THE QUATERNARY PERIOD.
THE POST-PLIOCENE PERIOD.
Later than any of the Tertiary formations are various detached
and more or less superficial accumulations, which are generally
spoken of as the _Post-Tertiary formations_, in accordance with
the nomenclature of Sir Charles Lyell--or as the _Quaternary
formations_, in accordance with the general usage of Continental
geologists. In all these formations we meet with no _Mollusca_
except such as are now alive--with the partial and very limited
exception of some of the oldest deposits of this period, in which
a few of the shells occasionally belong to species not known
to be in existence at the present day. Whilst the _Shell-fish_
of the Quaternary deposits are, generally speaking, identical
with existing forms, the _Mammals_ are sometimes referable to
living, sometimes to extinct species. In accordance with this,
the Quaternary formations are divided into two groups: (1) The
_Post-Pliocene_, in which the shells are almost invariably referable
to existing species, but some of the _Mammals are extinct_; and
(2) the _Recent_, in which _the shells and the Mammals alike
belong to existing species_. The Post-Pliocene deposits are often
spoken of as the Pleistocene formations (Gr. _pleistos_, most;
_kainos_, new or recent), in allusion to the fact that the great
majority of the living beings of this period belong to the species
characteristic of the "new" or Recent period.
The _Recent_ deposits, though of the highest possible interest,
do not properly concern the palæontologist strictly so-called, but
the zoologist, since they contain the remains of none but existing
animals. They are "Pre-historic," but they belong entirely to
the existing terrestrial order. The _Post-Pliocene_ deposits, on
the other hand, contain the remains of various extinct Mammals;
and though Man undoubtedly existed in, at any rate, the later
portion of this period, if not throughout the whole of it, they
properly form part of the domain of the palæontologist.
The Post-Pliocene deposits are extremely varied, and very widely
distributed; and owing to the mode of their occurrence, the ordinary
geological tests of age are in their case but very partially
available. The subject of the classification of these deposits
is therefore an extremely complicated one; and as regards the age
of even some of the most important of them, there still exists
considerable difference of opinion. For our present purpose, it
will be convenient to adopt a classification of the Post-Pliocene
deposits founded on the relations which they bear in time to the
great "Ice-age" or "Glacial period;" though it is not pretended
that our present knowledge is sufficient to render such a
classification more than a provisional one.
In the early Tertiary period, as we have seen, the climate of the
northern hemisphere, as shown by the Eocene animals and plants,
was very much hotter than it is at present--partaking, indeed, of a
sub-tropical character. In the Middle Tertiary or Miocene period,
the temperature, though not so high, was still much warmer than
that now enjoyed by the northern hemisphere; and we know that the
plants of temperate regions at this time flourished within the
Arctic circle. In the later Tertiary or Pliocene period, again,
there is evidence that the northern hemisphere underwent a further
progressive diminution of temperature; though the climate of Europe
generally seems at the close of the Tertiary period to have been
if anything warmer, or at any rate not colder, than it is at
the present day. With the commencement of the Quaternary period,
however, this diminution of temperature became more decided; and
beginning with a temperate climate, we find the greater portion
of the northern hemisphere to become gradually subjected to all
the rigours of intense Arctic cold. All the mountainous regions
of Northern and Central Europe, of Britain, and of North America,
became the nurseries of huge ice-streams, and large areas of the
land appear to have been covered with a continuous ice-sheet.
The Arctic conditions of this, the well-known "Glacial period,"
relaxed more than once, and were more than once re-established
with lesser intensity. Finally, a gradual but steadily progressive
amelioration of temperature took place; the ice slowly gave way,
and ultimately disappeared altogether; and the climate once more
became temperate, except in high northern latitudes.
The changes of temperature sketched out above took place slowly
and gradually, and occupied the whole of the Post-Pliocene period.
In each of the three periods marked out by these changes--in
the early temperate, the central cold, and the later temperate
period--certain deposits were laid down over the surface of the
northern hemisphere; and these deposits collectively constitute the
Post-Pliocene formations. Hence we may conveniently classify all
the accumulations of this age under the heads of (1) _Pre-Glacial_
deposits, (2) _Glacial_ deposits, and (3) _Post-Glacial_ deposits,
according as they were formed before, during, or after the "Glacial
period." It cannot by any means be asserted that we can definitely
fix the precise relations in time of all the Post-Pliocene deposits
to the Glacial period. On the contrary, there are some which
hold a very disputed position as regards this point; and there
are others which do not admit of definite allocation in this
manner at all, in consequence of their occurrence in regions
where no "Glacial Period" is known to have been established.
For our present purpose, however, dealing as we shall have to do
principally with the northern hemisphere, the above classification,
with all its defects, has greater advantages than any other that
has been yet proposed.
I. PRE-GLACIAL DEPOSITS.--The chief pre-glacial deposit of Britain
is found on the Norfolk coast, reposing upon the Newer Pliocene
(Norwich Crag), and consists of an ancient land-surface which
is known as the "Cromer Forest-bed."
This consists of an ancient soil, having embedded in it the stumps
of many trees, still in an erect position, with remains of living
plants, and the bones of recent and extinct quadrupeds. It is
overlaid by fresh-water and marine beds, all the shells of which
belong to existing species, and it is finally surmounted by true
"glacial drift." While all the shells and plants of the Cromer
Forest-bed and its associated strata belong to existing species,
the Mammals are partly living, partly extinct. Thus we find the
existing Wolf (_Canis lupus_), Red Deer (_Cervus elaphus_), Roebuck
(_Cervus capreolus_), Mole (_Talpa Europtoea_), and Beaver (_Castor
fiber_), living in western England side by side with the
_Hippopotamus major, Elephas antiquus, Elephas meridionalis,
Rhinoceros Etruscus_, and _R. Megarhinus_ of the Pliocene period,
which are not only extinct, but imply an at any rate moderately
warm climate. Besides the above, the Forest-bed has yielded the
remains of several extinct species of Deer, of the great extinct
Beaver (_Trogontherium Cuvieri_), of the Caledonian Bull or "Urus"
(_Bos primigenius_), and of a Horse (_Equus fossilis_), little
if at all distinguishable from the existing form.
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." They vary much in their exact nature in different
districts, but they universally consist of one, or all, of the
following members:--
1. _Unstratified_ clays, or loams, containing numerous angular
or sub-angular blocks of stone, which have often been transported
for a greater or less distance from their parent rock, and which
often exhibit polished, grooved, or striated surfaces. These
beds are what is called _Boulder-clay_, or _Till_.
2. Sands, gravels, and clays, often more or less regularly
_stratified_, but containing erratic blocks, often of large size,
and with their edges _unworn_, derived from considerable distances
from the place where they are now found. In these beds it is
not at all uncommon to find fossil shells; and these, though of
existing species, are generally of an Arctic character, comprising
a greater or less number of forms which are now exclusively found
in the icy waters of the Arctic seas. These beds are often spoken
of as "Stratified Drift."
3. _Stratified_ sands and gravels, in which the pebbles are _worn_
and rounded, and which have been produced by a rearrangement of
ordinary glacial beds by the sea. These beds are commonly known
as "Drift-gravels," or "Regenerated Drift".
Some of the last-mentioned of these are doubtless post-glacial;
but, in the absence of fossils, it is often impossible to arrive at
a positive opinion as to the precise age of superficial accumulations
of this nature. It is also the opinion of high authorities that a
considerable number of the so-called "cave-deposits," with the
bones of extinct Mammals, truly belong to the Glacial period,
being formed during warm intervals when the severity of the Arctic
cold had become relaxed. It is further believed that some, at
any rate, of the so-called "high-level" river-gravels and
"brick-earths" have likewise been deposited during mild or warm
intervals in the great age of ice; and in two or three instances
this has apparently been demonstrated--deposits of this nature,
with the bones of extinct animals and the implements of man,
having been shown to be overlaid by true Boulder-clay.
The fossils of the undoubted Glacial deposits are principally
shells, which are found in great numbers in certain localities,
sometimes with _Foraminifera_, the bivalved cases of Ostracode
Crustaceans, &c. Whilst some of the shells of the "Drift" are such
as now live in the seas of temperate regions, others, as previously
remarked, are such as are now only known to live in the seas of
high latitudes; and these therefore afford unquestionable evidence
of cold conditions. Amongst these Arctic forms of shells which
characterise the Glacial beds may be mentioned _Pecten Islandicus_
(fig. 254), _Pecten Groenlandicus, Scalaria Groenlandica, Leda
truncata, Astarte borealis, Tellina proxima, Nattra clausa_,
&c.
[Illustration: Fig. 254.--Left valve of _Pecten Islandicus_, Glacial
and Recent.]
III. POST-GLACIAL DEPOSITS.--As the intense cold of the Glacial
period became gradually mitigated, and temperate conditions of
climate were once more re-established, various deposits were
formed in the northern hemisphere, which are found to contain
the remains of extinct Mammals, and which, therefore, are clearly
of Post-Pliocene age. To these deposits the general name of
_Post-Glacial_ formations is given; but it is obvious that, from
the nature of the case, and with our present limited knowledge,
we cannot draw a rigid line of demarcation between the deposits
formed towards the close of the Glacial period, or during warm
"interglacial" periods, and those laid down after the ice had
fairly disappeared. Indeed it is extremely improbable that any
such rigid line of demarcation should ever have existed; and it
is far more likely that the Glacial and Post-Glacial periods,
and their corresponding deposits, shade into one another by an
imperceptible gradation. Accepting this reservation, we may group
together, under the general head of "Post-Glacial Deposits,"
most of the so-called "Valley-gravels," "Brick-earths," and
"Cave-deposits," together with some "raised beaches" and various
deposits of peat. Though not strictly within the compass of this
work, a few words may be said here as to the origin and mode of
formation of the Brick-earths, Valley-gravels, and Cave-deposits,
as the subject will thus be rendered more clearly intelligible.
Every river produces at the present day beds of fine mud and
loam, and accumulations of gravel, which it deposits at various
parts of its course--the gravel generally occupying the lowest
position, and the finer sands and mud coming above. Numerous
deposits of a similar nature are found in most countries in various
localities, and at various heights above the present channels of
our rivers. Many of these fluviatile (Lat. _fluvius_, a river)
deposits consist of fine loam, worked for brick-making, and known
as "Brick-earths;" and they have yielded the remains of numerous
extinct Mammals, of which the Mammoth (_Elephas primigenius_) is
the most abundant. In the valley of the Rhine these fluviatile
loams (known as "Loess") attain a thickness of several hundred
feet, and contain land and fresh-water shells of existing species.
With these occur the remains of Mammals, such as the Mammoth and
Woolly Rhinoceros. Many of these Brick-earths are undoubtedly
Post-Glacial, but others seem to be clearly "inter-glacial;" and
instances have recently been brought forward in which deposits
of Brick-earth containing bones and shells of fresh-water Molluscs
have been found to be overlaid by regular unstratified boulder-clay.
The so-called "Valley-gravels," like the Brick-earths, are fluviatile
deposits, but are of a coarser nature, consisting of sands and
gravels. Every river gives origin to deposits of this kind at
different points along the course of its valley; and it is not
uncommon to find that there exist in the valley of a single river
two or more sets of these gravel-beds, formed by the river itself,
but formed at times when the river ran at different levels, and
therefore formed at different periods. These different accumulations
are known as the "high-level" and "low-level" gravels; and a
reference to the accompanying diagram will explain the origin
and nature of these deposits (fig. 255). When a river begins
to occupy a particular line of drainage, and to form its own
channel, it will deposit fluviatile sands and gravels along its
sides. As it goes on deepening the bed or valley through which
it flows, it will deposit other fluviatile strata at a lower
level beside its new bed. In this way have arisen the terms
"high-level" and "low-level" gravels. We find, for instance, a
modern river flowing through a valley which it has to a great
extent or entirely formed itself; by the side of its immediate
channel we may find gravels, sand, and loam (fig. 255, 2 2')
deposited by the river flowing in its present bed. These are
_recent_ fluviatile or alluvial deposits. At some distance from
the present bed of the river, and at a higher level, we may find
other sands and gravels, quite like the recent ones in character
and origin, but formed at a time when the stream flowed at a higher
level, and before it had excavated its valley to its present
depth. These (fig. 255, 3 3') are the so-called "_low-level_
gravels" of a river. At a still higher level, and still farther
removed from the present bed of the river, we may find another
terrace, composed of just the same materials as the lower one,
but formed at a still earlier period, when the excavation of
the valley had proceeded to a much less extent. These (fig. 255,
4 4') are the so-called "_high-level_ gravels" of a river, and
there may be one or more terraces of these.
[Illustration: Fig. 255.--Recent and Post-Pliocene Alluvial Deposits.
1, Peat of the recent period; 2, Gravel of the modern river:
2', Loam of the modern river; 3. Lower-level valley-gravel with
bones of extinct Mammals (Post-Pliocene); 3', Loam of the same
age as 3; 4. Higher-level valley-gravel (Post-Pliocene); 4',
Loam of the same age as 4; 5. Upland gravels of various kinds
(often glacial drift); 6, Older rock. (After Sir Charles Lyell.)]
The important fact to remember about these fluviatile deposits
is this--that here the ordinary geological rule is reversed. The
high-level gravels are, of course, the highest, so far as their
actual elevation above the sea is concerned; but geologically the
lowest, since they are obviously much older than the low-level
gravels, as these are than the recent gravels. How much older
the high-level gravels may be than the low-level ones, it is
impossible to say. They occur at heights varying from 10 to 100
feet above the present river-channels, and they are therefore
older than the recent gravels by the time required by the river
to dig out its own bed to this depth. How long this period may
be, our data do not enable us to determine accurately; but if
we are to calculate from the observed rate of erosion of the
actually existing rivers, the period between the different
valley-gravels must be a very long one.
The lowest or recent fluviatile deposits which occur beside the
bed of the present river, are referable to the Recent period,
as they contain the remains of none but living Mammals. The two
other sets of gravels are Post-Pliocene, as they contain the
bones of extinct Mammals, mixed with land and fresh-water shells
of existing species. Among the more important extinct Mammals
of the low-level and high-level valley-gravels may be mentioned
the _Elephas antiquus_, the Mammoth (_Elephas primigenius_),
the Woolly Rhinoceros (_R. Tichorhinus_), the Hippopotamus, the
Cave-lion, and the Cave-bear. Along with these are found
unquestionable traces of the existence of Man, in the form of
rude flint implements of undoubted human workmanship.
The so-called "Cave-deposits," again, though exhibiting peculiarities
due to the fact of their occurrence in caverns or fissures in the
rocks, are in many respects essentially similar to the older
valley-gravels. Caves, in the great majority of instances, occur
in limestone. When this is not the case, it will generally be
found that they occur along lines of sea-coast, or along lines
which can be shown to have anciently formed the coast-line. There
are many caves, however, in the making of which it can be shown
that the sea has had no hand; and these are most of the caves
of limestone districts. These owe their origin to the solvent
action upon lime of water holding carbonic acid in solution.
The rain which falls upon a limestone district absorbs a certain
amount of carbonic acid from the air, or from the soil. It then
percolates through the rock, generally along the lines of jointing
so characteristic of limestones, and in its progress it dissolves
and carries off a certain quantity of carbonate of lime. In this
way, the natural joints and fissures in the rock are widened, as
can be seen at the present day in any or all limestone districts.
By a continuance of this action for a sufficient length of time,
caves may ultimately be produced. Nothing, also, is commoner
in a limestone district than for the natural drainage to take
the line of some fissure, dissolving the rock in its course. In
this way we constantly meet in limestone districts with springs
issuing from the limestone rock--sometimes as large rivers--the
waters of which are charged with carbonate of lime, obtained by
the solution of the sides of the fissure through which the waters
have flowed. By these and similar actions, every district in which
limestones are extensively developed will be found to exhibit
a number of natural caves, rents, or fissures. The first element,
therefore, in the production of cave-deposits, is the existence
of a period in which limestone rocks were largely dissolved, and
caves were formed in consequence of the then existing drainage
taking the line of some fissure.
Secondly, there must have been a period in which various deposits
were accumulated in the caves thus formed. These cavern-deposits
are of very various nature, consisting of mud, loam, gravel,
or breccias of different kinds. In all cases, these materials
have been introduced into the cave at some period subsequent to,
or contemporaneous with, the formation of the cave. Sometimes
the cave communicates with the surface by a fissure through which
sand, gravel, &c., may be washed by rains or by floods from some
neighbouring river. Sometimes the cave has been the bed of an
ancient stream, and the deposits have been formed as are fluviatile
deposits at the surface. Or, again, the river has formerly flowed
at a greater elevation than it does at present, and the cave
has been filled with fluviatile deposits by the river at a time
prior to the excavation of its bed to the present depth (fig.
256). In this last case, the cave-deposits obviously bear exactly
the same relation in point of antiquity to recent deposits, as
do the low-level and high-level valley-gravels to recent
river-gravels. In any case, it is necessary for the physical
geography of the district to change to some extent, in order
that the cave-deposits should be preserved. If the materials
have been introduced by a fissure, the cave will probably become
ultimately filled to the roof, and the aperture of admission
thus blocked up. If a river has flowed through the cave, the
surface configuration of the district must be altered so far
as to divert the river into a new channel. And if the cave is
placed in the side of a river-valley, as in fig. 256, the river
must have excavated its channel to such a depth that it can no
longer wash out the contents of the cave even in high floods.
[Illustration: Fig 256.--Diagrammatic section across a river-valley
and cave. _a a_, Recent valley-gravels near the channel (b) of
the existing river; c, Cavern, partly filled with cave-earth;
_d d_, High-level gravels, filling fissures in the limestone,
which perhaps communicate in some instances with the cave, and
form a channel by which materials of various kinds were introduced
into it; _e e_, Inclined beds of limestone.]
If the cave be entirely filled, the included deposits generally
get more or less completely cemented together by the percolation
through them of water holding carbonate of lime in solution. If
the cave is only partially filled, the dropping of water from
the roof holding lime in solution, and its subsequent evaporation,
would lead to the formation over the deposits below of a layer of
stalagmite, perhaps several inches, or even feet, in thickness.
In this way cave-deposits, with their contained remains, may
be hermetically sealed up and preserved without injury for an
altogether indefinite period of time.
In all caves in limestone in which deposits containing bones are
found, we have then evidence of three principal sets of changes.
(1.) A period during which the cave was slowly hollowed out by
the percolation of acidulated water; (2.) A period in which the
cave became the channel of an engulfed river, or otherwise came
to form part of the general drainage-system of the district; (3.)
A period in which the cave was inhabited by various animals.
As a typical example of a cave with fossiliferous Post-Pliocene
deposits, we may take Kent's Cavern, near Torquay, in which a
systematic and careful examination has revealed the following
sequence of accumulations in descending order:--
(a) Large blocks of limestone, which lie on the floor of the
cave, having fallen from the roof, and which are sometimes cemented
together by stalagmite.
(b) A layer of black mould, from three to twelve inches thick,
with human bones, fragments of pottery, stone and bronze implements,
and the bones of animals now living in Britain. This, therefore,
is a _recent_ deposit.
(c) A layer of stalagmite, from sixteen to twenty inches thick,
but sometimes as much as five feet, containing the bones of Man,
together with those of extinct Post-Pliocene Mammals.
(d) A bed of red cave-earth, sometimes four feet in thickness,
with numerous bones of extinct Mammals (Mammoth, Cave-bear, &c.),
together with human implements of flint and horn.
(e) A second bed of stalagmite, in places twelve feet in thickness,
with bones of the Cave-bear.
(f) A red-loam and cave-breccia, with remains of the Cave-bear
and human implements.
The most important Mammals which are found in cave-deposits in
Europe generally, are the Cave-bear, the Cave-lion, the Cave-hyæna,
the Reindeer, the Musk-ox, the Glutton, and the Lemming--of which
the first three are probably identical with existing forms, and
the remainder are certainly so--together with the Mammoth and
the Woolly Rhinoceros, which are undoubtedly extinct. Along with
these are found the implements, and in some cases the bones, of
Man himself, in such a manner as to render it absolutely certain
that an early race of men was truly contemporaneous in Western
Europe with the animals above mentioned.
IV. UNCLASSIFIED POST-PLIOCENE DEPOSITS.--Apart from any of the
afore mentioned deposits, there occur other accumulations--sometimes
superficial, sometimes in caves--which are found in regions where
a "Glacial period" has not been fully demonstrated, or where
such did not take place; and which, therefore, are not amenable
to the above classification. The most important of these are
known to occur in South America and Australia; and though their
numerous extinct Mammalia place their reference to the Post-Pliocene
period beyond doubt, their relations to the glacial period and
its deposits in the northern hemisphere have not been precisely
determined.