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may lie directly on "solid formations" or on banks of "river
terrace gravel". Alluvium components are subject to rapid changes
in thicknesses and often arranged in "lens-shaped" masses.
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 Shooters 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 Shooters 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