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THE ESSEX NATURALIST.
Re-emergence after the Lenham stage occurred during the
Pliocene, and continued with fluctuations of submergence into
the Early Holocene. The successive emergencies and sub-
mergencies of the land are a resultant of complex causes : (1)
general, or eustatic, changes in the level of the sea, as more
or less water is transferred to the land as ice, or as the floor
of the oceans may rise or fall ; (2) violent tectonic movements ;
(3) gentle isostatic adjustments of the crust due to increase
or reduction of load by sedimentation or denudation, or
variations in the thickness of ice ; (4) where minor local changes
are concerned, the shrinkage and slumping of deposits also
comes into play.
For myself, I think the emergence of the London Basin was
not exclusively eustatic, but may have been associated with the
sinking area about Lowestoft and Southwold (probably extending
into the North Sea), in which the shallow-water estuarine deposits
of the Norwich Crag were accumulated. That is to say, I
think that the emergence and the submergence were at least
in some part compensating, or isostatic. It is sufficient for my
purpose to leave it at "emergence" and "submergence"
without attempting to disentangle the intricacies of the problem.
In any case the Lenham sea-bed emerged as a plain gently
inclined to the North-East towards the Suffolk coast, which
remained below sea-level. We may therefore assume that
over the surface of this inclined plain the young Thames would
begin to find its way, and the record of its early history must
be inferred from a series of high-level river gravels. The highest
gravel, at 500 ft. O.D., is at Stanmore, but Wooldridge [1927]
has shown good reason for believing that this is a relic of marine
shingle of the Pliocene sea, and does not come within the
fluviatile group. The same is true of the gravel capping Shooters
Hill (424 ft. O.D.) and also of another outlier near Rochester
(400). From these outliers, supported by the slightly later
Pebble Gravel on the Laindon Hills (385), one may conclude
that the land level above the position of the Thames Estuary
was nearly 400 ft. and would not give a suitable gradient for the
river.
Static Deposits.—There are a variety of stratified deposits,
formed under water, namely, marine, lacustrine and fluviatile ;
there are also aeolian deposits such as wind-born loess, talus,
desert sands, etc., and the varied group of Glacial deposits.
Besides these there are many superficial accumulations formed
of constituents previously existing near the site, but which have
become re-arranged and lost their original characteristics.
This particularly happens under rigorous climatic conditions