THE GLACIAL HISTORY OF NORTHERN AND CENTRAL EUROPE
The literature of the glacial period in Europe is stupendous and is,
further, of a highly contradictory nature. Space does not permit of
any summary of the great conflict between the monoglacialists and
the polyglacialists; it is sufficient to say that the latter often
went to extremes and so laid themselves open to defeat, but the
twofold nature of the glaciation is now widely accepted. It must
be understood, however, that the following summary represents the
views of a certain section of geologists only, views which are not
universally held. In the British Isles especially, where the remains
of the maximum glaciation completely dominate those of all the
others, the theory of a single glaciation still largely prevails.
When ice began to accumulate on the rising Scandinavian plateau
it naturally formed at first on the Norwegian mountains near the
Atlantic, which was the chief source of snowfall. These mountain
glaciers spread rapidly down the steep seaward slopes to the west and
more slowly down the gentler landward slopes to the east. At this
stage the centre of the ice-sheet, and consequently the centre of
the glacial anticyclone, as soon as the latter developed a definite
existence, lay quite near the Norwegian coast. Under anticyclonic
conditions the circulation of the winds round the centre is in the
same direction as the motion of the hands of a watch, combined with
an outward inclination at an angle of about thirty to forty-five
degrees. Consequently, while the centre lay in Norway, due north of
the Alps, the prevailing winds in the latter must have been from
north-east, and therefore very cold. Accordingly, this stage is
probably contemporaneous with the Gunz glaciation of the Alps. In the
same way, over the North Sea area the winds must have been easterly,
causing the currents which piled up the great shell-banks of the East
Anglian coast, already referred to as marking the end of the Tertiary
and beginning of the Quaternary period.
But the ice which reached the northern North Sea broke up into
icebergs not far from the coast, and floated away, while that which
moved east into the north of Sweden could only be dissipated by
melting and ablation, processes which we have reason to believe went
on very slowly. Hence ice began to accumulate and spread over a wide
area east of the main Scandinavian mountain chain. Fresh snow was
deposited directly on this ice-surface, until it gradually overtopped
the mountains which originally gave rise to it, and reversed the
flow, so that the ice actually moved uphill across the mountain
chain. As the center of the ice-sheet moved eastward the glacial
anticyclone moved with it, and this new position to the eastward
caused an alteration in the direction of the prevailing winds over
the rest of Europe. The Alps were now south-west of the anticyclonic
centre, and the winds in that district accordingly became easterly
instead of north-easterly. Of course, the glacial anticyclone was
now more intense, but in summer in central Europe easterly winds are
naturally so much warmer than north-easterly winds that at first this
increase in intensity was not enough to counterbalance the change in
direction, and there was a slight improvement in the Alpine climate.
In the same way, over the North Sea district the prevailing winds had
now become south-easterly instead of easterly, which would make for a
slight rise of temperature, as also would the occasional depressions
which would be able to make their way in from the westward, bringing
warm moist air from the Atlantic and occasional rainfall. By this
time the process of elevation had converted the North Sea floor into
an extensive plain.
From Sweden and the Gulf of Bothnia the ice spread out in all
directions, extending in the east to the foot of the Ural Mountains,
which formed an independent centre of glaciation; in the south-east
over a large part of European Russia, where it reached as far south
as latitude 40° in the Dnieper valley; in the south over almost the
whole of Germany as far as the Riesengebirge and Harz Mountains;
and in the south-east over the whole of Holland and the North Sea
basin. It should be noted that Holland and Denmark were glaciated,
not by Norwegian ice, but by ice from the Baltic sheet which had
crossed southern Sweden. The North Sea glacier extended across East
Anglia as far as Cambridge, while a northern branch of it swept
across Caithness and the Orkney and Shetland Islands, but most of the
British Isles were glaciated from independent centres—the Scottish
Highlands, the Pennines, Cumberland, Wales and northern Ireland.
With the growth of the glaciated area, and particularly with its
extension south-westward across the North Sea, the Alpine climate
again became very severe, and the local glaciers and Piedmont
ice-sheets of the Alps reached their maximum development in the
Mindelian. At the same time the central plateau of France developed
a local plateau glacier of its own, and the Pyrenees underwent their
first and greatest glaciation, no traces of the Gunzian having been
found in this range.
The British Isles show an interesting outward migration of the local
centres of maximum ice-development. The Scandinavian glacier which
invaded East Anglia extended arctic anticyclonic conditions across
the North Sea, and induced a heavy snowfall over the high lands of
Great Britain. These, in consequence, developed independent glaciers,
which on their eastern sides fused with the Scandinavian glacier
and, partly by deflecting its flow, partly by intercepting some of
its snowfall, pushed it back into the North Sea plain. The Scottish
glaciers became strong enough to encroach on Ireland, partly in the
north-east, and partly by way of the Irish Sea and St. George’s
Channel (then a valley) on to the south-east. This further extension
of the cold area enabled the Irish glaciers to develop, and these
in turn pushed back the Scottish glaciers until Ireland was solely
glaciated by Irish ice.
The southern margin of the ice-sheet did not extend beyond the Thames
valley, but at some stage the English Channel carried floating ice,
which formed the deposits of ice-borne boulders, of which that at
Selsey is a well-known example.
This great ice-sheet nowhere formed marked terminal moraines, but its
deposits fade away in thin beds of stiff boulder-clay. This absence
of moraines is probably connected with the great thickness of the
ice-sheets, which did not leave any appreciable nunataks or rocky
“islands” exposed in its path, so that there was nothing to give
rise to detritus on the surface of the ice. All the transportation
had to be carried on beneath the ice-sheets, and these, penetrating
into comparatively low latitudes where the sun is powerful in summer,
would suffer gradual melting and ablation for some distance from
their margins. Near the actual ice-limit the motion must have been
slow and the thickness of the ice small, so that conditions were
against the accumulation of thick beds of detritus.
On the borders of the ice-sheet the climate cannot have been
over-rigorous, for pre-Chellean man was able to live almost up to the
ice-edge. The air must have been extremely cold, and there was a belt
of high arctic climate round the ice, but in the south and south-west
this appears to have been very narrow, and sub-arctic conditions,
no worse than those in which many races live to-day, prevailed not
very far from the ice. The configuration of the ice-surface largely
explains this. A high steeply sloping wall of ice causes intensely
violent winds, carrying dense clouds of drift-snow—blizzards, in
fact, similar to those now experienced in parts of Antarctica under
similar circumstances, which sweep the land bare of all life for a
considerable distance. But a low and gradually sloping surface, such
as seems to have existed near the borders of the maximum glaciation,
favours instead comparatively gentle winds without much drift snow.
It is only on the north-west ice-ridge, where ice-cliffs fronted
the sea and where severe storms from the Atlantic were frequent in
winter, that blizzards occurred.
When the land in Scandinavia began to sink under the ice-load more
rapidly than the supply of snow could build up the surface of the
ice-sheet the force which pushed out the ice in all directions from
the centre gradually died away, and the ice-masses over the North
Sea area—now probably again below sea-level—and the low grounds of
Europe were left derelict, with no resources but the snowfall on
their own surfaces. Under these conditions they melted away more
or less rapidly. While these derelict ice-masses were still large,
the auxiliary peripheral centres in the Alps, Pyrenees and British
Isles maintained an independent existence for a while, probably with
fluctuations similar to those which marked the close of the last
glaciation in the Alps, though the evidence of these has now been
wiped away. It is even likely that the beginnings of the weakening
of the central source of supply helped the British ice to divert the
Scandinavian ice into the North Sea. Had there been any powerful
rivers bearing great masses of detritus from the south, as there
are in Siberia, some of these derelict ice-sheets might have been
preserved for a time, at least, as “fossil ice,” but in western
Europe conditions were not favourable for this.
With the disappearance of the ice-sheets the general climate of
Europe must have passed through a series of stages of amelioration,
of which traces can be found here and there, though the details
are lost to us. Ultimately temperate conditions again prevailed;
and for a very long time, approaching a quarter of a million years,
Europe cannot have differed greatly from present climatic conditions.
In Scandinavia the mammoth roamed in forests of birch, pine and
spruce; further south the mammoth is absent, and we find instead more
southern forms—_Elephas antiquus_, resembling the Indian elephant,
_Rhinoceros merckii_, a southern form, the sabre-toothed tiger,
cave-lion, cave-bear and cave-hyæna, wolf, beaver, horse and various
forms of deer, while the flora included even such warmth-loving trees
as the fig. Obviously, during part of this interglacial period, the
climate must have been even warmer than the present.
Let us glance for a moment at the probable conditions. One of
the dominant features in the present weather of Europe is the
accumulation of floating ice in the Arctic basin. This keeps the
temperature low and the pressure high—forms in fact during the spring
and summer months a temporary glacial anticyclone similar in kind
to, though of less intensity than, that which has been described as
covering the Scandinavian ice-sheet. This anticyclone maintains on
its southern edges a belt of easterly winds, and these winds enter
into the general circulation of the earth. Their effect is to push
southward the permanent storm-centres normally situated near Iceland
and the Aleutian Islands, and it is these storm-centres which play
a large part in causing the rainy weather of northern and central
Europe. But occasionally—as in the remarkable spring and summer of
1921—these conditions break down. The Arctic Ocean becomes unusually
ice-free and warm, the pressure falls, and in consequence the
storm-centres move northward. Europe comes under the influence of
the permanent anticyclones of sub-tropical latitudes, rain-bearing
storms pass far to the northward, and we have a dry warm summer of
the Mediterranean type.
This is presumably what happened during the long warm Mindel-Riss
interglacial. For some reason, possibly connected with a temporary
widening and deepening of the Bering Strait, the waters of the Arctic
Ocean became warmer and the amount of floating ice less. Pressure
became lower in the polar basin and therefore higher over the
Atlantic and Europe, and fine warm conditions prevailed in Europe as
the normal climate instead of only as an occasional event.
This warm interval was finally brought to a close by the renewed
elevation of Scandinavia, and the ice-sheets began to develop again,
heralded by a period of dry steppe climate. This time, however, the
conditions were different; the elevation was not so great, and was
more local. Hence the resulting glaciation was less intense; it
filled the Baltic basin and extended some distance on to the North
German plain and into Holland. It failed to reach the coast of
Britain, but that it extended some way across the North Sea plain is
indicated by the peculiar distribution of the Newer Drift of Britain,
to be referred to later. In the north of Norway the slope of the ice
towards the sea was very steep, so that many of the coastal hills
extended above it as nunataks. The ice extended into the channel
between the mainland and the Lofoten Islands (then a peninsula), but
according to Ahlmann these islands were an independent centre of
local glaciation, as the British Isles had been during the preceding
period, and the local ice met the main ice-sheet in the fiords. On
the coast of Nordland sufficient land lay bare to harbour a small
Arctic flora, and Vaero, the southernmost island of Lofoten, had only
small hanging snow-banks.
The interpretation of the British glacial deposits is still very
much under discussion, but it seems probable that the Scottish
highlands formed a subsidiary centre which glaciated the whole of
Scotland and north-east England, sending a stream south-eastward,
which was prevented from spreading across the North Sea plain by
the presence of Scandinavian ice to the east and impinged on the
coast of Yorkshire and Lincolnshire, just reaching the northern
extremity of Norfolk. Many British geologists regard this development
as the concluding phase of a single glaciation of Britain, but the
differences in the amount of weathering undergone are against such
an interpretation. At the same time there were local glaciers in
Cumberland, Wales and Ireland.
In England limits of this glaciation are characterized by a
well-marked series of end-moraines, which indicate that the ice
carried much surface detritus, and probably ended in a steep cliff.
In Scandinavia, on the other hand, the centre of glaciation again
lay over the low ground well to the east of the mountains, and the
ice which reached Germany and Denmark was still largely free of
surface detritus, and so did not form marked end-moraines. There
was a difference, however. On this occasion, owing to the local
nature of the elevation in Scandinavia, the ice-sheet did not extend
its borders so far to the eastward, and the glaciation of Asia, as
described in Chapter VII, was slight. Europe came more under the
influence of cold north-easterly and northerly winds, and life on
the ice-borders was not so easy as during the preceding glaciation.
Man could still live near the ice, but he took to making his home in
caves, and to clothing himself in skins for warmth.
After the ice had reached its Rissian maximum of glaciation the
climate improved somewhat. The ice-edge retreated, leaving Denmark
and the German coast, and vacating the Baltic basin, but not
disappearing altogether from Scandinavia. At Rixdorf, near Berlin,
there is a bed of gravel deposited in this “interglacial,” containing
numerous and well-preserved bones of the mammoth, woolly rhinoceros,
aurochs, bison, horse, reindeer, red deer and other species of
_Cervus_, musk ox and wolf—a cold temperate to sub-arctic fauna.
In south Germany fresh-water mollusca indicate that the summers in
that district were almost as warm as at present, but the winters
were probably severe. As described in the preceding chapter this
“interglacial” was the time of loess formation _par excellence_, with
a continental climate and steppe conditions over much of central
Europe.
Investigations at Skærumhede in Denmark show that this recession of
the ice was accompanied by, and presumably due to, a fall in the
level of the land relatively to that of the sea, for at the beginning
of the oscillation the land lay about 350 feet above its present
level, sinking gradually to only 30 feet above present. Even at its
best during this interglacial the climate was almost sub-arctic in
Denmark. In northern Finland, on the eastern edge of the ice-sheet,
there was also an “interglacial,” with a slight improvement in the
climate accompanying a temporary submergence. But in Scandinavia
there are no traces of any interglacial deposits of this period, and
considering the cold climates which prevailed in Denmark and North
Germany, it seems probable that Scandinavia continued to be glaciated
during the whole period.
The mode of life among Mousterian men, who lived during this
“interglacial,” also points to a severe climate. For at this time man
did not live in the open, but in caves and rock-shelters, and the
practice of wearing the fur skins of animals as a protection against
the cold, begun in the preceding Rissian glacial period, was not
discontinued.
After the temporary subsidence had ceased, elevation again set in,
causing a readvance of the ice-sheets and glaciers. The limits fell
short of those of the preceding maximum, and the climate was not
so severe, but in its general character it resembled that of the
preceding maximum, but was much stormier, and there were probably
frequent blizzards of the Antarctic type, carrying drift-snow. The
new ice-sheet carried more surface detritus than its predecessors,
presumably because all the high ground was not covered, and it
formed high terminal moraines. The close association of cold ice
and irregular masses of bare sand and stones, strongly heated by the
summer sun, set up a belt of powerful convection very favourable for
the development of blizzards; possibly there was something in the
nature of an ice-cliff down which the cold winds could blow with
great strength. At any rate, man found the near neighbourhood of
the ice unpleasant, and went, so that there are no contemporaneous
human implements near the moraines. The limits of the Scandinavian
ice-sheet ran from the Norwegian coast across Denmark from north
to south, through North Germany and northern Russia, and included
Finland. The ice probably did not extend far across the North Sea
plain, and in the British Isles there was no ice-sheet, but the high
mountains of Scotland, Ireland, Wales and Cumberland bore small
local glaciers, which were long enough to reach the sea in the
Scottish highlands. The Alps bore considerable glaciers, indicating
a depression of the snow-line of about 3500 feet, corresponding to a
temperature 11° F. lower than the present.
After this ice-development had reached its maximum limits and
remained there for perhaps a thousand years, retreat set in, and
the Scandinavian ice once more withdrew from Germany and Denmark to
the Baltic basin. But its edge was never far from the German coast,
and occasionally readvanced across it, for numerous fossiliferous
deposits are intercalated in boulder-clay. The fauna and flora, which
are well known, point to an arctic climate. At its best the mean
temperature of July rose to about 50° F., and there was a vegetation
period of three or four months with an average temperature of about
40° F., but these relatively mild conditions lasted at most for a few
decades or perhaps a century at a time, and the winters were severe
throughout. The duration of the whole of this “Baltic Interstadial”
was from one to two thousand years.
Next followed the final readvance of the ice forming the great
“Baltic” moraines which fringe the Baltic coast of Germany, turning
northward in the west into Denmark and in the east into Finland.
There was a corresponding re-development of glaciers in the Alps
(Bühl stage) and in the mountains of Ireland and Scotland, though
these probably failed to reach the sea even in Scotland. This period
gave us a repetition of the climate of the preceding maxima. In this
case we have definite evidence of the presence of a belt of easterly
winds on the southern side of the ice-sheet, in a series of “barkans”
or fossil dunes in Holland, Germany and Galicia. These dunes were
formed of fine ice-deposited material, and they are crescent-shaped,
with their convexities to the east, indicating that they were built
by strong easterly winds. A moment’s consideration will show the
truth of the latter statement. Suppose there is an isolated round
hillock of sand exposed to strong easterly winds. The sand grains
will travel up the easterly windward slope of the hillock and roll
down the westerly leeward side. In this way the whole hillock will
advance very slowly westwards. But in the centre, where the hillock
reaches its greatest height, the grains will take longer to reach
the highest point than near the edges, where they have not to rise
so high. At the edges a strong gust will carry some of the heavier
grains right over the dune, while nearer the centre they will be left
half-way, and when the gust ceases will perhaps roll back to their
original position. In this way the margins of the dune will advance
westward more rapidly than the centre, producing the crescent shape
with the convex side to the east. At the time of their formation
these dunes must have had their steepest side to the westward, but
the westerly winds which have prevailed during the last few thousand
years have succeeded in modifying that detail, without destroying
the general shape of the dunes, and the steepest slopes are now on
the eastern side. The preservation of the original shape, in spite
of the subsequent development of westerly winds, is due in part to
the coating of vegetation, which protected the dunes as soon as more
favourable conditions occurred, and probably in part to the lesser
velocity of the westerlies. If the period of east winds and dune
formation had been long enough, we might have had another deposit of
loess, but it was short, and vegetation, which is necessary to the
genesis of true loess, had no time to establish itself before the
climate changed again with the final retreat of the ice. The climate
of this period in Rumania has been ably described by G. Murgoci:
“In general the prevailing climate of the time of the formation of
loessoid soils and blown sands must have been that which is named by
E. de Martonne the aralian climate, a dry climate with some rain in
spring to call forth a poor and transient vegetation and to maintain
the flowing water in rivers and lakes. The temperature with great
extremes, in summer up to 120° F. and in winter below 20° F., was the
characteristic of this climate; the atmosphere was very dry in the
hot season, but in the rest of the year there was some humidity in
the air and moisture in the soil, the water of the subsoil being not
very deep. The atmospheric precipitation in this region was caused
by the south-west wind just as at present; but the dominant wind
giving the character of a dry continental climate was the north-east
wind (Crivat) which has left its traces in the fossil dunes of the
Baragan.”
A history of the changes of climate in Europe which followed the
maximum of the last readvance of the ice-sheet must be left to later
chapters.
BIBLIOGRAPHY
Brooks, C. E. P. “The correlation of the Quaternary deposits of the
British Isles with those of the continent of Europe.” _Ann. Rep.
Smithsonian Inst._, 1917, pp. 277-375. [Full list of references.]
Penck, A., and Brückner, E. “Die Alpen in Eiszeitalter.” 3 Vols.
_Leipzig_, 1901-9.
Gagel, C. “Die Beweise für eine mehrfache Vereisung
Norddeutschlands in diluvialer Zeit.” _Geol. Rundschau_, 4, 1913,
p. 39.
Wahnschaffe, F. “Die Oberflächengestaltung des norddeutschen
Flachlandes.” Stuttgart, 1910.
Svastos, R. “Le postglaciare dans l’Europe centrale du nord et
orientale.” _Ann. Sci. Univ. Jassy_, 4, 1908, p. 48.