As is well known, the chalk consists mainly of the shells or
outer frames of minute one-celled creatures (Thalamophores) which
float in the ocean, and form a deep ooze at its bottom with their
discarded skeletons. What depth this ocean must have been is
disputed, and hardly concerns us. It is clear that it must have
taken an enormous period for microscopic shells to form the thick
masses of chalk which cover so much of southern and eastern
England. On the lowest estimates the Cretaceous period, which
includes the deposit of other strata besides chalk, lasted about
three million years. And as people like to have some idea of the
time since these things happened, I may add that, on the lowest
estimate (which most geologists would at least double), it is
about three million years since the last stretches of the
chalk-ocean disappeared from the surface of Europe.
But while our chalk cliffs conjure up a vision of England lying
deep--at least twenty or thirty fathoms deep-- below a warm
ocean, in which gigantic Ammonites and Belemnites and sharks ply
their deadly trade, they also remind us of the last phase of the
remarkable life of the earth's Middle Ages.
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