Geologists are agreed in
assigning to this event the date of March, 709, when great inundations
occurred in the Bay of Avranches on the French coast; they are not
equally unanimous as to the cause, but science now rejects the theory of
a raising of the sea-level and that of a general subsidence of the
island. The most reasonable explanation appears to be that the
overpowering force of a tidal wave suddenly swept away barriers whose
resistance had been for ages surely though imperceptibly diminishing,
and that the districts thus left unprotected proved to be below the
sea-level--owing, as regards the forests, to gradual subsidence easily
explicable in the case of undrained, swampy soil; and, as regards the
rocks, to the fact that the newly exposed surface consisted of
accumulations of already disintegrated deposits.
It is unquestionable that before the inroad of the sea the inlet in the
south-west of the island known as Rocquaine Bay was enclosed by two
arms, the northern of which terminated in the point of Lihou; on which
still stand the ruins of an old priory, while the southern ended in the
Hanois rocks, on which a lighthouse has been erected.
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