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the sequence may be entirely absent. Where maximum and minimum
depths are stated, this implies a true variation of the thickness
of the beds in question and, with the exception of the superficial
deposits, is not to be ascribed to thinning due to erosion.
Superficial deposits in the Borough are distributed over the
surface of the "solid formations" and may be considered as being
of (a) "made" ground, (b) alluvium, and (c) gravel, the latter
deposit being of the Pleistocene or "drift" series. At some places
these deposits fill deep hollows in the surface of the "solid" attaining
a maximum thickness of about 70 feet and at others are
arranged as banks. More usually they form spreads of comparatively
uniform thickness, or remain caps to the higher ground.
Situated below these are the "solid" Eocene formations of Claygate,
London, Blackheath and Woolwich and Reading beds followed
by that of Thanet Sand. Underlying the whole Eocene strata
is the Chalk or Cretaceous formation.
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 exceptiontally 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 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.