View report page

36
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.
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 sand and clay with abundant shells. The base of
the Woolwich and 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
are exposed around Woolwich, Charlton, Greenwich and north of
Lewisham, while at Plumstead Marshes the eroded beds are overlain
by superficial deposits.
Normally up to 55 feet thick, the beds reach a maximum thickness
in the south east of the area.
Chalk—This formation, almost entirely composed of soft, white
limestone with flints, is present everywhere in the area underlying
the Eocene strata except when it comes to the surface at Woolwich
and at the Greenwich Fault on both sides of the River Ravensbourne
in the vicinity of the Blackheath Hill, Brookmill Road and
Loampit Vale areas.
Its thickness, which can be from 550 to 740 feet, is subject to
considerable variation due to pre-Eocene erosion, but the formation
is usually divided into Upper, Middle and Lower chalk beds,
and these are often sub-divided in accordance with the types and
species of fossils found therein.
The accompanying sketch map gives a clearer picture of the
various geological formations found within the Borough.