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36
Superficial Deposits
Made—Made ground, consisting of artificial accumulations of
debris and dumped materials from excavations have been spread
over a number of areas of the district especially where the material
has been used to raise the surface level of alluvial areas to reduce
flooding in times of exceptionally high tides and heavy rainfall.
Alluvium—Such a term is applied to the deposits up to 30
feet in depth forming the low-lying areas bordering the Thames
and its tributaries. Originally marshland, these areas are still
liable to flood should the river overflow its banks except where
artificial means of protection have been adopted.
Consisting of alternations of mud or silt, clay, shell marl, peat,
sand and gravel, any or all of which may be present, the deposits
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 platforms 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 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.