ICE AND ITS WORK
In the last chapter we have spoken of the moulding of the surface of the
earth by means of running water and the agents summed up in the term
"weathering." The process is sometimes called "normal erosion," to
distinguish it from that other form of surface moulding in which ice and
frost play a prominent part. At the present time ice, in the form of
ice-sheets or glaciers, is confined to relatively small areas of the
globe, so that we are justified in regarding its action as exceptional
when compared with the work of running water. It is, however, well known
that this limitation of the field of action of ice is very recent, and
that during a period which geologically is only yesterday, a much
greater part of the surface than at present was ice-clad.
In point of fact, much of Europe, especially the northern parts and
those regions which lie close to the lofty mountain chains, much of
North America, and, probably, considerable parts of the southern
hemisphere, were subjected to the action of ice so recently that the
processes of normal erosion have not had time to obliterate, hardly even
to blur, the tracks which the ice left.
The results of the great extension of ice action in that period which
geologists call Pleistocene were twofold. In the first place, as the
result of the presence of the ice-sheet, we have vast accumulations of
débris spread over the lower grounds. These accumulations sometimes form
great sheets of boulder clay; sometimes they are collected into the
curious sandy and gravelly mounds called kames which in parts of,
_e. g._ Scotland, have a great extension; sometimes they have formed
great heaps of material at the entrances of valleys. Again, these
deposits have sometimes blocked valleys and so formed lakes, and they
have supplied the post-glacial rivers with a vast amount of material
which has been used to scour out the river-beds, and has been often
re-sorted and re-arranged by running water.
Secondly, the fact that the northern region and the high grounds
further south, in both Europe and North America, have been recently clad
in ice is associated with many peculiarities of surface form, some of
which have exercised a marked influence on human settlements and ways of
communication.
These peculiarities of surface moulding have been the object of
singularly detailed study in late years, and from this detailed study
many interesting facts have emerged. It may be well to state at once
that this study has been largely stimulated by the fact that there is at
present a great want of unanimity of opinion as to the exact cause of
these peculiarities of form. According to one school ice is a more
powerful eroding agent than water; according to another its action is
largely conservative, and its power of erosion is slight as compared
with that of water.
The beginnings of a possible solution of the problem are perhaps to be
seen in the suggestions of those who seek the causes of the peculiar
features of glaciated regions in the way in which running water works
when it is controlled and modified by the existence of ice; but we must
admit that, on the whole, the conflict is still hot and many members of
the opposing schools will have no compromise.
To the geographer, however, the very fierceness of the controversy has
been useful. The question as to the exact part played respectively by
water and by ice in surface moulding is really a question for the
geologist. It is, however, of great importance to the geographer that
recently glaciated surfaces should be studied from every point of view,
for from this detailed study are emerging many important
generalisations. We shall, therefore, in this chapter only touch very
lightly upon the actual points in dispute, but shall lay stress upon the
interesting facts admitted by both parties.
When the conception of a just-vanished period of great glaciation was
being established by the labours of many geologists, stress was
naturally laid upon the obvious resemblances between parts of, _e. g._
Scotland and Wales, and those parts of the Alps which have been exposed
by the retreat of the existing glaciers. Thus we find that most of the
text-books emphasise the occurrence of perched blocks, of erratics,
_i. e._ of blocks of rock which must have been carried from a distance,
of the phenomenon of crag and tail, of giants' kettles, and so on. All
these are of more geological than geographical importance; they do not
in themselves greatly affect the distribution of other phenomena over
the surface. We shall not, therefore, stop to consider them in detail.
It is otherwise with those indications of recent glaciation which have
been studied within the last few years, and they demand the geographer's
most careful consideration.
The most active discussion has taken place in regard to the peculiar
features of the valleys in recently-glaciated districts, and we shall
discuss especially this point.
We have already described the general features presented by valleys
which owe their origin to running water. In such valleys, as we have
seen, the longer the forces work the more nearly is the valley floor
reduced to an even slope, whose angle decreases in passing from the
mountain to the plain track. In the ordinary river valley the shape of
the valley approximates to that of a V, that is, the valley narrows
downwards, the river occupying the narrowest region.
Again, as a general rule there is no great difference of level between
the tributary valleys--at least at their extremities--and the main
valley, that is, there is no sharp discordance between the two. While,
however, the "mature" river valley shows a gentle, continuous slope, we
usually find that "young" rivers, at least in their mountain track, show
an alternation of plain and gorge, which is very easily observed in any
hilly region.
In other words, we find that, owing to the inclination of the rocks, or
to their varying hardness, or to other causes, particular reaches are
less easily eroded than others. These form waterfalls, which ultimately,
as we have seen, give place to gorges. Beyond the waterfall the
diminishing slope checks the rapidity of flow, and the stream tends to
widen out, and also to throw down its load of débris, so that an
alluvial plain may be formed.
One other character of an ordinary river valley may be noted. It heads,
as we have seen, in a collecting basin, which receives the surface
runnels and the outflows of the springs which form the beginning of the
river.
Let us now turn to the valleys in a recently glaciated country. We omit
any description of existing glaciers; these will be found described in
the volume on the Alps, and further, photography and the picture
postcard have rendered the main features of a glacier familiar to every
one. Almost every large railway station now shows fine coloured
photographs of some of the important Swiss glaciers.
Taking, then, a valley known to have been occupied by a Pleistocene
glacier, we find the following features. As contrasted with an ordinary
river valley, the glacial valley is usually flat-bottomed, a condition
described as U-shaped to point the contrast with the river valley.
Examples in Great Britain and elsewhere are frequent, but some of the
Alpine valleys show the phenomenon in a very striking form. Two good
examples are the Aar valley at Meiringen, and the Lauterbrunnen valley
at the village of the same name. Both have been rendered more or less
familiar by constant photographing (see fig. 7).
The reason why they have been so much photographed leads us to consider
another peculiarity of the glaciated valley. In both the cases named a
steep cliff wall rises from either side of the broad, flat valley
floor, and from the summit of this cliff the lateral streams leap into
the main valley by often superb waterfalls. This is a very important
feature of glaciated valleys--the fact that their tributaries are
markedly _discordant_, that is, that there is marked difference of level
between the beds of the side and main streams.
Because the side valleys lie high above the main they are said to
"hang," and are called hanging valleys, while the main valley is said to
be over-deepened. The rocky height over which the water springs may be
called the junction step, as an attempt to translate the French term
_gradin de confluence_ which is applied to it.
Incidentally we may note that in the Alps the junction step is of great
human importance. Its presence gives the water the power which is used
in lighting the Alpine villages with electricity, and in driving the
trains which often carry the tourist to those villages. In the French
and Italian Alps especially, the power is being more and more used to
supply the motive force for various minor manufactures, notably for the
production of nitrogenous manure from the air.
[Illustration: Fig. 6.--A diagrammatic cross-section of a recently
glaciated valley. AB, the mountain slope which rose above the ancient
glacier and has therefore retained the sharp, unrounded forms due to
ordinary weathering. BC, the shelf or shoulder, formerly covered by the
ice, and therefore strewn with glacial débris. It now usually forms a
pasture or alp. The dotted line connecting CC shows the probable form of
the pre-glacial valley; CD, the rocky wall of the existing U-shaped
valley on whose floor the river now flows.]
Associated with the hanging valleys of Alpine regions is the presence
of a curious shelf, shoulder, or "bench," which frequently lies on the
top of the cliff from which the lateral streams spring (see figs. 6 and
7). Any one who has done some walking in the Alps, must have noticed a
peculiar and often trying feature of any walk which leads up the side of
the valley. This is that the walk begins with a very steep ascent,
where the road or track zig-zags to and fro. After this steep and trying
climb the walker reaches a broad shelf (BC in figs. 6 and 7), where the
slope is much less, and where the extent of relatively level ground
gives room for the erection of a huge hotel, or perhaps only of a group
of chalets. This shelf is covered with fine herbage, destined to be
cropped by the cows of the community.
[Illustration: Fig. 7.--An actual cross-section of the Lauterbrunnen
valley. The vertical and horizontal scales are the same. B marks the
edge of the cliff wall over which the streams leap in cascades. A is the
position of the stream at the bottom of the U-shaped valley. BC marks
the position of the shelf, largely occupied by the pastures or alps.
Above them are rocky, unsmoothed slopes.]
If the traveller continue his walk he will find that above this pasture
ground or _alp_ the slopes are again steep up to the mountain summits.
Possibly, however, his walk has been to see a famous waterfall from
above, and he will find that the streams which flow with relative
slowness over the comparatively gentle slopes of the alp or shelf, will
at some point tumble over the region up which he climbed, probably in a
series of leaps or cascades.
The U-shaped valley, the "hanging" tributaries, the shelf or shoulder
running along the upper part of the cliff wall which bounds the main
valley, all these are striking features of glaciated regions. We shall
not here discuss the probable causes of this striking "break of slope,"
so different from the characteristically continuous slopes of an
ordinary mature river valley. As has been indicated, it is here that
active controversy rages. It is, however, important to note that the
shoulder or bench of which we have spoken was almost certainly once
covered by the ice, its gentle slope indicating the original valley
floor, before over-deepening took place.
The reason why pasture now grows upon it is that it is covered with fine
glacial débris, which makes fertile soil. The fertile soil, which is
often irrigated by milky water from existing glaciers, combined with the
effect of altitude upon the plants, produces rich pasturage, and makes
cattle-rearing an important alpine industry.
The next interesting feature of glaciated regions is the occurrence of
those curious mountain forms which have special names in nearly every
recently-glaciated region. Those gigantic arm-chair-shaped notches, high
up on the mountain sides, which the Welsh call cwms, the Scotch corries,
the French cirques, and the Germans kare, are very widespread in the
Highlands of Scotland, in the mountains of Wales, in the Tyrol, and in
other parts of the Alps (though they are not common in the Central
region), and in North America as well as elsewhere.
A cirque (fig. 8) is shaped something like an office arm-chair. The
floor has only a gentle downward slope, and often lodges a lake; or in
other cases it is marshy, showing that a lake was once present. The back
and sides are steep and precipitous. In some instances, if several
cirques occur near together, the side walls may be eroded through, so
that a shelf is produced, as one might produce a bench by putting two
chairs side by side, and cutting away the contiguous arms. Very often,
as one may easily see in the Highlands of Scotland, a series of cirques
occur, one above the other, so that a climber proceeding from the valley
floor upwards has a succession of steep "pitches," to use a
mountaineering term, alternating with easy if wet walks across the
floors of the successive cirques.
[Illustration: Fig. 8.--Diagram showing two glacial cirques.]
It quite often happens in the case of high mountains in the Alps that
the topmost of such a series of cirques still retains a glacier, what is
called a dead glacier, that is, one which has practically ceased to
move.
In other cases, again, we may find that what should be the flat floor
of the cirque has been largely eaten away, as it were, by a huge rounded
trough, which occupies what would be the extreme front of the seat of
the arm-chair. In this trough a stream runs, and the trough has the
characteristic U-shaped rounding characteristic of glacial forms.
Further, at the top of the wall of the trough a bench or shelf exists,
which is obviously the remains of the old cirque floor. In the case of
all characteristic glacial cirques, however, the special feature is that
the flat bottom of the cirque is discontinuous with the valley below;
they are not parts of the same system of drainage. What we may call an
unconformity appears between the two regions, more or less marked
according as running water has or has not had time to begin the work of
the removal of the unconformity.
The immediate human importance of these corries or cirques is not so
apparent as in the case of hanging valleys, but they must be mentioned,
if only because of their extraordinary abundance in glaciated regions,
and especially in Great Britain. There are two views as to their origin,
and we shall indicate both here without making any attempt to decide
which is the correct one. A very full and clear statement of one
position will be found in an article by Prof. Garwood in the
_Geographical Journal_ for September 1910, while previous articles by
Prof. Davis and others in this journal formulate the opposed view.
To the first school the corrie is simply in origin the collecting basin
of a pre-glacial stream, such a basin tending to acquire, roughly
speaking, a flattish bottom and somewhat steep sides. With the onset of
the ice the floor of the basin was protected by the ice from further
erosion, while the frost ate back the wall and so steepened it, and the
glacier carried away all débris as it formed. At a later stage the lower
part of the glacier disappeared and only the cirque glacier was left. It
continued its protective action, while below the powerful torrents
hollowed out a trough. This process was perhaps repeated several times,
with the final result that the protected cirque was left as a
much-modified remnant of pre-glacial conditions, while the valley below
was powerfully eroded by the glacial torrents. Thus a cirque lying above
an existing valley is to be regarded as the beheaded end of an old
valley, preserved by its ice covering, while below the old valley has
been fundamentally modified by the scour of the glacial torrents. On
this view the sharp distinction between the two angles of slope marks
the distinction between the work of ice (protective) and the work of
water (erosive). A series of cirques means a succession of glacial and
interglacial periods.
According to the other school, for whom ice is a more powerful eroding
agent than water, the cirque was produced by the ice, its presence or
absence, in _e. g._ the Alps, being determined by the shape of the
pre-glacial mountains. Cirques are believed to have been produced by the
ice wherever the form of the mountains conduced to the accumulation of
snow, and the occurrence of a series of cirques, and of the troughs
which seem sometimes to eat into their floors, is ascribed to the
successive retreat of the great ice-plough, _i. e._ to the action of the
retreating ice itself, and not to the water which flows from beneath it.
* * * * *
Another striking feature of many glacial valleys is a very marked want
of continuity in the slope of the main valley. Not only do the side
valleys "hang" over the main valleys, but, further, this main valley
itself often consists of relatively level reaches alternating with rocky
bars, through which the river has sometimes later cut a gorge. Examples
of this are very frequent. The famous gorge of the Aar above Meiringen
is a river gorge cut through a rocky bar of this kind.
The Pyrenees are somewhat less familiar, both to tourists and in the
form of pictures, but there, also, the same thing occurs. Above the
health resort of Cauterets lies the little Lac de Gaube, whose mouth is
blocked by a rocky bar through which the little torrent is cutting a
tiny gorge. If the tourist crosses the lake in a boat and begins to walk
up the valley above it, he will find that it has the form of a
staircase, the huge steps being separated from one another by broad
plateaux, which are flat and swampy, and have obviously been occupied by
lakes not long ago. Above each plateau there is a rocky wall, almost
precipitous, down which the stream flows in cascades. In other parts of
the Pyrenees the same phenomenon occurs, and the lakes sometimes
persist, lying one above the other in a series.
[Illustration: Fig. 9.--Profile of the Maderaner thal in Switzerland, to
show the staircase arrangement peculiar to recently glaciated valleys.
(From Garwood.)]
The phenomenon is so common that it markedly affects human life in the
Alps. The "landings," as the French call them, usually afford good
pasture ground, while those which lie at no great elevation can be
cultivated. Further, as the ground is level there is room for houses or
even for a considerable village. The intervening region or step is too
rocky to give level ground for human habitations or for pasture and
cultivation. Where the river has had time to cut a gorge, the road must
leave the stream, and can often be constructed only with difficulty. The
result is that an Alpine valley often consists of a chain of villages,
linked together by a difficult mule track or path. The abundant
water-power, however, makes mechanical traction relatively easy, and we
have sometimes the curious condition that a mule track is replaced by a
railway, without the intervention of a road fit for wheeled traffic.
We need not stop to discuss the probable cause of this step and stair
arrangement, which presents much the same problem as the series of
cirques at the head of the valley. It is enough to indicate that
according to one group of physical geographers the flat landings are due
to the way in which the gradually decreasing glacier protected its bed
from erosion, while the torrent which issued from it eroded very rapidly
below; according to another school the landings are due to direct
glacial erosion. There are other observers, again, who lay especial
stress upon the modifications of the erosive powers of running water,
due to the presence of the ice. For us it is of interest to notice that,
as has been already indicated, the staircase effect occurs also, though
on a smaller scale, in the case of mountain streams generally, some of
which must be post-glacial in origin. In other words, there seems to be
fundamental similarity between the work of ice and of water, the
differences being differences of degree rather than of kind, and due
largely to the varying fluidity of the two.
There is still one other feature of glaciated regions to which reference
must be made. This is the occurrence of peculiarly open passes in
considerable numbers across mountain regions which have been recently
glaciated. In the geography books and in some maps, the Alps, for
example, are represented as a great barrier, shutting off the fertile
plains of Italy from the countries of Central Europe. But history shows
that they have never been such a barrier, and the phrase of "splendid
traitor" has been applied to the whole mountain range, in order to
emphasise its total inadequacy as a barrier, either to armed or to
peaceful invasion.
Since the time of Napoleon I public attention has been focussed upon a
few great Alpine passes, notably the Mont Cenis, the Simplon and the St.
Gothard, which are crossed by great carriage roads, now functionally
replaced by railway tunnels beneath. But we must not forget that in
addition to these and the other great passes there are almost
innumerable ways of crossing the Alps on foot, and the presence either
of Hospices or of small inns on many of the smaller passes shows that
they are constantly used at the present time, in spite of railway
tunnels and carriage roads elsewhere. Even a pass relatively so
difficult as the Théodule, was used by very large numbers of Italian
peasants during the time when work on the Simplon railway made great
demands on Italian labour.
Any one of the passes, great or small, shows in outline the same
characters. There is a steep ascent, often steeper on the Italian than
on the other side, then a broad, windswept, open summit, sometimes
almost level, below which the rapid descent begins. Not infrequently a
lake, or lakes, may be found near the summit.
On a smaller scale the same phenomenon occurs in such glaciated regions
as Scotland, the relatively low connections between one valley system
and another greatly facilitating communication, and usually carrying
both road and railway, where the latter exists. Such connections between
two drainage systems (that is, the existence of a very low divide
between the two) only exist on a small scale outside glaciated regions,
so that they, with all their effects upon communications, must be
largely ascribed to ice-action. We shall describe one case in a little
detail, with the proviso that while no one denies the frequency of such
passes in glaciated regions, some authorities believe that their
production was due more to glacial torrents than to the erosive action
of ice itself.
A very pretty example is the picturesque pass known as the Gemmi, which
is traversed only by a mule path, and connects Kandersteg, and thus the
lake of Thun and the town of Berne, with the Rhone valley, which the
path enters at the village of Leuk. The walk proper is, however, over at
the Baths of Leuk, a small health resort lying at the foot of the great
Gemmiwand, a wall of rock over 1,600 feet in height on the summit of
which is the Gemmi pass. Readers of Mark Twain's _A Tramp Abroad_ will
remember his interesting description of the crossing of the pass, which
is part of the regulation tour in Switzerland.
The excursion may be very briefly described. The traveller starts from
the village of Kandersteg, and almost immediately begins a steep climb,
which after a rise of over 2,000 feet leads him over a ridge to a
pasture, once swept by an avalanche. Another short but steep rise (note
the staircase arrangement) leads him to the lonely Daubensee, a little
lake which is frozen for more than half the year and has no outlet. It
is itself fed by a glacier lying to the traveller's right, the Laemmern
glacier, which is shrinking and exposing more and more of its old bed.
Even to the most inexperienced traveller it is obvious that this present
day shrinkage is, as it were, the last remnant of a shrinkage which has
been going on for a prolonged period, so that the route by which the
traveller ascended from Kandersteg is but a remnant of the bed of the
old glacier. The point of special interest, however, is that at the end
of the Daubensee the traveller leaves the glacial valley by which he has
ascended, and passing through a great notch or gateway in a wall of
rock, begins the almost precipitous descent to Leukerbad, which lies at
his feet, 1,600 feet below. It is this notch which makes the pass, and
it is fundamentally a breach in the mountain wall which separates the
drainage of the Rhine from that of the Rhone. Comparing small things
with great we may note that this gateway presents some resemblance to
the Tyne and Aire Gaps in the Pennines, already mentioned, which may
also have been modified by ice-action.
The explanation given is as follows:--At the time when the glaciation
reached its maximum height the mass of ice in what is now the Laemmern
glacier was so great that it could not be contained within its own
valley. The ice was piled up so high that it over-rode the watershed,
rose up beyond the containing wall of its own valley, and pushed a long
arm over the valley wall, down into the Rhone valley. This tongue of
ice, either by its own erosive power, or because of the glacial and
sub-glacial streams which it produced, wore out a notch in the wall as
it crossed, and it is this notch which makes the pass. As the glacier
gradually shrank, it could no longer send this tributary over the wall
into the valley below, and was constrained to send all its drainage into
its own valley, that is ultimately into the Rhine. But the Gemmi pass
persists as a proof of its former magnitude, of the fact that once part
of the Laemmern drainage reached the Mediterranean instead of the North
Sea, that there was once a communication between the Rhine and the Rhone
drainage systems.
Many at least of the great Alpine passes are believed to have been
produced in this way, and therefore we must add to the peculiarities of
recently-glaciated countries, the fact that passes are likely to be
frequent across their hills and valleys, owing to the power which ice
possesses, when enormously developed, of rising above valley walls, and
streaming down into another valley system. Some of the great Alpine
passes, perhaps, arose in other ways, but this brief description may be
of interest as suggesting one, probably common, mode of origin.
If we sum up what has been said as to the special features of glaciated
regions, we may note that their valleys tend to be U-shaped, and to be
discontinuous with their tributary valleys, which "hang" over them. On
the top of the cliff from which these tributary streams leap is a shelf,
which is clearly a portion of the floor of the pre-glacial valley and is
covered by glacial débris. At the heads of the valleys there are often
cirques or plateaux, which again are markedly discordant, hanging high
above the valley below. In the main valley itself there are similar
discordances, giving rise to a staircase arrangement. Finally, different
valley systems often communicate with each other by passes, natural
highways which hang high above both valley systems alike.
Obviously, however, we might replace this detailed summary by the simple
statement that whereas in a region subjected only to the action of
running water, there is a marked tendency to continuity of slopes
throughout, a tendency more and more marked the longer the water acts,
in glaciated regions there is an equally obvious discordance, a
discontinuity of slope, most marked where water has not had time to
begin its smoothing action. As every glaciated valley which we can study
in detail has been subjected to the action both of ice and of water, it
is a simple deduction that the discontinuity is due to the differential
action of the two. This is the point of geographical importance, and to
the geographer it is of minor importance to know whether it is the
passive resistance of the ice which has caused the discontinuity, or
whether it is the water which has been unable to keep pace with the
activity of the ice.
* * * * *
There is one other point which must be alluded to even in this very
brief consideration of the effect of the ice age upon the physical
geography of the glaciated regions. This is the fact that it greatly
modified the numbers and distribution of plants and animals throughout
the areas affected. Obviously the covering of ice must have rendered a
large part of Europe uninhabitable both for man and for the vast
majority of animals and plants. In Europe, therefore, as also in North
America, there must have been a southward sweep of all living organisms,
driven from their original habitat by the onset of the cold period. But
the conditions in the two continents differed greatly.
In North America, especially in the east, there are no transverse chains
of mountains, there is no southern sea until the Gulf of Mexico is
reached in lat. 30°, and even here Florida almost touches the tropic,
and Mexico extends far beyond it. In this continent, therefore, the
plants and animals, though driven far to the south, still found room to
live and multiply, and had no great obstacle to cross either in their
southward journey, or when they strove to re-annex their old territory
as the cold conditions passed away again.
It is a curious fact that the forest trees of eastern Asia and of
eastern North America show a remarkable resemblance to one another, and
both regions are very rich in species and in genera. It is believed that
this rich North American flora is a remnant of pre-glacial conditions,
and that its persistence is due to the ease with which the trees
obtained an asylum to the south during the period when the climate was
most severe.
In Europe, in spite of the fact that the winter climate is much milder
than in corresponding latitudes in North America, the number of kinds of
forest trees is much less, there is little resemblance to those of Asia
and the eastern United States, and the trees have generally a less
southern aspect. This is the more remarkable in that trees of southern
facies introduced from China and Japan and from the United States thrive
admirably in Europe, showing that there is no climatic obstacle to their
presence there. To mention only a few examples, the Tree of Heaven
(_Ailanthus glandulosa_), so very common, even as a wild tree in many
parts of the continent of Europe, was introduced from China, while the
beautiful _Sophora japonica_, so frequently planted in towns, comes, as
its name indicates, from Japan, and the various species of those
beautiful flowering trees known as Catalpa are either American or
Asiatic. The western plane (_Platanus occidentalis_), another favourite
town tree, comes from the United States, and other American trees which
are found very abundantly in towns in the warmer parts of Europe are the
black walnut and the honey locust (_Gleditschia tricanthos_). Perhaps
more striking than any of these is the case of the so-called false
acacia (_Robinia pseudacacia_), which is as common over a great part of
the continent of Europe as hawthorn bushes or wild roses are with us,
and yet is a North American species, introduced less than three hundred
years ago. Generally, we may say that all the more beautiful trees now
growing in the warmer parts of Europe come either from eastern Asia or
from the United States. In other words, the Ice Age seems to have
greatly impoverished the flora of Europe. To a less extent this is also
true of western North America, which has fewer species of trees than the
east.
Why had the ice this impoverishing effect upon Europe? The topography of
the continent supplies the answer. In the first place, in Europe there
are numerous transverse chains of mountains. The Pyrenees, the Alps, the
Caucasus, each with its load of ice, each with glaciers deploying on the
low ground at its feet, must have been obstacles in the way of the
southern migration alike of plants and of animals. Again, even if these
obstacles were passed or turned, the great inland sea formed another
barrier further south. In consequence of this difficulty in finding
asylums the pre-glacial plants and animals must have perished in
considerable numbers, and thus a general impoverishment took place. One
must not of course exaggerate. A proportion of the pre-glacial forms did
succeed in living through the period of stress, but many must have been,
as it were, squeezed out of Europe or out of existence by the
unfavourable climatic conditions.
As the climate improved the lands swept bare once again became
inhabitable, and there was a recolonisation by movements from the south
and from the east. We shall indicate later how man himself came from
the south and the east to colonise the west and north, but his movements
were only part of a great series which included also those of plants and
animals.