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Gravels—Although four types of gravel deposits have been
distinguished, the most important are the River Gravels which form
part of the ancient alluvial deposits of the Thames and its tributaries.
These deposits, up to 40 feet in thickness, extend up the sides of the
valley from the bed of the Thames and lie on the gentle slopes or
series of platform or terraces of "solid formations".
The gravel is largely composed of subangular and well-rounded
flints, together with quartz and quartzite pebbles. Sand is frequently
interbedded with the gravel and, in places, constitutes the bulk of
the deposit while occasional seams of peat are to be found.
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 bluish-grey
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