THE OLIGOCENE AND MIOCENE PERIODS.
(i) _The Oligocene Beds._
_Classification._ The Oligocene Beds of Britain are classified as
follows:--
Upper Wanting
Middle Hempstead Beds
{ Bembridge Beds
Lower { Osborne Beds
{ Headon Beds
_Description of the strata._ Little need be said of the deposits of
this period, either in Britain or abroad, except to remark that they
show the further spread of continental conditions over the regions now
occupied by land. The British deposits are now seen in the Hampshire
Basin only, and have been spoken of as the fluvio-marine series, as
many of the strata were laid down in continental sheets of water,
while the true marine sediments are thin and infrequent.
The lithological characters of deposits formed under these conditions
naturally vary greatly, consisting of different kinds of mechanical
sediments occasionally mixed with thin freshwater marls and
limestones. On the Continent similar conditions prevailed, though the
occurrence of fairly wide tracts of level surface is indicated by the
widespread distribution of beds of brown coal or lignite, and the
coarse and thick Oligocene 'nagelfluh' of Switzerland points to the
elevation of mountain ranges in the neighbourhood.
_The flora and fauna._ The remarks made concerning the Eocene flora
and fauna are generally applicable to those of Oligocene times, except
that the Oligocene fossils bear a still closer resemblance to living
forms, and the Nummulites are no longer dominant.
(ii) _The Miocene Period._ Beds of Miocene age are wanting in Britain,
and on the Continent they occur in isolated basins deposited in
gulf-like prolongations of the ocean, never very far from land. A
description of the strata and their fossil contents would be of little
use for our present purposes, and the remarks made concerning the
Oligocene beds will apply to the Miocene strata also.
The period was mainly remarkable on account of the important physical
changes which occurred, to which we must devote some consideration.
Commencing with the British area, we find in the south evidence of the
separation of the London and Hampshire Basins at this time, for the
Oligocene beds of Hampshire are tilted up on the south side of an
anticline, which separates the Hampshire Basin from that of London,
indicating that the movement was post-Miocene, while in Kent, beds of
Pliocene age rest on the denuded top of the chalk, showing that the
elevation and denudation which accompanied it were pre-Pliocene; the
great Wealden anticline is thus seen to be of Miocene age. On the
north side of the London Basin the line of demarcation between Eocene
and Mesozoic beds runs approximately parallel to the strike of the
latter in that part of Britain, and this points to the elevation of
the Mesozoic strata which gave them their present south-easterly dip
about the same period, though in the absence of Oligocene rocks it
cannot be definitely stated that the movement was altogether
post-Oligocene. The present physical geography of considerable parts
of Britain must date from Miocene times.
Important as the changes were in Britain, they were slight as compared
with those which affected Europe and many parts of Asia. The great
mountain chains of the Old World received their maximum uplift during
this great period of earth-movement, and orogenic structures were
impressed upon the rocks of many regions, for the Tertiary Mountain
Chains of the Old World have an Alpine structure impressed upon them
as the result of intense lateral pressure, accordingly we find the
Eocene strata lifted far above their original level to heights of
8,000 feet in the Alps and over 12,000 feet in the Himalayas. Away
from these marked uplifts epeirogenic movements caused the
disappearance of the seas of earlier Eocene times, so that towards the
close of the Miocene Period, the main features of the Eurasian
continent were much as they are now. The present drainage-systems must
have originated at the same time, and the sculpture of our continent
has been carried on more or less continuously by subaerial agents from
Miocene times to the present day. That any addition to the total area
of land was made is doubtful. The land which appears to have existed
to the west of Britain during Cretaceous and Eocene times finally
disappeared beneath the waters of the Atlantic Ocean, and the movement
probably gave rise to the prominent submarine feature which now exists
at some distance from the coast of Ireland. A great marine period is
now existent in our ocean areas, but so far as the existing
continents are concerned, we are living on the fourth continental
period which practically came into existence in Miocene times.
The strike of the uplifted strata naturally coincides on the whole
with the axes of the major uplifts, and accordingly we find the
Mesozoic and early Tertiary strata folded around axes which have a
prevalent east and west direction, with others which have a trend at
right angles to this. The strike of the British Mesozoic rocks seems
to have been determined by each of these sets of movements, so that
although it is east and west in the south of England, it runs north
and south in the eastern counties north of the Thames.
In America, although epeirogenic movements had occurred before Miocene
times, with the formation of wide continental tracts, these appear to
have been of the nature of plains, diversified by extensive inland
sheets of water, and uplift of orogenic character converted these
plains into uneven tracts in Miocene times. Many of the movements in
America, which like those of Europe are still progressing with
enfeebled power, differ from those of Eurasia, giving rise to raised
monoclinal blocks rather than to violent folds of Alpine character, as
seen in the western territories of North America, and as proved also
by the differential movements which are now known to affect the
Atlantic coast of that continent.
Accompanying these changes in the earth's crust were others which
affected the climate, at any rate locally. The warm climate of Eocene
times gradually gave way to a cooler climate in Oligocene times, and
this lowering of temperature was still further advanced in Miocene
times, though there is evidence that the temperature of those parts of
Europe which have strata representative of the Miocene period was
higher than it is at the present day.
Owing to the changes which occurred in Miocene times, the area of
sedimentation was extensively shifted to our present oceans, and
accordingly we find that the times subsequent to those of the Miocene
uplifts are marked by scattered accumulations of continental
character, with a few insignificant marine strata seldom found far
inland from the present coast-lines.