For not only is the clay deeper as you travel eastward, but--and
this is a matter to which geologists attach great importance--the
character of the shells differs in different parts of the clay.
You must know that certain sorts of shells live in deep water, and
certain in shallow. You may prove this to yourselves, on a small
scale, whenever you go to the seaside. You will find that the shell
which crawl on the rocks about high-water mark are different from
those which you find at low-tide mark; and those again different
from the shells which are brought up by the oyster-dredgers from the
sea outside. Now, the lower part of the clay, near here, contains
shallow-water shells: but if you went forty miles to the eastward,
you would find in the corresponding lower beds of the clay, deep-
water shells, and far above them, shallow-water shells such as you
find here: a fact which shows plainly that this end of the clay sea
was shallowest, and therefore first filled up.
But again--and this is a very curious fact--between the time of the
Plastic clays and sands, with their oyster-beds and black pebbles,
and that of the London clay, great changes had taken place. The
Plastic clay and sands were deposited during a period of earthquake,
of upheaval and subsidence of ancient lands; and therefore of
violent currents and flood waves, seemingly rushing down from, or
round the shores of that Wealden island to the south of us, on the
shore of which island Odiham once stood.
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