Page:VCH Surrey 1.djvu/49

 GEOLOGY mammal, bird, turtles, crocodiles and fish, chiefly from sections at Croydon 1 and Dulwich. 2 These fossils, like those of the London Clay, indicate a climate considerably warmer than that which now prevails in the district. The Blackheath or Oldhaven Beds, which come next above the Woolwich and Reading Beds in the sequence, need not detain us long, as they attain their chief development to the eastward of the county boundary, and thin out westward soon after crossing it, finally disap- pearing at Croydon. Small ' outliers ' of these beds are scattered over the Chalk Downs to the southward of Croydon, up to the very crest of the escarpment south of Caterham. They consist of pebble-beds of extremely well-rounded flints more or less intermingled with sand. They seem to have been accumulated as shingle-banks in a shallow sea some little distance from the shore. Though in some places containing estuarine shells they yield more marine fossils than the Woolwich and Reading Beds, and thus herald the submergence which brought the waters of the sea once more over the whole of the south-east of England. LONDON CLAY With the deepening of this sea during the subsequent stage a thick and widespread mass of marine clay was deposited, which extends with- out much change throughout the London Basin and reappears to the south-westward of the Wealden dome in Sussex and Hampshire, having evidently once been continuous over all the intervening tract. This deposit, which from the fact of its underlying the metropolis is known as the ' London Clay,' occupies a wide area in Surrey ; and though concealed by newer deposits in the north-west of the county, it is continuous either at or beneath the surface in all that part of Surrey which lies to the northward of the outcrop of the Lower London Ter- tiaries. In composition it is a tenacious bluish-grey clay, weathering brown at the surface, containing layers of nodular concretions of clayey limestone. These nodules generally show shrinkage-cracks lined with calcite or aragonite, giving them a divided appearance, whence they are termed septaria ; they are often very fossiliferous. For a few feet at its base the London Clay generally shows an admixture of green and yellow sand, with rounded pebbles of flint, and part of this ' basement-bed ' is sometimes indurated into tabular rocky masses. The topmost layers of the clay are also intermingled with sand, thus passing gradually upward into the overlying Lower Bagshot Sand ; but otherwise its composition is remarkably uniform. Its thickness in Surrey ranges from about 300 to about 400 feet, increasing gradually from west to east. Its fossils, not everywhere present and obtained more abundantly in the neighbouring counties of Middlesex and Kent than in Surrey, include extinct mammals, birds, turtles, croco- 1 See H. M. Klaassen, Proc. Geol. Assoc., vol. viii. (1883) No. 4, pp. 236-242. 8 See C. Rickman, Quart. Journ. Geol. Soc. t vol. xvii. p. 6 ; and Mem. Geol. Survey, ' Geology of London,' pp. 211-213. 15