phenomena because they so strongly corroborate the conclusions
arrived at a number of years ago by glacialists in our own country.
Many additional examples might be cited from other parts of Europe,
but those already given may serve to show that at least one epoch of
interglacial conditions supervened during the Pleistocene period.
Before leaving this part of my subject, however, I may point out the
significant circumstance that long before much was known of glaciation,
and certainly before the periodicity of ice-epochs had been recognised,
Collomb had detected in the Vosges conspicuous evidence of two
successive glaciations.[BN]
[BN] _Preuves de l'existence d'anciens glaciers dans les vallées des
Vosges_, 1847, p. 141.
Having shown that alike in the regions formerly occupied by the great
northern ice-sheet, and in the Alpine Lands of central and southern
Europe, alternations of cold and genial conditions characterised the
so-called Glacial period, we may now glance at the evidence supplied
by those Pleistocene deposits that lie outside of the glaciated
areas. Of these we have a typical example in the river-accumulations
of the Rhine Valley between Bâle and Bingen. Here and there these
deposits have yielded remains of extinct and no longer indigenous
mammals and relics of Palæolithic man--one of the most interesting
deposits from which mammalian remains have been obtained being the
Sands of Mosbach, between Wiesbaden and Mayence. The fauna in question
is characteristically Pleistocene, nor can it be doubted that the
Mosbach Sands belong to the same geological horizon as the similar
fluviatile deposits of the Seine, the Thames, and other river-valleys
in western Europe. Dr. Kinkelin has shown,[BO] and with him Dr.
Schumacher agrees,[BP] that the Mosbach deposits are of interglacial
age; while Dr. Pohlig has no hesitation in assigning them to the same
horizon.[BQ] It is true there are no glacial accumulations in the
region where they occur, but they rest upon a series of unfossiliferous
gravels which are recognised as the equivalents of the fluvio-glacial
and glacial deposits of the Vosges, the Black Forest, the Alps, etc.
These gravels are traced at intervals up to considerable heights above
the Rhine, and contain numerous erratics, some of which are several
feet in diameter, while a large proportion are not at all water-worn,
but roughly and sharply angular. The blocks have unquestionably been
transported by river-ice, and imply therefore cold climatic conditions.
The overlying Mosbach Sands have yielded not only _Elephas antiquus_
and _Hippopotamus major_, but the reindeer, the mammoth, and the
marmot--two strongly contrasted faunas, betokening climatic changes
similar to those that marked the accumulation of the river-deposits
of the Thames, the Seine, etc. Of younger date than the Mosbach Sands
is another series of unfossiliferous gravels, which, like the older
series, are charged with ice-floated erratics. The beds at Mosbach are
thus shown to be of interglacial age: they occupy the same geological
horizon as the interglacial beds of Switzerland and other glaciated
tracts in central and northern Europe.
[BO] Kinkelin: _Bericht über die Senckenberg. naturf. Ges. in Frankfurt
a. M._, 1889.
[BP] Schumacher: _Mittheilungen d. Commission für d. geolog.
Landes-Untersuch. v. Elsass-Lothringen_, Bd. ii., 1890, p. 184.
[BQ] _Zeitschr. d. deutsch. geolog. Ges._, 1887, p. 806.
To this position must likewise be assigned the Pleistocene
river-alluvia of other districts. There is no other horizon, indeed,
on which these can be placed. That they are not of post-glacial age
is shown by the fact that in many places the angular gravels and
flood-loams of the Glacial period overlie them. And that they cannot
all belong to pre-glacial times is proved by the frequent occurrence
underneath them of glacial or fluvio-glacial accumulations. It is
quite possible, of course, that here and there in the valleys of
western and southern Europe some of the Pleistocene alluvia may be of
pre-glacial age. But in the main these alluvia must be regarded as
the equivalents of the glacial and interglacial deposits of northern
and Alpine districts. This will appear a reasonable conclusion
when we bear in mind that long before the Pliocene period came to a
close the climate of Europe had begun to deteriorate. In England, as
we know, glacial conditions supervened almost at the advent of the
Pleistocene period. And the same was the case in the Alpine Lands of
the south. Again, in the glaciated areas of north and south alike, the
closing stage of the Pleistocene was characterised by cold climatic
conditions. And thus in those regions the glacial and interglacial
epochs were co-extensive with that period. It follows, therefore,
that the Pleistocene deposits of extra-glacial areas must be the
equivalents of the glacial and interglacial accumulations elsewhere.
If we refused to admit this we should be puzzled indeed to tell what
the rivers of western and southern Europe were doing throughout the
long-continued Glacial period. There is no escape from the conclusion
that the Pleistocene river-alluvia and cave-accumulations must be
assigned to the same general horizon as the glacial and interglacial
deposits. This is now admitted by Continental palæontologists who find
in the character of Pleistocene organic remains abundant proof that
the old river-alluvia and cave-accumulations were laid down under
changing climatic conditions. Did neither glacial nor interglacial
deposits exist, the relics of the Pleistocene flora and fauna met with
in extra-glacial regions would yet lead us to the conclusion that
after the close of the Pliocene period, extremely cold and very genial
climates alternated up to the dawn of the present. Thus during one
stage of the Pleistocene "clement winters and cool summers permitted
the wide diffusion and intimate association of plants which have
now a very different range. Temperate and southern species like the
ash, the poplar, the sycamore, the fig-tree, the judas-tree, etc.,
overspread all the low-grounds of France as far north at least as
Paris. It was under such conditions that the elephants, rhinoceroses,
hippopotamuses, and the vast herds of temperate cervine and bovine
species ranged over Europe, from the shores of the Mediterranean up to
the latitude of Yorkshire, and probably even further north still, and
from the borders of Asia to the western ocean. Despite the presence of
numerous fierce carnivora--lions, hyænas, tigers, and others--Europe
at that time, with its shady forests, its laurel-margined streams,
its broad and deep-flowing rivers--a country in every way suited
to the needs of a race of hunters and fishers--must have been no
unpleasant habitation for Palæolithic man." But during another stage
of the Pleistocene period, the climate of our continent presented the
strongest contrast to those genial conditions. At that time "the dwarf
birch of the Scottish Highlands, and the Arctic willow, with their
northern congeners, grew upon the low-grounds of middle Europe. Arctic
animals, such as the musk-sheep and the reindeer, lived then, all the
year round, in the south of France; the mammoth ranged into Spain and
Italy; the glutton descended to the shores of the Mediterranean; the
marmot came down to the low-grounds at the foot of the Apennines; and
the lagomys inhabitated the low-lying maritime districts of Corsica
and Sardinia. The land and freshwater molluscs of many Pleistocene
deposits tell a similar tale: high alpine, boreal, and hyperborean
forms are characteristic of those deposits in central Europe; even in
the southern regions of our continent the shells testify to a former
colder and wetter climate. It was during the climax of these conditions
that the caves of Aquitaine were occupied by those artistic men who
appear to have delighted in carving and engraving."[BR] Such, in brief,
is the testimony of the Pleistocene flora and fauna of extra-glacial
regions. It is from the deposits in these regions, therefore, that
we derive our fullest knowledge of the life of the period. But a
comparison of their organic remains with those that occur in the
glacial and interglacial deposits of alpine and northern lands shows
us that the Pleistocene accumulations of glacial and extra-glacial
countries are contemporaneous--for there is not a single life-form
obtained from interglacial beds which does not also occur in the
deposits of extra-glacial regions. The converse is not true--nor
is that to be wondered at, for interglacial deposits have only been
sparingly preserved. In regions liable to glaciation such superficial
accumulations must frequently have been ploughed up and incorporated
with ground-moraine. It was only in the extra-glacial tracts that
alluvia of interglacial age were at all likely to be preserved in
any abundance. To appreciate fully the climatic conditions of the
Pleistocene period, therefore, it is necessary to combine the evidence
derived from the glaciated areas with that obtained from the lands
that lay beyond the reach of the ice-plough. The one is the complement
of the other, and this being so, it is obvious that any attempted
explanation of the origin of the Glacial period which does not fully
realise the importance of the interglacial phase of that period cannot
be accepted.
[BR] _Prehistoric Europe_, p. 67.
But if the climatic changes of Pleistocene times are the most
important phenomena which the geologist who essays to trace the
history of that period is called upon to consider, he cannot ignore
the evidence of contemporaneous geographical mutations. These are so
generally admitted, however, that it is only necessary here to state
the well-known fact that everywhere throughout the maritime tracts of
the glaciated lands of Europe and North America frequent changes in
the relative level of land and sea took place during Pleistocene and
post-glacial times.
I must now very briefly review the evidence bearing on the climatic
conditions of post-glacial times. And first, let it be noted that the
closing stage of the Pleistocene period was one of cold conditions,
accompanied in north-western Europe by partial depression of the land
below its present level. This is shown by the late-glacial marine
deposits of central Scotland and the coast-lands of Scandinavia. The
historical records of the succeeding post-glacial period are furnished
chiefly by raised beaches, river- and lake-alluvia, calcareous tufas,
and peat-bogs. An examination of these has shown that the climate, at
first cold, gradually became less ungenial, so that the Arctic-alpine
flora and northern fauna were eventually supplanted in our latitude by
those temperate forms which, as a group, still occupy this region. The
amelioration of the climate was accompanied by striking geographical
changes, the British Islands becoming united with themselves and the
opposite coasts of the continent. The genial character of the climate
at this time is shown by the great development of forests, the remains
of which occur under our oldest peat-bogs. Not only did trees then grow
at greater altitudes in these regions than is at present the case,
but forests ranged much further north, and flourished in lands where
they cannot now exist. In Orkney and Shetland, in the far north of
Norway, and even in the Faröe Islands and in Iceland relics of this old
forest-epoch are met with. In connection with these facts reference may
be made to the evidence obtained from certain raised beaches on both
sides of the N. Atlantic, and from recent dredgings in the intervening
sea. The occurrence of isolated colonies of southern molluscs in our
northern seas, and the appearance in raised beaches of many forms which
are now confined to the waters of more southern latitudes, seem to show
that in early post-glacial times the seas of these northern latitudes
were warmer than now. And it is quite certain that the southern forms
referred to are not the relics of any pre-glacial or interglacial
immigration. They could only have entered our northern seas after the
close of the Glacial period, and their evidence taken in connection
with that furnished by the buried trees of our peat-bogs, leads to the
conclusion that a genial climate supervened after the cold of the last
glacial epoch and of earliest post-glacial times had passed away.
To this genial stage succeeded an epoch of cold humid conditions,
accompanied by geographical changes which resulted in the insulation
of Britain and Ireland--the sea encroaching to some extent on what are
now our maritime regions. The climate was less favourable to the growth
of forests, which began to decay and to become buried under widespread
accumulations of growing peat. At this time glaciers reappeared in the
glens of the Scottish Highlands, and here and there descended to the
sea. The evidence for these is quite conspicuous, for the moraines are
found resting on the surface of post-glacial beaches. Thus my friend
Mr. L. Hinxman, of the Geological Survey, tells us that at the foot
of Glen Thraill well-formed moraines are seen in section reposing on
beach-deposits at the distance of about three-quarters of a mile above
the head of Loch Torridon.[BS] The evidence of this recrudescence of
glacial conditions in post-glacial times is not confined to Scotland. I
believe it will yet be recognised in many other mountain-regions; but
already Prof. Penck has detected it in the valleys of the Pyrenees.[BT]
Dr. Kerner von Marilaun has also described similar phenomena in the
higher valleys of Tyrol, while Professor Brückner has obtained like
evidence in the Salzach region.[BU]
[BS] For Scottish post-glacial glaciers see J. Geikie: _Scottish
Naturalist_, Jan., 1880; _Prehistoric Europe_, pp. 386,407; Penck:
_Deutsche geographische Blätter_, Bd. vi., p. 323; _Verhand. d. Ges. f.
Erdkunde, Berlin_, 1884, Heft 1; Hinxman: _Trans. Edin. Geol. Soc._,
vol. vi., p. 249.
[BT] "Die Eiszeit in den Pyrenäen": _Mitth. d. Vereins. f. Erdkunde_,
Leipzig, 1883.
[BU] Kerner: _Mitth. k. k. geograph. Ges. Wien_, 1890, p. 307;
_Sitzungsb. d. kais. Akad. d. Wissensch. in Wien_, Bd. c., Abth. i.,
1891; Brückner: _X. Jahresbericht d. geograph. Ges. v. Bern_, 1891.
I have elsewhere traced the history of the succeeding stages of the
post-glacial period, and brought forward evidence of similar but less
strongly-marked climatic changes having followed upon those just
referred to, and my conclusions, I may add, have been supported by
the independent researches of Professor Blytt in Norway. But these
later changes need not be considered here, and I shall leave them out
of account in the discussion that follows. It is sufficient for my
present purpose to confine attention to the well-proved conclusion
that in early post-glacial times genial climatic conditions obtained,
and that these were followed by cold and humid conditions, during the
prevalence of which considerable local glaciers reappeared in certain
mountain-valleys.[BV]
[BV] For a full statement of the evidence see _Prehistoric Europe_,
chaps. xvi., xvii.
We speak of Pleistocene or Glacial and of Post-glacial periods as if
the one were more or less sharply marked off from the other. Of course,
that is not the case, and in point of fact it would be for many reasons
preferable to include them under some general term. Taken together they
form one tolerably well-defined cycle of time, characterised above all
by its remarkable climatic changes--by alternations of cold and genial
conditions, which were most strongly contrasted in the earlier stages
of the period. It is further worthy of note that various oscillations
of the sea-level appear to have taken place again and again both in the
earlier and later stages of the cycle.
We may now proceed to inquire whether the phenomena we have been
considering can be accounted for by movements of the earth's crust--a
view which has recently received considerable support, more especially
in America. I need hardly say that the view in question is no novelty.
Many years ago, while our knowledge of the Pleistocene phenomena was
somewhat rudimentary, it was usual to infer that glaciation had been
induced by elevation of the land. This did not seem an unreasonable
conclusion, for above our heads, at a less or greater elevation,
according to latitude, an Arctic climate prevails. One could not doubt,
therefore, that if a land-surface were only sufficiently uplifted
it would reach the snow-line, and become more or less extensively
glaciated. But with the increase of our knowledge of Pleistocene
and post-glacial conditions, such a ready interpretation failed to
satisfy, although not a few geologists have continued to defend
the "earth-movement hypothesis," as accounting fairly well for the
phenomena of the Glacial period. By these staunch believers in the
adequacy of that view, it has been pointed out that elevation might not
only lift lands into the region of eternal snow, but, by converting
large areas of the sea-bed into land, would greatly modify the
direction of ocean-currents, and thus influence the climate. What might
not be expected to happen were the Gulf Stream to be excluded from
northern regions? What would be the fate of the temperate latitudes of
North America and Europe were that genial ocean-river to be deflected
into the Pacific across a submerged Isthmus of Panama? The possibility
of such changes having supervened in Pleistocene times has often been
present to my mind, but I long ago came to the conclusion that they
could not account for the facts. Moreover, I have never been able to
meet with any evidence in favour of the postulated "earth-movements."
Having carefully studied all that has been advanced of late years in
support of the hypothesis in question I find myself more than ever
constrained to oppose it, not only because it is grounded on no basis
of fact, but because it altogether fails to explain the conditions that
obtained in Pleistocene and post-glacial times.
There are various forms in which the hypothesis has appeared, and these
I shall now consider seriatim, and with such brevity as may be. It
has been maintained, for example, that at the advent of the Glacial
period vast areas of northern and north-western Europe, together with
enormous regions in the corresponding latitudes of North America, stood
several thousand feet higher than at present. But when we ask what
evidence can be adduced to prove this we get no satisfactory reply.
We are simply informed that a glacial climate must have resulted from
great elevation, and that the latter, therefore, must have taken place
at the beginning of the Glacial period. Some writers, however, have
ventured to give reasons for their faith. Thus Mr. W. Upham, pointing
to the evidence of the fiords of North America, and to the fact that
drowned river-valleys have been traced outwards across the 100-fathoms
line of the marginal plateau to depths of over 3500 feet, maintains
that the whole continent north of the Gulf of Mexico stood at the
commencement of the Glacial period some 3000 feet at least higher than
now. Of course he cites the fiords of Europe as evidence of a similar
great upheaval for the northern and north-western regions of our
Continent. Mr. Upham even favours the notion that during glacial times
a land-connection probably existed between North America and Europe,
by way of the British Islands, Iceland, and Greenland. When "this
uplifting attained its maximum, and brought on the Glacial period," he
says, "North America and north-western Europe stood 2500 to 3000 feet
above their present height."[BW]
[BW] _American Geologist_, vi., p. 327.
That fiords are simply submerged land-valleys has long been recognised:
that they have been formed mainly by the action of running water--just
in the same way as the mountain-valleys of Norway and Scotland--has
been the belief for many years of most students of physical geology.
But it is hard to understand why they should have been cited by Mr.
Upham in support of his contention, seeing that their evidence seems to
militate strongly against the very hypothesis he strives to maintain.
No one acquainted with the physical features and geological structure
of Scotland and Norway can doubt that the valleys which terminate
in fiords are of great geological antiquity. Their excavation by
fluviatile action certainly dates back to a period long anterior to
the advent of the Ice Age. And a like tale is told by the fiords and
drowned valley-troughs of North America, which cannot be referred to
so recent a period as post-Tertiary times. Those who are convinced
that our continental areas have persisted throughout long æons of
geological time, and that rivers frequently have survived great
geological revolutions--cutting their way across mountain-elevations
as fast as these were uplifted--will readily believe that some of the
submarine river-troughs of North America, such as that of the Hudson,
may belong even to Secondary times.[BX] It would be hard to say at
what particular date the excavation of the Scottish Highland valleys
commenced--but it was probably during the later part of the Palæozoic
era. The process has doubtless been retarded and accelerated frequently
enough, during successive movements of depression and elevation, but it
was practically completed before the beginning of Pleistocene times,
and that is all that we may trouble about here. Precisely the same
conclusion holds good for Norway: and such being the case it is obvious
that the question of the origin and age of the fiords has no bearing
on the problem of the glacial climate and its cause. In point of fact
the evidence, as already remarked, tells against the "earth-movement
hypothesis," for it shows us that, during a period when Europe and
North America stood several thousand feet higher, and extended much
further seawards, rivers, and not glaciers, were the occupants of our
mountain-valleys. It was not until all those valleys had come to assume
much the appearance they now present that general glaciation supervened.
[BX] Professor Dana inclines to date the erosion of the Hudson trough
so far back as the Jura-Trias period.--_American Journ. Science_, xl.,
p. 435.