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 A HISTORY OF SURREY Surrey and is subject to much variation in this respect as well as in the local development of its different members. 1 LOWER LONDON TERTIARIES The Thanet Sand, which forms the lowest portion of the group, is a fine light-coloured slightly clayey sand, having at its base a band of green-coated chalk flints derived from the erosion or dissolution of the Chalk. 2 This sand is best developed in the county of Kent, but extends thence into Surrey,' where its narrow outcrop fringes the Chalk in an indented belt running from Addington past Croydon, Sutton and Epsom, and thinning away gradually westward towards the Mole Valley until no longer traceable. Numerous small outlying patches of this and overlying divisions are also found capping the Chalk Downs some distance to the southward of the main outcrop, being the relics of the sheet which has once extended over the whole of the Chalk of Surrey. The scanty fossils which the Thanet Sand has yielded in our county support the evidence of the more plentiful organic remains which it contains in Kent in proving that the bed is essentially of marine origin. The Woolwich and Reading Beds, so named from the localities at which the different forms of these very variable deposits are typically de- veloped, constitute the middle division of the Lower London Tertiaries, and either overlie the Thanet Sands, or where these are absent rest directly upon the Chalk. Their outcrop stretches across the middle of the county from west-south-west to east-north-east in a narrow belt along the northern edge of the Downs, and in West Kent sweeps north- ward to the banks of the Thames between Erith and Greenwich, as shown on the map, and re-enters the north-eastern corner of Surrey for a limited space in the neighbourhood of Peckham and Dulwich, along with the Thanet Sands, encircling a small ' inlier ' of Chalk which reaches the surface at the county boundary west of Greenwich. These beds have been laid down in the estuary of a large river, which probably flowed from west to east. Like most estuarine deposits, their composition varies from place to place ; in the western part of the county they consist chiefly of lenticular alternations of plastic clay and coarse and fine sand, generally of bright tints, the clay often red and mottled, and the sand green, yellow, or greenish-grey. Almost the only fossils of the beds of this type are the plant-remains which occur in some of the laminated clays. Gradually changing eastward, the series at the eastern border of the county is mainly composed of light-coloured sands and finely-bedded grey clay, often crowded with estuarine shells and sometimes with layers of oysters compacted into rock, with pebble-beds of rolled flints towards the base, and occasionally with thin seams of lignite. Besides shells and plant- remains, the beds of this character have yielded traces of an extinct The most important contributions to our knowledge of the Eocene deposits of the county were made by the late Prof. J. Prestwich in a series of papers contributed to the Geological Society between 1847-57. 2 See W. Whitaker, Mem. Geol. Survey, ' Geology of London' (1889), vol. i. pp. 103- 106. 14