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. 



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