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penetrate a few inches or reach a depth of 10 feet beyond which the
brown-grey mottling indicates incomplete oxidization. When water
has stood on the clay for a long period the zone of decomposition
may extend to a depth of 40 feet.
At regular 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 thickness
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 Woodwich 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 greencoated
flints at the base (known as the Bull Head Bed), these beds