determined not only by temperature, but by the amount and distribution
of the snow-fall. Nevertheless, the position of the snow-line in
Europe during the Ice Age enables us to form a rough estimate of the
temperature. At present in middle Europe the temperature falls 1° F.
for every 300 feet of ascent. Hence if we take the average depression
of the snow-line in glacial times at 3000 feet, that would correspond
approximately to a lowering of the temperature by 10°.[BC] This may
not appear to be much, but, as Penck points out, were the mean annual
temperature to be lowered to that extent it would bring the climate of
northern Norway down to southern Germany, and the climate of Sweden to
Austria and Moravia, while that of the Alps would be met with over the
basin of the Mediterranean.
[BC] Professor Brückner thinks the general lowering of temperature
may not have exceeded 5-1/2° to 7° F. _Verhandlungen der 73
Jahresversammlung der schweizerischen Naturforschenden Gesellschaft in
Davos_, 1890.
Let it be noted further that this lowering of the temperature--this
displacement of climatic zones, was experienced over the whole
continent--extending on the one hand south into Africa, and on the
other east into Asia. But while the conditions in northern and central
Europe were markedly glacial, further south only more or less isolated
snow-capped mountains and local glaciers appeared--such, for example,
as those of the Sierra Nevada, the Apennines, Corsica, the Atlas, the
Lebanon, etc. In connection with these facts we may note also that
the Azores were reached by floating ice; and I need only refer in a
word to the evidence of cold wet conditions as furnished by the plant
and animal remains of the Pleistocene tufas, alluvia, and peat of
southern Europe. Again in north Africa and Syria we find, in desiccated
regions, widespread fluviatile accumulations, which, in the opinion of
a number of competent observers, are indicative of rainy conditions
contemporaneous with the Glacial period of Europe.
When we compare the conditions of the Ice Age with those of the present
we are struck with the fact that the former were only an exaggeration
of the latter. The development of glaciation was in strict accordance
with existing conditions. Thus in Pleistocene times North America
was more extensively glaciated than northern Europe, just as to-day
Greenland shows more snow and ice than Scandinavia. No traces of
glaciation have been observed as yet in northern Asia or in northern
Alaska, and to-day the only glaciers and ice-sheets that exist in
northern regions are confined to the formerly glaciated areas. Again,
in Pleistocene Europe glacial phenomena were more strongly developed
in the west than in the east. Large glaciers, for example, existed in
central France, and a considerable ice-flow poured into the basin of
the Douro. But in the same latitudes of eastern Europe we meet with
few or no traces of ice-action. Again, the Vosges appear to have been
more severely glaciated than the mountains of middle Germany; and so
likewise the old glaciers of the western Alps were on a much more
extensive scale than those towards the east end of the chain. Similar
contrasts may be noted at the present day. Thus we find glaciers in
Norway under lat. 60°, while in the Ural Mountains in the same latitude
there is none. The glaciers of the western Alps, again, are larger than
those in the eastern part of the chain. The Caucasus region, it is
true, has considerable glaciers, but then the mountains are higher.
Now turn for a moment to North America. The eastern area was covered
by one immense ice-sheet, while in the mountainous region of the west
gigantic glaciers existed. In our own day we see a similar contrast.
In the north-east lies Greenland well-nigh drowned in ice, while the
north-west region on the other hand, although considerably higher and
occurring in the same latitude, holds only local glaciers. We may
further note that at the present day very dry regions, even when these
are relatively lofty and in high latitudes, such as the uplands of
Siberia, contain no glaciers. And the same was the case in the Glacial
period. These facts are sufficient to show that the conditions of
glacial times bore an intimate relation to those that now obtain. Could
the requisite increase of precipitation and lowering of temperature
take place, we cannot doubt that ice-sheets and glaciers would reappear
in precisely the same regions where they were formerly so extensively
developed. No change in the relative elevation of the land would be
required--increased precipitation accompanied by a general lowering of
the snow-line for 3000 or 3500 feet would suffice to reintroduce the
Ice Age.
From the foregoing considerations we may conclude:--(1) That the
cold of the Glacial period was a general phenomenon, due to some
widely-acting cause--a cause sufficient to influence contemporaneously
the climate of Europe and North America; (2) that glaciation in our
continent increased in intensity from east to west, and from south to
north; (3) that where now we have the greatest rainfall, in glacial
times the greatest snow-fall took place, and the snow tended most to
accumulate; (4) that in the extreme south of Europe, and in north
Africa and west Asia, increased rain precipitation accompanied lowering
of temperature, from which it may be inferred that precipitation in
glacial times was greater generally than it is now.
Having considered the climatic conditions that obtained at the climax
of the Glacial period, I have next to recapitulate what is known
as to the climatic changes of Pleistocene times. It is generally
admitted that the glacial conditions of which I have been speaking
were repeated twice, some say three times, during the Pleistocene
period; while others maintain that even a larger number of glacial
episodes may have occurred. Two glacial epochs, at all events, have
been recognised generally both in Europe and North America. These were
separated by an interglacial stage of more genial conditions, the
evidence for which is steadily increasing. No one now calls in question
the existence of interglacial deposits, but, as their occurrence is
rather a stumbling-block in the way of certain recently resuscitated
hypotheses, some attempt has been made to minimise their importance--to
explain them away, in fact. It has been suggested, for example--(and
the suggestion is by no means new)--that the deposits in question
only show that there were local oscillations during the advance and
retreat of the old ice-sheets and glaciers. This, however, is not the
view of those who have observed and described interglacial beds--who
know the nature of the organic remains which they have yielded, and
the conditions under which the beds must have been accumulated. I need
not refer to the interglacial deposits of our own country further
than to remark that they certainly cannot be explained away in that
summary fashion. The peat and freshwater beds that lie between the
lower and upper tills in the neighbourhood of Edinburgh, for example,
are of themselves sufficient to prove a marked and decided change of
climate. No mere temporary retreat and re-advance of the ice-sheet
will account for their occurrence. The lower till is unquestionably
the bottom-moraine of an ice-sheet which, in that region, flowed
towards the east. When the geographical position of the deposits in
question is considered it becomes clear that an easterly flow of ice
in Mid-Lothian proves beyond gainsaying that during the accumulation
of the lower till all Scotland was drowned in ice. But when water once
more flowed over the land-surface--when a temperate flora, composed
of hazels and other plants, again appeared, it is obvious that the
ice-sheet had already vanished from central Scotland. This is not the
case of a mere temporary recession of the ice-front. It is impossible
to believe that a temperate or even cold-temperate flora could have
flourished in central Scotland at a period when thick glacier-ice
mantled any portion of our Lowlands. Again, in the upper till we read
the evidence of a recurrence of extreme glacial conditions--when
central Scotland was once more overwhelmed by confluent ice-streams
coming from the Highlands and the southern Uplands. Similar evidence
of recurrent glacial conditions, I need hardly remind you, has been
detected in other parts of the country. We are justified, then, in
maintaining that our interglacial beds point to distinct oscillations
of climate--oscillations which imply a long lapse of time. Continental
observers are equally convinced that the interglacial epoch, of which
so many interesting relics have been preserved over a wide region, was
marked at its climax by a temperate climate and endured for a long
period. The interglacial beds of northern and central Europe form
everywhere marked horizons in the glacial series.
Geologists sometimes forget that in every region where glacial
accumulations are well developed, good observers had recognised an
upper and lower series of "drift-deposits" long before the idea of two
separate glacial epochs had presented itself. Thus, in north Germany,
so clearly is the Upper differentiated from the Lower Diluvium that
the two series had been noted and mapped as separate accumulations
for years before geologists had formulated the theory of successive
ice-epochs.[BD] The division of the German Diluvium into an upper
and a lower series is as firmly established as any other well-marked
division in historical geology. The stratigraphical evidence has been
much strengthened, however, by the discovery between upper and lower
boulder-clays of true interglacial beds, containing lignite, peat,
diatomaceous earth, and marine, brackish, and freshwater molluscs,
fish, etc., and now and again bones of Pleistocene mammals.[BE]
A similar strongly-marked division characterises the glacial
accumulations of Sweden, as has been clearly shown by De Geer,[BF] who
thinks that the older and younger epochs of glaciation were separated
by a protracted period of interglacial conditions. In short, evidence
of a break in the glacial succession has been traced at intervals
across the whole width of the Continent, from the borders of the North
Sea to central Russia. M. Krischtafowitsch has recently detected in the
neighbourhood of Moscow[BG] certain fossiliferous interglacial beds,
the flora and fauna of which indicate a warmer and moister climate than
the present. The interglacial stage, he says, must have been of long
duration, and separated in Russia as in western Europe two distinct
epochs of glaciation.
[BD] Wahnschaffe: _Forschungen zur deutschen Landes- und Volkskunde von
Dr. A. Kirchhoff_, Bd. vi., Heft 1.
[BE] For interglacial beds of north Germany see Helland: _Zeitschr.
d. deutsch. geol._ Ges., xxxi., 879; Penck: _Ibid._, xxxi., 157;
_Länderkunde von Europa_ (Das deutsche Reich), 1887, 512; Dames:
_Samml. gemeinverständl. wissensch. Vorträge, von Virchow u.
Holtzendorff:_ xx. Ser., 479 Heft; Schröder: _Jahrb. d. k. geol.
Landensanst. f._ 1885, p. 219. For further references see Wahnschaffe,
_op. cit._ I have not thought it worth while in this paper to refer
to the interglacial deposits of our own islands. A general account of
them will be found in my _Great Ice Age_, and _Prehistoric Europe_. The
interglacial phenomena of the Continent seem to be less known here than
they ought to be.
[BF] _Zeitschrift d. deutsch. geolog. Gesellschaft_, Bd. xxxvii, p. 197.
[BG] _Anzeichen einer interglaziären Epoche in Central-Russland_,
Moskau, 1891.
No mere temporary retreat and re-advance of the ice-front can account
for these phenomena. The occurrence of remains of the great pachyderms
at Rixdorf, near Berlin, and the character of the flora met with in
the interglacial beds of north Germany and Russia are incompatible
with glacial conditions in the low-grounds of northern Europe. The
interglacial beds, described by Dr. C. Weber[BH] as occurring near
Grünenthal, in Holstein, are among the more recent discoveries of
this kind. These deposits rest upon boulder-clay, and are overlaid by
another sheet of the same character, and belong, according to Weber,
to "that great interglacial period which preceded the last ice-sheet
of northern Europe." The section shows 8 feet of peat resting on
freshwater clay, 2 feet thick, which is underlaid by some 10 feet of
"coral sand," with bryozoa. The flora and fauna have a distinctly
temperate facies. It is no wonder, then, that Continental geologists
are generally inclined to admit that north Germany and the contiguous
countries have been invaded at least twice by the ice-sheets of two
separate and distinct glacial epochs. This is not all, however. While
every observer acknowledges that the Diluvium is properly divided
into an upper and a lower series, there are some geologists who have
described the occurrence of three, and even more boulder-clays--the
one clearly differentiated from the other, and traceable over wide
areas. Is each of these to be considered the product of an independent
ice-sheet, or do they only indicate more or less extensive oscillations
of the ice-front? The boulder-clays are parted from each other by thick
beds of sand and clay, in some of which fossils have occasionally
been detected. It is quite possible that such stratified beds were
deposited during a temporary retreat of the ice-front, which when it
re-advanced covered them up with its bottom-moraine. On the other
hand, the phenomena are equally explicable on the assumption that
each boulder-clay represents a separate epoch of glaciation. Until
the stratified beds have yielded more abundant traces of the life of
the period, our judgment as to the conditions implied by them must be
suspended. It is worthy of note in this connection, however, that in
North America the existence of one prolonged interglacial epoch has
been well established, while distinct evidence is forthcoming of what
Chamberlin discriminates as "stages of deglaciation and re-advancing
ice."[BI]
[BH] _Neues Jahrbuch f. Mineralogie, Geologie, u. Palæontologie_, 1891,
Bd. ii., pp. 62, 228; 1892, Bd. i., p. 114.
[BI] _Sixth Annual Report, U. S. Geol. Survey_, 1884-5, P. 315.
When we turn to the Alpine Lands, we find that there also the
occurrence of former interglacial conditions has been recognised.
The interglacial deposits, as described by Heer and others, are well
known. These form as definite a geological horizon as the similar
fossiliferous zone in the Diluvium of northern Germany. The lignites,
as Heer pointed out, represent a long period of time, and this is still
further illustrated by the fact that considerable fluviatile erosion
supervened between the close of the first and the advent of the later
glacial epoch. No mere temporary retreat and re-advance of the ice will
account for the phenomena. Let us for a moment consider the conditions
under which the accumulations in question were laid down. The glacial
deposits underlying the lignite beds contain, amongst other erratics,
boulders which have come from the upper valley of the Rhine. This
means, of course, that the ancient glacier of the Rhine succeeded in
reaching the Lake of Zurich; and it is well known that it extended at
the same time to Lake Constance. That glacier, therefore exceeded sixty
miles in length. One cannot doubt that the climatic conditions implied
by this great extension were excessive, and quite incompatible with
the appearance in the low-grounds of Switzerland of such a flora as
that of the lignites. The organic remains of the lignite beds indicate
a climate certainly not less temperate than that which at present
characterises the district round the Lake of Zurich. We may safely
infer, therefore, that during interglacial times the glaciers of the
Alps were not more extensively developed than at present. Again, as
the lignites are overlaid by glacial deposits, it is obvious that the
Rhine glacier once more reached Lake Zurich--in other words, there was
a return of the excessive climate that induced the first great advance
of that and other Swiss glaciers. That these advances were really due
to extreme climatic conditions is shown by the fact that it was only
under such conditions that the Scandinavian flora could have invaded
the low-grounds of Europe, and entered Switzerland. It is impossible,
therefore, that the interglacial flora could have flourished in
Switzerland while the immigration of these northern plants was taking
place.
Lignites of the same age as those of Dürnten and Utznach occur
in many places both on the north and south sides of the Alpine
chain. At Imberg, near Sonthofen, in Bavaria, for example, they are
described by Penck[BJ] as being underlaid and overlaid by thick
glacial accumulations. The deposits in question form a terrace along
the flanks of the hills, at a height of 700 feet above the Iller.
The flora of the lignite has not yet been fully studied, but it is
composed chiefly of conifers, which must have grown near where their
remains now occur--that is at 3000 feet, or thereabout, above the sea.
It is incredible that coniferous forests could have flourished at
that elevation during a glacial epoch. A lowering of the mean annual
temperature by 3° C. only would render the growth of trees at that
height almost impossible, and certainly would be insufficient to cause
the glaciers of Algau to descend to the foot of the mountains, as we
know they did--a distance of at least twenty-four miles. The Imberg
lignites, therefore, are evidence of a climate not less temperate than
the present. More than this, there is clear proof that the interglacial
stage was long continued, for during that epoch the Iller had time to
effect very considerable erosion. The succession of changes shown by
the sections near Sonthofen are as follows.
1. The Iller Valley is filled with glacier-ice which flows out upon the
low-grounds at the base of the Alps.
2. The glacier retreats, and great sheets of shingle and gravel are
spread over the valley.
3. Coniferous forests now grow over the surface of the gravels; and as
the lignite formed of their remains attains a thickness of ten feet in
all, it obviously points to the lapse of some considerable time.
4. Eventually the forests decay, and their débris is buried under new
accumulations of shingle and gravel.
5. The Iller cuts its way down through all the deposits to depths of
680 to 720 feet.
6. A glacier again descends and fills the valley, but does not flow so
far as that of the earlier glacial stage.
[BJ] _Die Vergletscherung der deutschen Alpen_, 1882, p. 256.
In this section, as in those at Dürnten and Utznach, we have
conclusive evidence of two glacial epochs, sharply marked off the one
from the other. Nor does that evidence stand alone, for at various
points between Lake Geneva and the lower valley of the Inn similar
interglacial deposits occur. Sometimes these appear at the foot of
the mountains, as at Mörschweil on Lake Constance; sometimes just
within the mountain area, as at Imberg; sometimes far in the heart
of the Alpine Lands, as at Innsbruck. Professor Penck has further
shown, and his observations have been confirmed by Brückner, Blaas,
and Böhm, that massive sheets of fluviatile gravel are frequently
met with throughout the valleys of the Alps, occupying interglacial
positions. These gravels are exactly comparable to the interglacial
gravels of the Sonthofen sections. And it has been demonstrated that
they occur on two horizons, separated the one from the other by
characteristic ground-moraine, or boulder-clay. The lower gravels
rest on ground-moraine, and the upper gravels are overlaid by sheets
of the same kind of glacial detritus. In short, three separate and
distinct ground-moraines are recognised. The gravels, one cannot doubt,
are simply the torrential and fluviatile deposits laid down before
advancing and retreating glaciers; and it is especially to be noted
that each sheet of gravel, after its accumulation, was much denuded
and cut through by river-action. In a word, as Penck and others have
shown, the valleys of Upper Bavaria have been occupied by glaciers at
three successive epochs--each separated from the other by a period
during which much river-gravel was deposited and great erosion of the
valley-bottoms was effected.
On the Italian side of the Alps, similar evidence of climatic changes
is forthcoming. The lignites and lacustrine strata of Val Gandino,
and of Val Borlezza, as I have elsewhere shown,[BK] are clearly of
interglacial age. From these deposits many organic remains have been
obtained--amongst the animals being _Rhinoceros hemitoechus_ and _R.
leptorhinus_. According to Sordelli, the plants indicate a climate
as genial as that of the plains of Lombardy and Venetia, and warmer
therefore than that of the upland valleys in which the interglacial
beds occur. Professor Penck informs me that some time ago he detected
evidence in the district of Lake Garda of three successive glacial
epochs--the evidence being of the same character as that recognised in
the valleys of the Bavarian Alps.
[BK] _Prehistoric Europe_, p. 303.
In the glaciated districts of France similar phenomena are met with.
Thus in Cantal, according to M. Rames,[BL] the glacial deposits belong
to two separate epochs. The older morainic accumulations are scattered
over the surface of the plateau of Archæan schistose rocks, and extend
up the slopes of the great volcanic cone of that region to heights
of 2300 to 3300 feet. One of the features of these accumulations
are the innumerable gigantic erratics, known to the country folk as
_cimetière des enragés_. Sheets of fluvio-glacial gravel are also
associated with the moraines, and it is worthy of note that both have
the aspect of considerable age--they have evidently been subjected to
much denudation. In the valleys of the same region occurs a younger
series of glacial deposits, consisting of conspicuous lateral and
terminal moraines, which, unlike the older accumulations, have a very
fresh and well-preserved appearance. With them, as with the older
moraines, fluvio-glacial gravels are associated. M. Rames shows that
the interval that supervened between the formation of the two series of
glacial deposits must have been prolonged, for the valleys during that
interval were in some places eroded to a depth of 900 feet. Not only
was the volcanic _massif_ deeply incised, but even the old plateau of
crystalline rocks on which the volcanic cone reposes suffered extensive
denudation in interglacial times. M. Rames further recognises that the
second glacial epoch was marked by two advances of the valley-glaciers,
separated by a marked episode of fusion, the evidence for which is
conspicuous in the valley of the Cère.
[BL] _Bull. Soc. Géol. de France_, 1884.
The glacial and interglacial phenomena of Auvergne are quite analogous
to those of Cantal. Dr. Julien has described the morainic accumulations
of a large glacier that flowed from Mont Dore. After that glacier had
retreated a prolonged period of erosion followed, when the morainic
deposits were deeply trenched, and the underlying rocks cut into.
In the valleys and hollows thus excavated freshwater beds occur,
containing the relics of an abundant flora, together with the remains
of elephant (_E. meridionalis_), rhinoceros (_R. leptorhinus_),
hippopotamus, horse, cave-bear, hyæna, etc.--a fauna comparable to
that of the Italian interglacial deposits. After the deposition of the
freshwater beds, glaciers again descended the Auvergne valleys and
covered the beds in question with their moraines.[BM]
[BM] _Des Phénomènes glaciaires dans le Plateau central de France, etc._
According to the researches of Martins, Collomb, Garrigou,
Piette, and Penck, there is clear evidence in the Pyrenees of two
periods of glaciation, separated by an interval of much erosion
and valley-excavation. Penck, indeed, has shown that the valleys
of the Pyrenees have been occupied at three successive epochs by
glaciers--each epoch being represented by its series of moraines and by
terraces of fluvio-glacial detritus, which occur at successively lower
levels.