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At irregular intervals, layers of septarian nodules (argillaceous
limestone often with calcium sulphate veins) are met with throughout
the clay and crystals of selenite and pyrites are common. The
lower part of the formation is frequently composed of fossiliferous
sand with black flint pebbles and occasional layers of sandstone
and is known as the Basement Bed.
The full thickness of London Clay is from 300 to 430 feet but,
in the Borough, this is only realised at Shooter's Hill. In the
Deptford, Greenwich and Woolwich areas the whole of the clay
has been removed by denudation exposing older formations.
Blackheath Beds—Very irregular in thickness, ranging up to
45 feet, these beds lying to the East of a line joining Sydenham
and Deptford often fill hollows eroded in the Woolwich & Reading
beds. They are composed of pebble-beds with a sandy matrix and
beds of fine, sharp, light-coloured quartzose sand in which are thin
layers of clay. The pebbles are well-rounded and consist mainly
of black flints with white spotted surfaces and occasional quartzites.
Locally the pebble beds are cemented into conglomerate and
the sand hardened to a sandstone. Fossils occur throughout the
division and, in places, the calcareous matter from shells forms the
cement of the conglomerate. In other places the cement is ferruginous
or siliceous.
Woolwich & Reading Beds.—These beds, with a normal
ness of from 25 to 80 feet, have been denuded from a tract which
follows the Thames from South Bermondsey to East Woolwich
and outcrops only at intervals at Plumstead and Eltham.
They comprise a variable group of white, grey or crimson sands,
sometimes hardened; red, green, grey or brown clays, frequently
mottled; pebbles formed of flint or occasionally quartz, locally
cemented into conglomerate; shell beds, concretions of carbonate
of lime, selenite, iron pyrites and lignite.
Although considered as one division, the Woolwich & Reading
beds are distinct; the Reading Beds are characterised by mottled
plastic clays, sands and pebbles and the Woolwich Beds consisting
of laminated sands and clays with abundant shells. The base of
the Woolwich & Reading Beds is relatively constant, being composed
of a green or greyish-green sand, somewhat clayey, with
flint pebbles and, at intervals, oyster shells.
Thanet Sand—Essentially of fine, buff or pale greenish-grey
sand, often silt, with a bed of green loam and generally unworn
green-coated flints at the base (known as the Bull Head Bed),
these beds are exposed around Woolwich, Charlton, Greenwich
and north of Lewisham, while at Plumstead Marshes the eroded
beds are overlain by superficial deposits.