LONDON CLAY. 25 



the sands and pebbles of the Woolwich and Reading Series. An 

 approximate line for the western and northern limits of the 

 Blackheath Beds may be drawn from Croydon and Thornton 

 Heath to Lewisham and the neighbourhood of Stratford and 

 thence north-eastwards roughly along the line of the Colchester 

 Road. 



LONDON CLAY. 



On account both of its thickness and of the area occupied 

 at the surface the London Clay is, as might be supposed, the 

 most important formation of the district. 



It consists of a stiff clay of dark grey or bluish-grey colour; 

 when exposed at the surface, however, it is brown owing to the 

 conversion of ferrous oxide to ferric oxide by weathering ; a few 

 feet down, where the process is not complete, the clay is fre- 

 quently mottled. 



The lowest part of the formation comprises a sandy bed with 

 black flint pebbles and occasional layers of sandstone, and is 

 known as the Basement Bed. Towards the top a varying 

 thickness of alternating seams of clay and sand form a passage 

 to the overlying Bagshot Sand. Recently it has been found 

 desirable to separate these passage beds under the name of 

 ' Claygate Beds,' 1 and in reading accounts of sections written 

 previously to 1912 it must be remembered that this upper 

 division is included under the term ' London Clay.' 



Although the main mass is a stiff clay, it sometimes presents 

 a silty appearance difficult to distinguish from alluvial clay that 

 has no doubt been largely derived from the London Clay. In 

 foundations for the Gaiety Restaurant this was the case, and 

 had it not been for a floor of septaria it would have been difficult 

 to decide that the London Clay had been reached. 



These septaria or cement stones, which are a characteristic 

 feature of the London Clay, occur in nodular layers at irregular 

 intervals, and often enclose fossils. They are concretions of 

 argillaceous limestone, and the cracks which arose on their 

 consolidation are filled withe ale -spar, forming septa. On decom- 

 position at or near the surface we often find that the calcareous 

 matter has formed small nodules or concretions of carbonate of 

 lime called 'race,' themselves often septarian. A strong and 

 continuous layer of septaria forms the foundation of the new 

 County Hall. 



There are only a few localities where the full thickness of the 

 London Clay is shown, by the presence of the overlying Claygate 

 Beds, to be present. At Harrow it is only about 220 ft. ; at 

 Hampstead the thickness is about 330 ft. on the south and 

 375 ft. on the north ; at Highgate probably about 350 ft. In 

 Essex the Claygate Beds have not yet been mapped, though 

 many sections which would now be referred to that division 

 have been described. A well-section at Brentwood seems to 



1 ' Summary of Progress for 1911 ' (Mem. Geol. Surv.), 1912, p. 31. 



