THE PLIOCENE BEDS.
_Classification._ The Italian Pliocene Beds which have long been known
have been divided into three stages, to which names have been applied
which are somewhat widely used, though the division of the British
deposits into the same three stages has not been made. The stages
are:--
Astian.
Plaisancean.
Zanclean.
The classification of the British deposits may be made as follows:--
Cromer "Forest" Series.
Weybourne Crag and Bure Valley Beds.
Chillesford Crag.
Norwich Crag and Red Crag.
Upper Coralline Crag.
Lower Coralline Crag.
As the English deposits are somewhat scattered it is difficult to make
out the exact order of succession, but the above shows the
classification which is adopted by the best authorities, the Norwich
Crag (or Fluvio-marine Crag as it is sometimes termed) being now
supposed to represent the upper portion of the Red Crag.
_Description of the strata._ The British deposits are chiefly found in
the counties of Norfolk and Suffolk, but isolated patches have been
detected in Kent and at St Erth in Cornwall; while the inclusion of
Pliocene fossils in the glacial deposits of Aberdeenshire and on the
west coasts and islands of Great Britain suggests the occurrence of
Pliocene beds beneath sea-level, around the British coasts, at no
great distance from the land.
The term 'Crag' has been applied to shelly sands of which the British
Pliocene beds are largely composed. The oldest British Pliocene strata
are supposed to be the Lenham Beds, occurring in 'pipes' on the Chalk
of the North Downs, which are referred to the Lower Coralline Crag,
and some writers believe that the St Erth beds of Cornwall are of
similar age[106]. The former are ferruginous sands, and the latter
shelly sands and clays. The higher beds of the Coralline Crag are
found in Suffolk, and are largely calcareous, being made of remains of
polyzoa, molluscs, and other invertebrates. They were probably
deposited in deeper water than the rest of the British Pliocene
strata, and contain a far larger percentage of carbonate of lime. The
Red Crag consists of ferruginous shelly sands, of the nature of
sand-banks, formed near land; while the Norwich Crag is of a still
more littoral character, and contains remains of land shells and the
bones of mammalia mingled with the marine shells of the coast. The
higher Pliocene deposits are also coastal accumulations, even the
so-called Forest bed being a deposit and not a true surface soil, as
proved by the observations of Mr Clement Reid. At the summit of the
Cromer 'Forest' Series, however, is a true freshwater bed. These
British deposits appear to have been laid down on a coast line which
formed one side of the estuary of a large river, of which the present
Rhine is the 'betrunked' portion (to use a term introduced by Prof. W.
M. Davis)[107].
[Footnote 106: See Clement Reid, _Nature_, 1886, p. 342; and Kendall
and Bell, _Quart. Journ. Geol. Soc._, vol. XLII. p. 201.]
[Footnote 107: See a paper by Mr F. W. Harmer, "On the Pliocene
Deposits of Holland, and their relationship to the English and Belgian
Crags," _Quart. Journ. Geol. Soc._, vol. LII. p. 748.]
On the European continent, marine Pliocene beds are found in Belgium
and Italy. The former deposits greatly resemble our Crags, whilst the
latter are of interest on account of the mixture of volcanic beds with
marine sediments in Sicily, showing that the formation of Etna
commenced in Pliocene times. Various deposits formed in inland basins
are found in France and Germany, but the most remarkable occur in the
Vienna basin, where Caspian conditions prevailed over large areas, and
the ordinary strata alternate with chemical deposits of which the
best-known are the celebrated rock salt masses of Wieliczka, near
Cracow. At the same time volcanic activity was rife to the south of
the Carpathian mountains. Other deposits, which are partly referable
to the Pliocene period, occur in Greece at Pikermi, and in India in
the Siwalik hills; these are celebrated for their remarkable mammals,
as are the Pliocene strata of the Western territories of North
America. The occurrence of marked earth-movements since Pliocene times
is indicated by the nature of the deposits of Barbadoes, where
radiolarian cherts have furnished two echinids which are described by
Dr Gregory as deep-sea forms. These beds were once referred to the
Miocene period, but there is good reason for assigning them to a later
date, and correlating them with the Pliocene beds of other areas, in
which case there must have been a considerable uplift in this region
since Pliocene times, a fact of great theoretical importance.
The climatic conditions of Pliocene times show steady fall of
temperature. The early Pliocene beds of Britain were deposited during
the prevalence of warmer temperatures than those which now exist in
the same area, but during later Pliocene times, the temperature was at
first similar to that now prevailing, and afterwards distinctly
colder, and we find in the upper Pliocene beds the remains of
organisms of a northern type. In the uppermost deposit of the Cromer
'Forest' Series, the arctic birch and arctic willow indicate the
commencement of the cold which culminated in the succeeding 'Great Ice
Age.'
_The flora and fauna._ Little need be said of the Pliocene fossils:
the flora approaches that of present times, and the invertebrates are
in most cases specifically identical with those now living. The
vertebrates alone differ markedly from living forms, being chiefly of
extinct species, and in many cases belonging to extinct genera. It is
interesting to find that the mammalian fauna of Pliocene times
resembles the existing fauna of the area in which the beds are found,
a fact long ago observed by Darwin. Thus the European Pliocene mammals
are like existing European forms, whilst in Australia the mammalian
terrestrial fauna consists of Marsupials, and in South America there
are Edentata of Pliocene age[108].
[Footnote 108: The Pliocene fauna of Britain is described by Mr
Searles V. Wood in the _Monographs of the Palæontographical
Society_.]