for central France. More than twenty years ago Dr. Julien brought
forward evidence to show that the region of the Puy de Dôme had
witnessed two glacial epochs.[CO] During the first of these epochs a
large glacier flowed from Mont Dore. After its retreat a prolonged
interglacial epoch followed, during which the old morainic deposits
and the rocks they rest upon were much eroded. In the valleys and
hollows thus excavated freshwater beds occur which have yielded
relics of an abundant flora, together with the remains of _Elephas
meridionalis_, _Rhinoceros leptorhinus_, etc. After the deposition of
these freshwater alluvia, glaciers again descended the valleys and
covered the interglacial beds with their moraines. Similar results have
been obtained by M. Rames from a study of the glacial phenomenon of
Cantal, which he shows belong to two separate epochs.[CP] The interval
between the formation of the two series of glacial accumulations must
have been prolonged, for the valleys during that interval were in some
places eroded to a depth of 900 feet. M. Rames further recognises
that the second glacial epoch was distinguished by two advances of
valley-glaciers, separated by a marked episode of fusion. Dr. Julien
has likewise noted the evidence for two episodes of fusion during the
first extension of the glaciers of the Puy de Dôme.
[CO] _Des Phénomènes glaciaires dans le Plateau central de la France_,
&c.; Paris, 1869.
[CP] Bull. _Soc. géol. de France_, 1884; see also M. Boule, _Bull. de
la Soc. philomath. de Paris_, 8^e sér. i., p. 87.
Two glacial epochs have similarly been admitted for the Pyrenees;[CQ]
but Dr. Penck some years ago brought forward evidence to show that
these mountains, like the Alps, have experienced three separate and
distinct periods of glaciation.[CR]
[CQ] Garrigou: _Bull. Soc. géol. de France_, 2^e sér. xxiv., p. 577.
Jeanbernat: _Bull. de Soc. d'Hist. nat. de Toulouse_, iv., pp. 114,
138. Piette: _Bull. Soc. géol. de France_, 3^e sér. ii., pp. 503, 507.
[CR] _Mitteilungen d. Vereins f. Erdkunde zu Leipzig_, 1883.
We may now return to Scotland, and consider briefly the changes that
followed upon the disappearance of the local ice-sheets and large
valley-glaciers of our mountain-regions. The evidence is fortunately
clear and complete. In the valley of the Tay, for example, at and below
Perth, we encounter the following succession of deposits:--
6. Recent alluvia.
5. Carse-deposits, 45 feet above sea-level.
4. Peat and forest bed.
3. Old alluvia.
2. Clays, etc., of 100-feet beach.
1. Boulder-clay.
The old alluvia (3) are obviously of fluviatile origin, and show us
that after the deposition of the clays, etc., of the 100-feet beach
the sea retreated, and allowed the Tay and its tributaries to plough
their way down through the marine and estuarine deposits of the
"third" glacial epoch. These deposits would appear to have extended
at first as a broad and approximately level plain over all the lower
reaches of the valleys. Through this plain the Tay and the Earn cut
their way to a depth of more than 100 feet, and gradually removed all
the material over a course which can hardly be less than 2 miles in
breadth below the Bridge of Earn, and considerably exceeds that in the
Carse of Gowrie. No organic remains occur in the "old alluvia," but
the deposits consist principally of gravel and sand, and show not a
trace of ice-action. Immediately overlying them comes the well-known
peat-bed (4). This is a mass of vegetable matter, varying in thickness
from a few inches up to 3 or 4 feet. In some places it seems to be
made up chiefly of reed-like plants and sedges and occasional mosses,
commingled with which are abundant fragments of birch, alder, willow,
hazel, and pine. In other places it contains trunks and stools of oak
and hazel, with hazel-nuts--the trees being rooted in the subjacent
deposits. It is generally highly compressed and readily splits into
laminæ, upon the surface of which many small reeds, and now and again
wing-cases of beetles, may be detected. A large proportion of the woody
débris--twigs, branches, and trunks--appears to have been drifted. A
"dug-out" canoe of pine was found, along with trunks of the same tree,
in the peat at Perth. The Carse-deposits (5), consisting principally of
clay and silt, rest upon the peat-bed. The occurrence in these deposits
of _Scrobicularia piperata_ and oyster-shells leaves us in no doubt as
to their marine origin. They vary in thickness from 10 up to fully 40
feet.[CS]
[CS] For a particular account of the Tay-valley Succession, see
_Prehistoric Europe_, p. 385.
A similar succession of deposits is met with in the valley of the
Forth,[CT] and we cannot doubt that these tell precisely the same tale.
I have elsewhere[CU] adduced evidence to show that the peat-bed, with
drifted vegetable débris, which underlies the Carse accumulations of
the Forth and Tay is on the same horizon as the "lower buried forest"
of our oldest peat-bogs, and the similar bogs that occur in Norway,
Sweden, Denmark, Schleswig-Holstein, Holland, etc. Underneath the
"lower buried forest" of those regions occur now and again freshwater
clays, charged with the relics of an arctic-alpine flora; and quite
recently similar plant remains have been detected in old alluvia at
Corstorphine, near Edinburgh. When the beds below our older peat-bogs
are more carefully examined, traces of that old arctic flora will
doubtless be met with in many other parts of these islands. It was
this flora that clothed north-western Europe during the decay of the
last district ice-sheets of Britain and the disappearance of the great
Baltic glacier.
[CT] _Proc. Roy. Soc. Edin._ 1883-84, p. 745; _Mem. Geol. Survey,
Scotland_, Explanation of Sheet 31.
[CU] _Prehistoric Europe_, chaps. xvi., xvii.
The dissolution of the large valley-glaciers of this country was
accompanied by a general retreat of the sea--all the evidence leading
to the conviction that our islands eventually became united to the
Continent. The climatic conditions, as evidenced by the flora of the
"lower buried forest," were decidedly temperate--probably even more
genial than they are now, for the forests attained at that time a much
greater horizontal and vertical range. This epoch of mild climate and
continental connection was eventually succeeded by one of submergence,
accompanied by colder conditions. Britain was again insulated--the
sea-level in Scotland reaching a height of 45-50 feet above present
high-water. To this epoch pertain the Carse-clays of the Forth and Tay.
A few erratics occur in these deposits, probably betokening the action
of floating ice, but the beds more closely resemble the modern alluvial
silts of our estuaries than the tenacious clays of the 100-feet
terrace. When the Carse-clays are followed inland, however, they pass
into coarse river-gravel and shingle, forming a well-marked high-level
alluvial terrace of much the same character as the yet higher-level
fluviatile terrace which is associated in like manner with the marine
deposits of the 100-feet beach.
Of contemporaneous age with the Carse-clays, with which indeed they are
continuous, are the raised beaches at 45-50 feet. These beaches occur
at many places along the Scottish coasts, but they are seldom seen at
the heads of our sea-lochs. When the sea stood at this level, glaciers
of considerable size occupied many of our mountain-valleys. In the west
they came down in places to the sea-coast, and dropped their terminal
moraines upon the beach-deposits accumulating there. Thus, in Arran[CV]
and in Sutherland,[CW] these moraines are seen reposing on the raised
beaches of that epoch. And I think it is probable that the absence of
such beaches at the heads of many of the sea-lochs of the Highland
area is to be explained by the presence there of large glaciers, which
prevented their formation.
[CV] _British Association Reports_ (1854): Trans. of Sections, p. 78.
[CW] L. Hinxman: Paper read before Edin. Geol. Soc., April 1892.
Thus, there is clear evidence to show that after the genial epoch
represented by the "lower buried forest," a recrudescence of glacial
conditions supervened in Scotland. Many of the small moraines that
occur at the heads of our mountain-valleys, both in the Highlands and
Southern Uplands, belong in all probability to this epoch. They are
characterised by their very fresh and well-preserved appearance.[CX] It
is not at all likely that these later climatic changes could have been
confined to Scotland. Other regions must have been similarly affected.
But the evidence will probably be harder to read than it is with
us. Had it not been for the existence of our "lower buried forest,"
with the overlying Carse-deposits, we could hardly have been able to
distinguish so readily between the moraines of our "third" glacial
epoch and those of the later epoch to which I now refer. The latter, we
might have supposed, simply marked a stage in the final retreat of the
antecedent great valley-glaciers.
[CX] _Prehistoric Europe_ (chaps. xvi., xvii.) gives a fuller statement
of the evidence.
I have elsewhere traced the history of the succeeding stages of
the Pleistocene period, and adduced evidence of similar, but less
strongly-marked, climatic changes having followed upon those just
referred to, and my conclusions have been supported by the independent
researches of Professor Blytt in Norway. But these later changes need
not be considered here. It is sufficient for my general purpose to
confine attention to the well-proved conclusion that after the decay
of the last district ice-sheets and great glaciers of our "third"
glacial epoch genial conditions obtained, and that these were followed
by cold and humid conditions, during the prevalence of which glaciers
reappeared in many mountain-valleys.
We have thus, as it seems to me, clear evidence in Europe of four
glacial epochs, separated the one from the other by protracted
intervals of genial temperate conditions. So far, one's conclusions
are based on data which cannot be gainsaid, but there are certain
considerations which lead to the suspicion that the whole of the
complex tale has not yet been unravelled, and that the climatic
changes were even more numerous than those that I have indicated. Let
it be noted that glacial conditions attained their maximum during
the earliest of our recognised glacial epochs. With each recurring
cold period the ice-sheets and glaciers successively diminished in
importance. That is one of the outstanding facts with which we have
to deal. Whatever may have been the cause or causes of glacial and
interglacial conditions, it is obvious that those causes, after
attaining a maximum influence, gradually became less effective in their
operation. Such having been the case, one can hardly help suspecting
that our epoch of greatest glaciation may have been preceded by an
alternation of cold and genial stages analogous to those that followed
it. If three cold epochs of progressively diminished severity succeeded
the epoch of maximum glaciation, the latter may have been preceded by
one or more epochs of progressively increased severity. That something
of the kind may have taken place is suggested by the occurrence
of the old moraine of that great Baltic glacier that preceded the
appearance of the most extensive _mer de glace_ of northern Europe.
The old moraine in question, it will be remembered, underlies the
lower diluvium. Unfortunately, the very conditions that attended the
glaciation of Europe render it improbable that any conspicuous traces
of glacial epochs that may have occurred prior to the period of maximum
glaciation could have been preserved within the regions covered by the
great inland-ice. Their absence, therefore, cannot be held as proving
that the lower boulder-clays of Britain and northern Europe are the
representatives of the earliest glacial epoch. The lowest boulder-clay,
I believe, has yet to be discovered.
It is in the Alpine Lands that we encounter the most striking evidence
of glacial conditions anterior to the epoch of maximum glaciation.
The famous breccia of Hötting has already been referred to as of
interglacial age. From the character of its flora, Ettinghausen
considered this accumulation to be of Tertiary age. The assemblage of
plants is certainly not comparable to the well-known interglacial flora
of Dürnten. According to the researches of Dr. R. von Wettstein,[CY]
the Hötting flora has most affinity with that of the Pontic Mountains,
the Caucasus, and southern Spain, and implies a considerably warmer
climate than is now experienced in the Inn Valley. This remarkable
deposit, as Dr. Penck pointed out some ten years ago, is clearly of
interglacial age. His conclusions were at once challenged, on the
ground that the flora had a Tertiary and not a Pleistocene facies;
consequently, it was urged that, as all glacial deposits were of
Pleistocene age, this particular breccia could not be interglacial.
But in this, as in similar cases, the palæontologist's contention
has not been sustained by the stratigraphical evidence, and Dr.
Penck's observations have been confirmed by several highly-competent
geologists, as by MM. Böhm and Du Pasquier. The breccia is seen in
several well-exposed sections resting upon the moraine of a local
glacier which formerly descended the northern flanks of the Inn
Valley, opposite Innsbruck, where the mountain-slopes under existing
conditions are free from snow and ice. Nor is this all, for certain
erratics appear in the breccia, which could only have been derived
from pre-existing glacial accumulations, and their occurrence in this
accumulation at a height of 1150 metres shows that before the advent of
the Hötting flora the whole Inn Valley must have been filled with ice.
The plant-bearing beds are in their turn covered by the ground-moraine
of a later and more extensive glaciation. To bring about the glacial
conditions that obtained before the formation of the breccia, the
snow-line, according to Penck, must have been at least 1000 metres
lower than now; while, to induce the succeeding glaciation, the
depression of the snow-line could not have been less than 1200 metres.
These observations have been extended to many other parts of the Alps,
and the conclusion arrived at by Professor Penck and his colleagues,
Professor Brückner and Dr. Böhm, is briefly this--that the maximum
glaciation of those regions did not fall in the "first" but in the
"second" Alpine glacial epoch.
[CY] _Sitzungsberichte d. Kais. Acad. d. Wissensch. in Wien,
mathem.-naturw. Classe_, Bd. xcvii. Abth. i., 1888.
The glacial phenomena of northern and central Europe are so
similar--the climatic oscillations which appear to have taken place
had so much in common, and were on so grand a scale--that we cannot
doubt they were synchronous. We may feel sure, therefore, that the
epoch of maximum glaciation in the Alps was contemporaneous with the
similar epoch in the north. And if this be so, then in the oldest
ground-moraines of the Alps we have the records of an earlier glacial
epoch than that which is represented by the lower boulder-clays of
Britain and the corresponding latitudes of the Continent. In other
words, the Hötting flora belongs to an older stage of the Glacial
period than any of the acknowledged interglacial accumulations of
northern Europe. The character of the plants is in keeping with this
conclusion. The flora has evidently much less connection with the
present flora of the Alps than the interglacial floras of Britain
and northern Europe have with those that now occupy their place. The
Hötting flora, moreover, implies a considerably warmer climate than
now obtains in the Alpine regions, while that of our interglacial beds
indicates a temperate insular climate, apparently much like the present.
The high probability that oscillations of climate preceded the advent
of the so-called "first" _mer de glace_ of northern Europe must lead to
a re-examination of our Pliocene deposits, with a view to see whether
these yield conclusive evidence against such climatic changes having
obtained immediately before Pleistocene times. By drawing the line of
separation between the Pleistocene and the Pliocene at the base of our
glacial series, the two systems in Britain are strongly marked off
the one from the other. There is, in short, a distinct "break in the
succession." From the Cromer Forest-bed, with its abundant mammalian
fauna and temperate flora, we pass at once to the overlying arctic
freshwater bed and the superjacent boulder-clay that marks the epoch of
maximum glaciation.[CZ] Amongst the mammalian fauna of the Forest-bed
are elephants (_Elephas meridionalis_, _E. antiquus_), hippopotamus,
rhinoceros, (_R. etruscus_), horses, bison, boar, and many kinds of
deer, together with such carnivores as bears, _Machærodus_, spotted
hyæna, etc. The freshwater and estuarine beds which contain this
extensive fauna rest immediately upon marine deposits (Weybourn
Crag), the organic remains of which have a decidedly arctic facies.
Here, then, we have what at first sight would seem to be another
break in the succession. The Forest-bed, one might suppose, indicated
an interglacial epoch, separating two cold epochs. But Mr. Clement
Reid, who has worked out the geology of the Pliocene with admirable
skill,[DA] has another explanation of the phenomena. It has long been
known that the organic remains of the marine Pliocene of Britain
denote a progressive lowering of temperature. The lower member of the
system is crowded with southern forms, which indicate warm-temperate
conditions. But when we leave the Older and pass upwards into the
Newer Pliocene those southern forms progressively disappear, while at
the same time immigrants from the north increase in numbers, until
eventually, in the beds immediately underlying the Forest-bed, the
fauna presents a thoroughly arctic facies. During the formation of
the Older Pliocene with its southern fauna our area was considerably
submerged, so that the German Ocean had then a much wider communication
with the seas of lower latitudes. At the beginning of Newer Pliocene
times, however, the land emerged to some extent, and all connection
between the German Ocean and more southern seas was cut off. When at
last the "Forest-bed series" began to be accumulated, the southern half
of the North Sea basin had become dry land, and was traversed by the
Rhine in its course towards the north, the Forest-bed representing the
alluvial and estuarine deposits of that river.
[CZ] In some places, however, certain marine deposits (_Leda myalis_
bed) immediately overlie the Forest-bed.
[DA] _Mem. of Geol. Survey_, "Pliocene Deposits of Britain." _See
postea_, footnote, p. 317.
Mr. Reid, in referring to the progressive change indicated by the
Pliocene marine fauna, is inclined to agree with Professor Prestwich
that this was not altogether the result of a general climatic change.
He thinks the successive dying out of southern forms and the continuous
arrival of boreal species was principally due to the North Sea
remaining fully open to the north, while all connection with southern
seas was cut off. Under such conditions, he says, "there was a constant
supply of arctic species brought by every tide or storm, while at the
same time the southern forms had to hold their own without any aid
from without; and if one was exterminated it could not be replaced."
Doubtless the isolation of the North Sea must have hastened the
extermination of the southern forms, but the change could not have been
wholly due to such local causes. Similar, if less strongly-marked,
changes characterise the marine Pliocene of the Mediterranean area,
while the freshwater alluvia of France, etc., furnish evidence in the
same direction.
The Cromer Forest-bed overlies the Weybourn Crag, the marine fauna of
which has a distinctly Arctic facies. The two cannot, therefore, be
exactly contemporaneous: the marine equivalents of the Forest-bed are
not represented. But Mr. Reid points out that several arctic marine
shells of the Weybourn Crag occur also in the Forest-bed, while certain
southern freshwater and terrestrial shells common in the latter are
met with likewise in the former, commingled with the prevailing arctic
marine species. He thinks, therefore, that we may fairly conclude that
the two faunas occupied adjacent areas. One can hardly accept this
conclusion without reserve. It is difficult to believe that a temperate
flora and mammalian fauna like those of the Forest-bed clothed and
peopled eastern England when the adjacent sea was occupied by arctic
molluscs, etc. Surely the occurrence of a few forms, which are common
to the Forest-bed and the underlying Crag, does not necessarily prove
that the two faunas occupied adjacent districts. Mr. Reid, indeed,
admits that some of the marine shells in the Forest-bed series may have
been derived from the underlying Crag. Were the marine equivalents of
the Forest-bed forthcoming we might well expect them to contain many
Crag forms, but the facies of the fauna would most probably resemble
that of the existing North Sea fauna. Again, the appearance in the
Weybourn Crag of a few southern shells common to the Forest-bed does
not seem to prove more than that such shells were contemporaneous
somewhere with an arctic marine fauna. But it is quite possible that
they might have been carried for a long distance from the south;
and, even if they actually existed in the near neighbourhood of an
arctic marine fauna, we may easily attach too much importance to their
evidence.[DB] I cannot think, therefore, that Mr. Reid's conclusion is
entirely satisfactory. After all, the Cromer Forest-bed rests upon the
Weybourn Crag, and the evidence as it stands is explicable in another
way. It is quite possible, for example, that the Forest-bed really
indicates an epoch of genial or temperate conditions, preceded, as it
certainly was eventually succeeded, by colder conditions.
[DB] The inference that the Forest-bed occupies an interglacial
position is strengthened by the evidence of certain marine deposits
which immediately overlie it. These (known collectively as the _Leda
myalis_ bed) occur in irregular patches, which, from the character of
their organic remains, cannot all be precisely of the same age. In one
place, for example, they are abundantly charged with oysters, having
valves united, and with these are associated other species of molluscs
that still live in British seas. At another place no oysters occur, but
the beds yield two arctic shells, _Leda myalis_ and _Astarte borealis_,
and some other forms which have no special significance. Professor Otto
Torell pointed out to Mr. Reid that these separate deposits could not
be of the same age, for the oyster is sensitive to cold and does not
inhabit the seas where _Leda myalis_ and _Astarte borealis_ flourish.
From a consideration of this and other evidence Mr. Reid concludes that
it is possible that the deposits indicate a period of considerable
length, during which the depth of water varied and the climate changed.
Two additional facts may be noted: _Leda myalis_ does not occur in
any of the underlying Pliocene beds, while the oyster is not found in
the Weybourn and Chillesford Crag, though common lower down in the
Pliocene series. These facts seem to me to have a strong bearing on
the climatic conditions of the Forest-bed epoch. They show us that the
oyster flourished in the North Sea before the period of the Weybourn
Crag--that it did not live side by side with the arctic forms of that
period--and that it reappeared in our seas when favourable conditions
returned. When the climate again became cold an arctic fauna (including
a new-comer, _Leda myalis_) once more occupied the North Sea.
If it be objected that this would include as interglacial what has
hitherto been regarded by most as a Pliocene mammalian fauna,[DC] I
would reply that the interglacial age of that fauna has already been
proved in central France. The interglacial beds of Auvergne, with
_Elephas meridionalis_, rest upon and are covered by moraines,[DD] and
with these have been correlated the deposits of Saint-Prest. Again, in
northern Italy the lignites of Leffe and Pianico, which, as I showed a
number of years ago,[DE] occupy an interglacial position, have likewise
yielded _Elephas meridionalis_ and other associated mammalian forms.
[DC] _Elephas meridionalis_ is usually regarded as a type-form of the
Newer Pliocene, but long ago Dr. Fuchs pointed out that in Hungary
this species is of quaternary age: _Verhandl. d. k. k. geolog.
Reichsanstalt_, 1879, pp. 49, 270. It matters little whether we
relegate to the top of the Pliocene or to the base of the Pleistocene
the beds in which this species occurs. That it is met with upon an
interglacial horizon is certain; and if we are to make the Pleistocene
co-extensive with the glacial and interglacial series we shall be
compelled to include in that system some portion of the Newer Pliocene.