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. 



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