprina, Cytherea, 

 Pholadomya, Cucullsea, Thracia, Nucula, Corbula, Sanguinolaria, all bi- 

 valve shells, and Ampullaria. The cliffs and the shore, west of the Recul- 

 vers, yield those of the uppermost and more sandy part of the formation ; 

 whilst at Pegwell Bay the lower marly part is also shown. At both places 

 there are fossil-bearing sandstones in the upper part. The Bekesbourne 

 cutting of the London, Chatham, and Dover Railway yielded many fossils 

 to Mr. Dowker, one of the members of the Society, who has described this 

 section in a former volume of the ' Geologist.' 



Where a section shows a good thickness of the Thanet beds above the 

 Chalk, the junction of the two formations is even ; where, however, there 

 is but little of the former, the junction is generally more or less wavy, and 

 there are often large funnel-shaped hollows, known as " pipes." These 

 have been slowly formed by the action of water containing earbonic acid 

 in solution, which has dissolved away the Chalk (carbonate of lime) in an 

 irregular and unequal manner, and the beds above have fallen down into 

 the hollows thus left. It is where the Chalk is but thinly covered, and 

 therefore where water would the more easily filter through to it, that one 

 would expect this action to occur to the greatest extent, and such is the 

 case. 



2. The Woolwich Beds are remarkable for their ever-changing structure, 

 especially in Kent, in which county, however, there is none of the brightly- 

 coloured mottled plastic clay, that occurs so generally in this formation in 

 the western part of the London basin. 



In the " far west " of Kent, near London, the Woolwich beds are about 

 50 or 60 feet thick, and consist of alternations of sands, beds of pebbles, and 

 clays with shells ; and at the bottom, a bed of greenish sand with pebbles, 

 sometimes resting somewhat unevenly on the underlying Thanet Sand. 



The shell-beds must have been deposited in a river or estuaiy, for some 

 of the shells are of a kind that could not have lived in the sea, — such as 

 the bivalve genera Unio, Cyrena, and the univalve Paludina, Neritina, Me- 

 lanin, and Melanopsis. There are also oysters, and other genera that show 

 that the beds are not altogether freshwater, but partly of brackish- water 

 origin. As is usually the case with freshwater beds, fossils are most abun- 

 dant in numbers, but not in kind, — few species, but hosts of individuals. 

 Mr. Prestwich infers that the river which deposited these shells, and the 

 clay that contains them, flowed down from an island, which he supposed 

 to have occupied at that time part of our present Wealden district. The 

 climate indicated by the shells is that of a temperate region, and the remains 

 of plants that are found here and there, point the same way. These shell- 

 beds are well shown in the large ballast pit, at Charlton, near Woolwich, 



