24 LONDON DISTRICT. 



Reading series and the later-deposited London Clay. Some 

 estuarine and freshwater species are survivors from the former, 

 certain marine species are new-comers and range into the London 

 Clay, while others returned with changed geographical conditions. 

 On the grounds that an incoming fauna is a more important 

 criterion of age than surviving species Dr. Stamp 1 correlates the 

 Blackheath Beds with the Lower Ypresian. A typical shell of 

 these beds is the common and variable bivalve Pectunculus 

 plumsteadiensis J. Sowerby, less inflated and with a less pro- 

 minent umbo than P. terebratularis Lamarck, from the Woolwich 

 Beds ; Neritina globulus Ferussac, a thin-shelled, sub-globose 

 univalve, with flattened spire and a sharp, semicircular mouth, 

 is also common and characteristic. We may note also the 

 globular Natica labellata Lamarck, with its well-developed 

 spire, and the mussel -like bivalve Modiola mitchelli Morris. 



The division is in the main a lithological one, a local accumu- 

 lation of marine and estuarine shingle and sand, derived no 

 doubt in part from pre-existing pebble -beds ; the light -grey 

 ovoid pebbles frequently referred to as ' quartzite ' are in many, 

 if not all cases, composed of sarsen stone ; 2 they occur sparingly 

 but may be found by careful search in almost every exposure. 



The wooded hill of Crohamhurst, south-east of Croydon, 

 and the fine scarp of the Addington Hills on the east are formed 

 of the Blackheath pebble -beds which attain a thickness of about 

 50 ft., and with some underlying sands perhaps 20 ft. more. 

 These in places overlap the Woolwich and Reading Beds and 

 rest directly on the Thanet Sand. At Park Hill, Croydon, the 

 pebble-bed, which is fossiliferous, occurs at the base with sand 

 above and rests in a hollow cut partly in the upper clay and 

 shell-bed and partly in the mottled clay of the Woolwich and 

 Reading Beds. At Beckenham the pebble -bed yields many large 

 specimens of oysters, some with the ligament preserved. 



Fine sections of the pebble -beds have been opened up in 

 the neighbourhood of Chislehurst by Camden Park, and in the 

 Sundridge Park Rock-Pit and adjacent railway cutting. Here 

 indurated layers of the pebble beds and sandstone occur at 

 various horizons and yield many fossils, including both freshwater 

 and marine forms. 



Northwards the Blackheath Beds are exposed in various 

 places from Blackheath to Erith and Crayford, the sections at 

 East Wickham being specially notable. On the extreme eastern 

 margin of the area, at Mounts Wood and Bean the whole series 

 does not exceed 5 feet in thickness and may be absent here and 

 there, though the pebble-beds come in again in force near 

 Cobham. 



From boring records it is difficult to distinguish between the 

 Blackheath Beds and the Basement Bed of the London Clay, or 



1 Proc. OeoL Assoc., vol. xxxi, 1920, p. 192. 



2 Baker, H. A., ' Quartzite Pebbles of the Oldhaven Beds,' OeoL Mag., 

 1920, pp. 62-70. 



