COLD AND ICE ON THE EARTH
The astronomers who look at the planet Mars tell us that at the
Northern and Southern Poles there are great areas of snow, very much
greater than the arctic regions of the earth, for the south polar
area alone occupies 11,330,000 square miles. But the geological
records of the earth show that our own arctic regions once extended
very much farther than they do at present, a fact which need not in
any way surprise us, for as we have already remarked, snow and ice
are very largely a matter of the nearness of the sea to the land.
We may put the same thing in another way by saying that winter
cold and summer heat depend largely on the distribution of sea and
land. Thus Venice, which is not very much farther from the North
Pole than Vladivostok, has an altogether different climate, and
inhabitants of the Shetland Isles have a very different kind of
climatic experience from those who delve by the frozen Yukon. There
is another consideration which is sometimes overlooked. We do not
think of the earth as a very warm body. But at its very coldest
part, where the thermometer goes down to seventy or ninety degrees
below freezing, it is several hundreds of degrees warmer than the
space outside the earth. Midway between the earth and the moon the
temperature must be 430° F. below freezing; so that if we take the
surface of the earth as a whole we may say that it is between four
hundred and five hundred degrees warmer than the space by which it is
surrounded. Every one knows what is happening when he warms his hands
at a fire. The fire being hotter than its surroundings is giving out
heat towards them, and the hands catch some of this radiated heat.
Similarly the earth is radiating heat, and the atmosphere round the
earth catches some of it. So also do the seas. While therefore it
is certain that the heat of the sun warms the earth and the air and
the sea, and so gives rise to currents of air and perhaps of water,
so also is it likely that the heat of the earth causes warm air to
rise, and so plays its part in forming the winds, the currents of
air, and the currents of water. When the earth was warmer than it is
now it had more and greater effects in this direction. It caused more
evaporation of the water, more clouds, and therefore more rain, and
in winter more snow.
Suppose, then, a period when there was very much more snow in winter
than now. As the snow accumulated layer on layer the lower part would
become squeezed into a mass half ice, half snow; and it is quite
likely that the heat of the summer sun (especially if, as we have
supposed, the atmosphere was much cloudier then than now) would be
unable to dissolve it. Thus the snow age would gradually merge into
an ice age, and we can imagine a period when a great deal of Europe
was covered with snow or ice as Greenland is to-day. What records
would it leave behind? Now, on slopes the sheet of snow would tend to
slip just as it slips from sloping house-roofs. In doing so it would
push before it any loose material which lay in front of it; and trees
or bushes, stone or soil, would be gradually pushed downhill. If the
slopes were steep enough the snow-sheets would occasionally break
off and sweep down as avalanches. Sometimes these great masses are
many thousands of square yards in area and fifty feet thick, and in
the late winter and early spring often do immense damage, carrying
away houses, trees, and great masses of rocks in their progress. They
leave their imprint not only in ground swept bare, but in huge mounds
of debris piled up in the valleys below.
But when the snow has taken the form of a glacier its record is left
in more unmistakable characters. Imagine a great mass of snow and
ice descending between the clefts of mountains to lower levels. As
it slips slowly down its valley, like a very slow river--slower than
a river of thickest mud or pitch or lava would be--earth, sand, mud,
gravel, boulders, and masses of rock sometimes are washed down on its
surface from the slopes on either side. Avalanches will occasionally
bring other contributions. Nearly all this rubbish accumulates on
the edges of the glacier nearest the slopes, and it is slowly borne
to the journey's end on the glacier's shoulders. Some of it falls
into rents or crevasses in the ice, and may be imprisoned there and
carried down as an inside passenger, or it may reach the rocky floor
over which the ice is sliding. Its progress then resembles that of
the Irish gentleman who was travelling in a Sedan chair out of which
the seat and the bottom had fallen, and who said that if 'twere not
for the fashion of the thing he'd as lief walk. The rubbish borne
onward on the surface of the glacier is known as _moraine-stuff_,
and the mounds of it at the edge of the glacier are called _lateral
moraines_. Where two glaciers unite like two rivers, their moraines,
right-hand and left-hand, will join, and in the new glacier a new
moraine will appear running down the middle, and so called a _medial
moraine_. Where a glacier has many tributaries bearing a good deal
of moraine-stuff, its surface may be like a bare plain so covered
with stones that the ice beneath can hardly be discerned except here
and there. At the end of the glacier where the ice melts, the heaps
of stones, ever adding to their numbers, are deposited in heaps, to
which are given the name of the _terminal moraine_.
With such tokens of their existence as this, glaciers, as will
readily be understood, leave visiting-cards in history that cannot
easily be mistaken. Even existing glaciers tell strange stories.
Nowadays glaciers are carefully measured and examined both in
Switzerland and in Canada. During the last decade of the nineteenth
century and the first decade of the twentieth the Swiss glaciers
were found to show signs of receding farther up their valleys. The
same thing has been observed about some of the Canadian glaciers.
There are several plausible reasons for this. Professor Schaeberle
says that the earth is growing cooler, and that in the temperate
regions the winter rainfall (which would turn to falls of snow
in the mountains) is less than it has been. It is certain that a
shortage of winter rain over a number of years in succession would
account for the shrinkage of the glaciers, but it is not by any
means certain that a number of dry winters will not be followed by
wet ones, in which case the glaciers would increase again. Some of
the glaciers show that during their existence they have shrunk and
lengthened alternately like gutta-percha in a variable climate. How
do we know that they have shrunk and lengthened? The _moraines_ of
which we have spoken give us the testimony. As a glacier shrinks
either in length or in breadth and depth it leaves the blocks of
rock at its edges stranded on the sides of the valley. Such perched
blocks or _erratics_ are the best of glacier marks, and their great
size, some of them as large as a cheap villa residence, is such that
no current of water could have brought them there. They are often
poised on the tops of crags, on the very edges of precipices, or on
steep slopes, where they could never have been left by any flood,
even had the flood been able to move them. The only thing that could
have carried them must have been a vehicle that moved very, very
slowly and deposited them very, very gently--in fact, glacier ice.
We can see blocks like this on the glaciers now, and others stranded
at the sides. In the Swiss valleys the scattered ice-borne boulders
may be seen by hundreds far above the glaciers and far beyond the
places where the glaciers end. We _know_ they must have been left
by glaciers, and by inference we surmise that when we find a valley
filled with them, then, though the valley may have no glacier now,
it must have once been occupied by one.
These erratic blocks, now found all over Europe, tell us a good deal
about the ice on which they were transported. The blocks that fall
on the edges of the glacier remain on the side where they descend.
Hence, if there is any notable difference in the composition of the
rocks on either side of the valley, the existence of this difference
will be preserved in the _moraines_. If, therefore, in a country
where the glaciers have disappeared we can trace the scattered
blocks up to their sources among the mountains, we can say what was
the track followed by the prehistoric glaciers. In Europe there are
several examples of the uses of this detective evidence. Thus the
peculiar blocks of the Valais mountains can be traced right on to
Lyons; and this shows us that the glacier from which the River Rhone
sprang extended once right across the east of France to Lyons, and
probably farther. It was therefore once at least 170 miles longer.
Similarly, blocks which are exactly like the characteristic rocks
peculiar to Southern Scandinavia are found in Northern Germany,
Belgium, and East Anglia; and we therefore believe that a great sheet
of ice once filled up the Baltic and the German Ocean, carrying with
it immense numbers of northern "erratics." In our own country, in
fact, glacier boulders are found in nearly every county, and show
that once the greater part of the country was buried under ice.
But, as we have said, it is not only on its shoulders, but in its
interior and beneath its base that a glacier rolls and pushes its
rubbish along. It is not all stones. Clay and earth mingle with
it, often enclosing the stones; and the debris left by extinct
glaciers of ages ago is sometimes called the boulder-clay. This is
the deposit, earthy and stony, that the glacier leaves on the floor
of the valley as it shrinks--unless the river which usually springs
from the end of glaciers sweeps it away. Most of the stones thus left
are smoothed or polished and covered with scratches or ruts, such
as would be made by rubbing against other hard pointed fragments of
stone. This is to be explained by the fact that these stones as they
were carried on by the glacier were rubbed on the floor of rock over
which the glacier was slipping. If their journey was long enough,
they stood a chance of being rubbed away altogether and of finishing
their existence as sand or mud. What the valley did to the glacier's
stones, the stones did to the valley. They scratched it and scored
it. Every promontory of rock which stood in the path of the ice had
its angles and corners ground away. The polish and the directions of
the scratches are especially remarkable, because, whether the marks
are mere lines or deep-worn ruts, they are all on smooth surfaces,
and they all run one way. That way is the direction in which the
glacier moved. How high a degree of polish or how deep the markings
may be depends a good deal on the kind of rock over which the glacier
moved. Tough, close-grained rocks, such as hard limestone, are
sometimes polished to look like marble. But there is a great deal of
difference between the smoothing effected by a river or a torrent and
that which is produced by a glacier, because the river tosses the
rocks and stones in all directions, polishes them on every side, and
leaves no distinctive parallel scratches or grooves on them. That can
only be done by glaciers which hold the rocks, the rubbers and the
rubbed, pressed firmly together and grind them continually in the
same way.
These scratchings or striations of rocks, the smoothed and grooved
surfaces, and the deposited boulder-clay and boulders enable us to
trace the march of great ice-sheets over regions of the earth which
are now of totally different aspect. From this kind of evidence
we have been able to find that the whole of Northern Europe was
once buried under a great expanse of snow and ice. The sheet was,
as we should expect, thickest in the north and west, and thinned
away southward and eastward. Over Scandinavia it was between 6000
and 7000 feet in thickness--as we can tell from the scratches on
the sides of the high mountains. Similar marks 3000 feet above the
sea-level in the Scottish Highlands lead us to believe that over
Scotland the glaciers were 5000 feet thick, and even as far south as
the Hartz Mountains in Germany it could not have been far short of
1500 feet in thickness. Imagine this great mass of ice ever slowly
moving and ever creeping solemnly down to the sea. By the markings
it left we can trace where the greater glaciers slid grandly along.
In Scandinavia it swept westwards to the Atlantic and eastwards to
the Gulf of Bothnia, then frozen as solid as the Pole. Southward the
ice ground its way across Denmark to the Low Countries and North
Germany. The Baltic was choked with ice, and so was the North Sea
as far south as London. Ice in that day flowed in glaciers from the
British Isles, eastwards from Scotland into the hollow of the North
Sea, and westwards down all the clefts of the mountains, burying the
western isles and breaking off in icebergs that drifted far into the
Atlantic. Sir Archibald Geikie says that the western margin of the
ice-fields from the south-west of Ireland to the North Cape of Norway
must have presented a vast wall of ice 1200 to 1500 miles long and
hundreds of feet high--like that great barrier which the Antarctic
explorers tell us frown on the waters that lap the boundaries of
the south polar land. Northern Europe must have been like North
Greenland of to-day. The rock scratches tell us (since even the
southern coast of Ireland is intensely ice-worn) that the edge of
ice must have extended some distance beyond Cape Clear, rising out
of the sea with a precipitous front that faced to the south. Thence
the ice-cliff swung eastwards, passing probably along the line of the
British Channel and keeping to the north of the valley of the Thames.
Its southern margin ran across what is now Holland and skirted the
high grounds of Westphalia, Hanover, and the Hartz Mountains--which
probably barred its further progress southward. "There is evidence
that the ice swept round into the lowlands of Saxony up to the chain
of the Erz, Riesen, and Sudenten Mountains, whence its southern limit
turned eastward across Silesia, Poland, and Galicia, and then swung
round to the north, passing across Russia by way of Kieff and Nijni
Novgorod to the Arctic Ocean."
In North America there are similar traces of the great ice-sheet,
one of the branches of which streamed southward into the basin of
the Mississippi, the second moving westward from Hudson Bay to the
Rockies, and southward to Iowa, and the third setting out from the
great mountain ranges of British Columbia. Right across North America
to-day for thousands of miles stretch accumulations and mounds of
rock which were pushed forward by the ice, and were dropped by the
glaciers when they reached "farthest south." These accumulations are
called, from their origin, the great "terminal moraines" of the North
American Ice Age.
It must not be thought that these great ice-sheets of both
hemispheres remained constant in extent and thickness. There were
periods of retreat and advance, of progress and shrinking, and the
shrinkings of these took place on a large scale, and perhaps lasted
for hundreds or thousands of years; so that mixed with the strata
of boulder-clay, which are the characteristic strata of the glacial
periods, are other strata of sand, ordinary clay, and even peat.
Remains of plants and animals are found in these strata, showing
that sometimes the glaciers retreated so far and for so long that
vegetation sprang up and animals lived on the ground that they had
covered--in the intervals when the cold of centuries was replaced by
other centuries of mild and equable climate.
At last, after many of these swallow-like retreats and advances,
the warmer climate at length came to stay, and the ice retreated
farther and farther to the north. It still remained among the
mountains, so that we might describe the glaciers of the Alps
and of the Canadian Rockies as the last relics existing to-day of
the great Ice Age of the past. The retreat to the Arctic Circle
left many other relics behind it, the great lakes, for example,
like Winnipeg and Manitoba, and the Great Salt Lake of Utah. All
were once mightier sheets, because during the Ice Age their waters
were held back. Other smaller lakes formed by the dumping-heaps or
terminal moraines of the glaciers still exist, and are especially
noticeable in Finland. During the later stages of the Ice Age the
level of the land was lower than it is now in Western Europe. When
the ground began to rise--in slow upheavals with long pauses for
rest--it left its impress in raised beaches, which can be seen on
both sides of Scotland and on the Norwegian coast. The climate grew
gradually milder, the animals and the plants followed in the train of
the retreating ice, and even the traces of man's existence began to
appear. The change was not sudden; it was so gradual that the Ice Age
slipped as imperceptibly as its own glaciers into the age in which
Man's activities in Europe began.