View report page

28
Solid Formations
Claygate Beds—A group of evenly-bedded alternations of fine
sands and clays with thicknesses in the range of 50 to 100 feet. At
the top of the sequence, seams of clay lie between the thicker beds
of sand while at the base, fine laminae of white sand alternate with
beds of clay.
Within the Borough, a syncline fold running from Mitcham
through Crystal Palace to Shooter's Hill brings a vestige of the
Claygate Beds to the surface at the latter locality.
London Clay—This is a formation of stiff dark grey or bluishgrey
clay which when exposed at the surface or underlying "drift"
becomes brown in colour due to weathering. The weathering may
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, 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.