GEOLOGY 



Chadwell and West Tilbury. They have been opened up in pits north 

 of Grays Thurrock station, and have been seen to a thickness of 53 

 feet by Mr. T. V. Holmes in one of the Deneholes in Hangman's Wood, 

 north of Little Thurrock. Being in this area largely covered by valley 

 gravel they have but little direct influence on the soil. Eastwards and 

 westwards they are concealed beneath the alluvial deposits bordering the 

 Thames, but over a great part of Essex they have been proved in borings 

 and sinkings as at Stratford, Loughton and elsewhere. 



Along the northern margin of the county their presence has not 

 everywhere been determined, and they have not been separated from the 

 Woolwich and Reading Beds on the Geological Survey map, because in 

 that area the strata are largely concealed by Glacial Drift, and it is known 

 that although they occur at Sudbury, the Thanet Beds are not present to 

 the north-west of the London Basin. 



WOOLWICH AND READING BEDS 



The Woolwich and Reading Beds comprise mottled clay, laminated 

 clay and sand, also shelly clays and lignite, and usually at the base 

 greenish-grey sand with flint pebbles. In composition and thickness 

 this formation is as usual variable, and it is known better in Essex from 

 the records of well-sections than from the surface exposures. The thick- 

 ness varies from 25 to 60 feet or more. 



The beds occur beneath the valley gravels at West Ham and also 

 at Beckton, and they come to the surface from near Wennington to 

 Aveley, Stiffbrd, Orsett and Stanford-le-Hope, where they may be seen 

 here and there in temporary excavations. 



Along the northern outcrop they appear at Roydon, and near Farn- 

 ham and Stanstead Montfitchet. Near Elsenham and Debden they 

 probably occur further north than the map indicates. They are seen 

 again north of Thaxted and at Castle Hedingham. 



Among the fossils Melania inquinata, Cerithium funatum^ Cyrena cunei- 

 formis and Osfrea bellavacina indicate estuarine conditions. 



BLACKHEATH OR OLDHAVEN BEDS 



The Blackheath (or Oldhaven) Beds consist locally of grey sands 

 with flint pebbles, and contain marine shells like those of the London 

 Clay, but they have been somewhat doubtfully recognized by Mr. 

 Whitaker in borings in south Essex. Thicknesses of 1 1 feet at Barking 

 Side, 50 feet at Stanford-le-Hope and 37 feet at Shoeburyness are thus 

 assigned to this formation, 1 which is well represented south of the 

 Thames in the pebble-beds of Blackheath, and in the sands of Oldhaven 

 or Bishopstone Gap. 



The occurrence of flint pebbles in the Blackheath Beds shows that 

 in some areas the Chalk must have been upraised to form cliffs. This 

 may have been in the area of the Weald from which the Chalk has since 

 been removed. 



1 See Whitaker, Grt/egy ef LoaJon, vol. i. p. 233. 

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