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. 7
