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• 11
them beneath the sea bordering" the coast of the three latter
counties ; we are therefore to conceive the whole of this tract
to be situated in a vast hollow in the chalk, which is
geologically termed the chalk basin of London. The layer
of sand which passes above this chalk from Hungerford to
Dover is the very sand from which rises the water which
supplies the wells of Tottenham. The fact appears to be
that the water which falls on the sand where it is superficial
(from Hungerford to within some distance of Tottenham),
together with that which passes into it from the chalk,
percolates the stratum of sand underlying the clay ; hence
where an opening is made through the clay, the water rises
nearly or quite to the surface, on the principle of seeking its
level—the level of the sand at the foot of the chalk hills and
of the clay at Tottenham being nearly the same (see
Geological Map). The sand lies in a hollow in the chalk
and the clay in a hollow in the sand.
Robinson's " History of Tottenham."