WOOLWICH AND READING BEDS. 19 



Allophane, 1 a hydrated silicate of alumina and lime, has been 

 found in a nodular form at Charlton, Plumstead, and other 

 localities in fissures of the Chalk at the junction with the 

 Thanet Sand. 



WOOLWICH AND READING BEDS. 



This formation is present everywhere beneath the London 

 Clay in the district under consideration. It overlies the Thanet 

 Beds in the southern and eastern tracts, but beyond the western 

 limit of that formation, as given above, rests directly upon the 

 Chalk. 



The Woolwich and Reading Beds comprise a variable group 

 of sands, pebble-beds and clays, with occasional hard calcareous 

 bands, concretionary masses of siliceous greywether sandstone, 

 and puddingstone. The sands are white, grey and occasionally 

 crimson. The clays, for the most part brightly coloured, espe- 

 cially in the Reading facies, include red and mottled ' plastic 

 clay ' and green, grey and brown bands. ' Race ' (concretions 

 of carbonate of lime), selenite, iron pyrites, lignite, and seams 

 of black lignitic clay occur in the formation. The pebble -beds 

 are formed almost wholly of flint pebbles, and are met with at 

 various horizons. 



The term ' Woolwich and Reading Beds ' was given by 

 Prestwich to indicate different phases of deposition. The 

 Woolwich Beds, which in East Kent are entirely marine, 

 become estuarine and fluviatile in character at Woolwich and 

 Dulwich, while in the neighbourhood of Reading the mass of 

 the formation appears to be of freshwater origin, the product 

 of an Eocene river, and to this set of strata the term Reading 

 Beds is applied, although an estuarine bed usually occurs at the 

 base. Many plant remains have been found at Reading, but 

 others have been met with in the strata at Lewisham and 

 Mottingham, so that the limit of the two phases is naturally 

 ,very indefinite. Moreover, the Reading plant-bed may be 

 slightly older than that in West Kent. 



The estuarine bed at the base is persistent throughout the 

 formation and is often referred to as the ' Bottom Bed ' to dis- 

 tinguish it from the ' Basement Bed ' of the London Clay. Like 

 that below the Thanet Sand, it consists of greenish sand with 

 unworn flints and flint pebbles, and an oyster bed with Ostrea 

 bellovacensis Lamarck. The top of the underlying strata usually 

 shows borings made by molluscs or other marine organisms. 

 Where the Thanet Beds are present the Bottom Bed rests 

 sometimes on an even surface of the sands, sometimes irregu- 

 larly on them, the flints, which are always pebbles, being 

 4 splashed into ' the sands. At Swanscombe the Bottom Bed 

 has yielded several fossils, mostly bivalves of a marine type; 2 



1 Or more probably Halloysite. 



Stamp, L. D., and S. Priest, ' Geology of the Swanscombe Outlier,' Proc. 

 Geol. Assoc., vol. xxxi, 1920, p. 188. 



