The so-called "Bridlington Crag" of Yorkshire, and the "Chillesford
Beds" of Suffolk, are probably to be regarded as also belonging
to this period; though many of the shells which they contain
are of an Arctic character, and would indicate that they were
deposited in the commencement of the Glacial period itself. Owing,
however, to the fact that a few of the shells of these deposits
are not known to occur in a living condition, these, and some
other similar accumulations, are sometimes considered as referable
to the Pliocene period.
II. GLACIAL DEPOSITS.--Under this head is included a great series
of deposits which are widely spread over both Europe and America,
and which were formed at a time when the climate of these countries
was very much colder than it is at present, and approached more
or less closely to what we see at the present day in the Arctic
regions. These deposits are known by the general name of the
_Glacial deposits_, or by the more specialised names of the Drift,
the Northern Drift, the Boulder-clay, the Till, &c.
These glacial deposits are found in Britain as far south as the
Thames, over the whole of Northern Europe, in all the more elevated
portions of Southern and Central Europe, and over the whole of
North America, as far south as the 39th parallel. They generally
occur as sands, clays, and gravels, spread in widely-extended
sheets over all the geological formations alike, except the most
recent, and are commonly spoken of under the general term of
"Glacial drift.
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