Exterior of the boulder--Travelled stones a difficult problem--Once
referred to the Deluge--Other theories--Novelty of the true
solution--Icebergs formed in three ways--Progress and scenery
of an iceberg--Its effects--Size of icebergs--Boulder-clay
had a glacial origin--This explanation confirmed by fossil
shells--Laws of the distribution of life--Deductions.
Has the reader, when wandering up the course of a stream, rod in hand
perhaps, ever paused at some huge rounded block of gneiss or granite
damming up the channel, and puzzled himself for a moment to conjecture
how it could get there? Or when rolling along in a railway carriage,
through some deep cutting of sand, clay, and gravel, did the question
ever obtrude itself how such masses of water-worn material came into
existence? Did he ever wonder at the odd position of some huge grey
boulder, far away among the hills, arrested as it were on the steep
slope of a deep glen, or perched on the edge of a precipitous cliff, as
though a push with the hand would hurl it down into the ravine below?
Or did he ever watch the operations of the quarryman, and mark, as each
spadeful of soil was removed, how the surface of the rock below was all
smoothed, and striated, and grooved?
These questions, seemingly simple enough, involve what was wont to
be one of the greatest problems of geology, and not many years have
elapsed since it was solved. The whole surface of the country was
observed to be thickly covered with a series of clays, gravels, and
sands, often abounding in rounded masses of rock of all sizes up to
several yards in diameter. These deposits were seen to cover all
the harder rocks, and to occur in a very irregular manner, sometimes
heaped up into great mounds, and sometimes entirely wanting. They were
evidently the results of no agency visible now, either on the land
or around our coasts. They had an appearance rather of tumultuous
and violent action, and so it was wisely concluded that they must be
traces of the great deluge. The decision had at least this much in its
favour, it was thoroughly orthodox, and accordingly received marked
approbation, more especially from those who wished well to the young
science of geology, but were not altogether sure of its tendencies.
But, alas! this promising symptom very soon vanished. As observers
multiplied, and investigations were carried on in different countries,
the truth came out that these clays and gravels were peculiarly a
northern formation; that they did not appear to exist in the south of
France, Italy, Asia Minor, Syria, and the contiguous countries. If,
then, they originated from the rushing of the diluvian waters, these
southern lands must have escaped the catastrophe, and the site of the
plains of Eden would have to be sought somewhere between the Alps
and the North Pole. This, of course, shocked all previous ideas of
topography; it was accordingly agreed, at least among more thoughtful
men, that with these clays and sands the deluge could have had nothing
to do.
Other theories speedily sprang up, endeavouring to account for the
phenomena by supposing great bodies of water rushing with terrific
force across whole continents, sweeping away the tops of hills,
tearing up and dispersing entire geological formations, and strewing
the ocean-bottom with scattered debris. But this explanation had the
disadvantage of being woefully unphilosophical and not very clearly
orthodox. Such debacles did not appear to have ever taken place in
any previous geologic era, and experience was against them. Besides,
they did not account for some of the most evident characteristics of
the phenomena, such as the northern character of the formation, the
long parallel striations of the rock surfaces, and the perching of
huge boulders on lofty hills, often hundreds of miles distant from the
parent rock. Geologists were completely at fault, and the boulder-clay
remained a mystery for years.
When we consider the physical aspects of the countries where the
question was studied, we cannot much wonder that the truth was so hard
to find. In the midst of corn-fields and meadows, one cannot readily
realize the fact that the spot where they stand has been the site of
a wide-spread sea; and that where now villages and green lanes meet
the eye, there once swam the porpoise and the whale, or monsters of
a still earlier creation, unwieldy in bulk and uncouth in form. Such
changes, however, must have been, for their traces meet us on every
hand. We have the sea dashing against our shores, and there seems
nothing at all improbable in the assertion that once it dashed against
our hill-tops. No one, therefore, has any difficulty in giving such
statements his implicit belief. But who could have dreamed that these
fields, so warm and sunny, were once sealed in ice, and sunk beneath
a sea that was cumbered with many a wandering iceberg? Who could have
imagined, that down these glens, now carpeted with heath and harebell,
the glacier worked its slow way amid the stillness of perpetual snow?
And yet strange as it may seem, such is the true solution of the
problem. The boulder-clay was formed during the slow submergence of our
country beneath an icy sea, and the rock-surfaces owe their polished
and striated appearance to the grating across them of sand and stones
frozen into the bottom of vast icebergs, that drifted drearily from the
north. That we may the better see how these results have been effected,
let us glance for a little at the phenomena observable in northern
latitudes at the present day.
Icebergs are formed in three principal ways:--1st, By glaciers
descending to the shore, and being borne seawards by land-winds; 2d,
By river-ice packed during spring, when the upper reaches of the rivers
begin to thaw; 3d, By coast-ice.
I. There is an upper stratum of the atmosphere characterized by intense
cold, and called the region of perpetual snow. It covers the earth
like a great arch, the two ends resting, one on the arctic, the other
on the antarctic zone, while the centre, being about 16,000 feet above
the sea,[2] rises directly over the tropics. Wherever a mountain is
sufficiently lofty to pierce this upper stratum, its summit is covered
with snow, and, as the snow never melts, it is plain that, from the
accumulations of fresh snow-drifts, the mountain-tops, by gradually
increasing in height and width, would become the supporting columns
of vast hills of ice, which, breaking up at last from their weight
and width, would roll down the mountain-sides and cover vast areas
of country with a ruin and desolation more terrible than that of any
avalanche. Olympus would really be superposed upon Ossa. By a beautiful
arrangement this undue growth is prevented, so that the hill-tops
never vary much in height above the sea. The cone of ice and snow
which covers the higher part of the mountain, sends down into each of
the diverging valleys a long sluggish stream of ice, with a motion so
slow as to be almost imperceptible. These streams are called glaciers.
As they creep down the ravines and gorges, blocks of rock detached
by the frosts from the cliffs above, fall on the surface of the ice,
and are slowly carried along with it. The bottom also of the glaciers
is charged with sand, gravel, and mud, produced by the slow-crushing
movement; large rocky masses become eventually worn down into
fragments, and the whole surface of the hard rock below is traversed
by long parallel grooves and striæ in the direction of the glacier's
course. Among the Alps, the lowest point to which the glacier descends
is about 8500 feet. There the temperature gets too high to allow of its
further progress, and so it slowly melts away, choking up the valleys
with piles of rocky fragments called moraines, and 'giving rise to
numerous muddy streams that traverse the valleys, uniting at length
into great rivers such as the Rhone, which enters the Lake of Geneva
turbid and discoloured with glacial mud.
[Footnote 2: The average height of the snow-line within the tropics
is 15,207 feet, but it varies according to the amount of land and sea
adjacent, and other causes. Thus, among the Bolivian Andes, owing to
the extensive radiation, and the ascending currents of air from the
neighbouring plains and valleys, the line stands at a level of 18,000
feet, while, on mountains near Quito, that is, immediately on the
equatorial line, the lowest level is 15,795.--See Mrs. Somerville's
_Physical Geography_, 4th edit. p. 314.]
In higher latitudes, where the lower limit of the snow-line descends to
the level of the sea, the glaciers are often seen protruding from the
shore, still laden with blocks that have been carried down from valleys
far in the interior. The action of storms and tides is sufficient to
detach large masses of the ice, which then floats off, and is often
wafted for hundreds of miles into temperate regions, where it gradually
melts away. Such floating islands are known as icebergs.
II. In climates such as that of Canada, where the winters are very
severe, the rivers become solidly frozen over, and, if the frost be
intense enough, a cake of ice forms at the bottom. In this way sand,
mud, and rocky fragments strewing the banks or the channel of the
stream, are firmly enclosed. When spring sets in, and the upper parts
of the rivers begin to thaw, the swollen waters burst their wintry
integuments, and the ice is then said to _pack_. Layer is pushed
over layer, and mass heaped upon mass, until great floes are formed.
These have often the most fantastic shapes, and are borne down by the
current, dropping, as they go, the mud and boulders, with which they
are charged, until they are stranded along some coast line, or melt
away in mid-ocean.
III. But icebergs are also produced by the freezing of the water of
the ocean. In high latitudes, this takes place when the temperature
falls to 28·5° of Fahrenheit. The surface of the sea then parts with its
saline ingredients, and takes the form of a sheet of ice, which, by
the addition of successive layers, augmented sometimes by snow-drifts,
often reaches a height of from thirty to forty feet. On the approach of
summer these ice-fields break up, crashing into fragments with a noise
like the thundering of cannon. The disparted portions are then carried
towards the equator by currents, and may be encountered by hundreds
floating in open sea. Their first form is flat, but, as they travel on,
they assume every variety of shape and size.
On the shores of brackish seas, such as the Baltic, or along a coast
where the salt water is freshened by streams or snow-drifts from the
land, sheets of ice also frequently form during severe frosts. Sand and
boulders are thus frozen in, especially where a layer of ice has formed
upon the sea-bottom.[3] The action of gales or of tides is sufficient
to break up these masses, which are then either driven ashore and
frozen in a fresh cake of ice, or blown away to sea. The bergs formed
in this way have originally a low flat outline, and many extend as
ice-fields over an area of many miles, while, at a later time, they may
be seen towering precipitously as great hills, some 200 or 300 feet
high.
[Footnote 3: I was informed by the late Mr. Hugh Miller, that a seam
of shale abounding in liassic fossils, had been found intercalated
among the boulder-clay beds in the vicinity of Eathie. He explained its
occurrence there by supposing that it had formed a reef along a shore
where ground-ice was forming; and so having been firmly frozen in, it
was torn up on the breaking of the ice, and deposited at a distance
among the mud at the sea-bottom.]
Few sights in nature are more imposing than that of the huge,
solitary iceberg, as, regardless alike of wind and tide, it steers
its course across the face of the deep far away from land. Like one
of the "Hrim-thursar," or Frost-giants of Scandinavian mythology,[4]
it issues from the portals of the north armed with great blocks of
stone. Proudly it sails on. The waves that dash in foam against its
sides shake not the strength of its crystal walls, nor tarnish the
sheen of its emerald caves. Sleet and snow, storm and tempest, are its
congenial elements. Night falls around, and the stars are reflected
tremulously from a thousand peaks, and from the green depths of
"caverns measureless to man." Dawn again arises, and the slant rays of
the rising sun gleam brightly on every projecting crag and pinnacle,
as the berg still floats steadily on; yet, as it gains more southern
latitudes, what could not be accomplished by the united fury of the
waves, is slowly effected by the mildness of the climate. The floating
island becomes gradually shrouded in mist and spume, streamlets
everywhere trickle down its sides, and great crags ever and anon fall
with a sullen plunge into the deep. The mass becoming top-heavy,
reels over, exposing to light rocky fragments still firmly imbedded.
These, as the ice around them gives way, are dropped one by one into
the ocean, until at last the iceberg itself melts away, the mists are
dispelled, and sunshine once more rests upon the dimpled face of the
deep.[5] If, however, before this final dissipation, the wandering
island should be stranded on some coast, desolation and gloom are
spread over the country for leagues. The sun is obscured, and the air
chilled; the crops will not ripen; and, to avoid the horrors of famine,
the inhabitants are fain to seek some more genial locality until the
ice shall have melted away; and months may elapse before they can
return again to their villages.
[Footnote 4: The account of the origin of these giants, as given in
the prose _Edda_, is very graphic, and may be not inaptly quoted
here:--"When the rivers that are called Elivagar had flowed far from
their sources," replied Har, "the venom which they rolled along;
hardened, as does dross that runs from a furnace, and became ice.
When the rivers flowed no longer, and the ice stood still, the vapour
arising from the venom gathered over it and froze to rime; and in this
manner were formed in Ginnungagap many layers of congealed vapour,
piled one over the other."--"That part of Ginnungagap," added Jafnhar,
"that lies towards the north, was thus filled with heavy masses of
gelid vapour and ice, whilst everywhere within were whirlwinds and
fleeting mists. But the southern part of Ginnungagap was lighted by
the sparks and flakes that flew into it from Muspellheim.... When the
heated blast met the gelid vapour, it melted into drops, and, by the
might of him who sent the heat, these drops quickened into life, and
took a human semblance. The being thus formed was named Ymir, from whom
descend the race of the Frost-giants (Hrim-thursar), as it is said in
the Völuspá, 'From Vidolph came all witches; from Vilmeith all wizards;
from Svarthöfdi all poison-seekers; and all giants from Ymir.'"--See
Mallet's _Northern Antiquities_, edit. Bohn, p. 402.]
[Footnote 5: That beautiful expression of Æschylus occurs to me, so
impossible adequately to clothe in English: ἁνηριθμον γελασμα κυματων.
Who that has spent a calm summer day upon the sea, has not realized its
force and delicate beauty?]
The iceberg melts away, but not without leaving well-marked traces of
its existence. If it disappear in mid-ocean, the mud and boulders, with
which it was charged, are scattered athwart the sea-bottom. Blocks
of stone may thus be carried across profound abysses, and deposited
hundreds of miles from the parent hill; and it should be noticed, that
this is the only way, so far as we know, in which such a thing could
be effected. Great currents could sweep masses of rock down into deep
gulfs, but could not sweep them up again, far less repeat this process
for hundreds of miles. Such blocks could only be transported by being
lifted up at the one place and set down at the other; and the only
agent we know of, capable of carrying such a freight, is the iceberg.
In this way, the bed of the sea in northern latitudes must be covered
with a thick stratum of mud and sand, plentifully interspersed with
boulders of all sizes, and its valleys must gradually be filled up as
year by year the deposit goes on.
But this is not all. The visible portion of an iceberg is only about
one-ninth part of the real bulk of the whole mass, so that if one be
seen 100 feet high, its lowest peak may perhaps be away down 800 feet
below the waves. Now it is easy to see that such a moving island will
often grate across the summit and along the sides of submarine hills;
and when the lower part of the berg is roughened over with earth and
stones, the surface of the rock over which it passes will be torn up
and dispersed, or smoothed and striated, while the boulders imbedded in
the ice will be striated in turn.
But some icebergs have been seen rising 300 feet over the sea; and
these, if their submarine portions sank to the maximum depth,
must have reached the enormous total height of 2700 feet--that is,
rather higher than the Cheviot Hills.[6] By such a mass, any rock or
mountain-top existing 2400 feet below the surface of the ocean would be
polished and grooved, and succeeding bergs depositing mud and boulders
upon it, this smoothed surface might be covered up and suffer no change
until the ocean-bed should be slowly upheaved to the light of day.
In this way, submarine rock surfaces at all depths, from the coast
line down to 2000 or 3000 feet, may be scratched and polished, and
eventually entombed in mud.
[Footnote 6: In the _American Journal of Science_ for 1843, p.
155, mention is made of an iceberg aground on the Great Bank of
Newfoundland. The average depth of the water was about 500 feet, and
the visible portion of the berg from 50 to 70 feet high, so that its
total height must have been little short of 600 feet, of which only a
_tenth_ part remained above water.]
[Illustration: Fig. 1. Iceberg grating along the sea-bottom and
depositing mud and boulders.]
And such has been the origin of the deep clay, which, with its included
and accompanying boulders, covers so large a part of our country. When
this arctic condition of things began, the land must have been slowly
sinking beneath the sea; and so, as years rolled past, higher and yet
higher zones of land were brought down to the sea-level, where floating
ice, coming from the north-west, stranded upon the rocks, and scored
them all over as it grated along. This period of submergence may have
continued until even the highest peak of the Grampians disappeared,
and, after suffering from the grinding action of ice-freighted rocks,
eventually lay buried in mud far down beneath a wide expanse of sea,
over which there voyaged whole argosies of bergs. When the process of
elevation began, the action of waves and currents would tend greatly to
modify the surface of the glacial deposit of mud and boulders, as the
ocean-bed slowly rose to the level of the coast line. In some places
the muddy envelope was removed, and the subjacent rock laid bare,
all polished and grooved. In other localities, currents brought in a
continual supply of sand, or washed off the boulder mud and sand, and
then re-deposited them in irregular beds; hence resulted those local
deposits of stratified sand and gravel so frequently to be seen resting
over the boulder clay. At length, by degrees, the land emerged from the
sea, yet glaciers still capped its hills and choked its valleys; but
eventually a warmer and more genial climate arose, plants and animals,
such as those at present amongst us, and some, such as the wolf, no
longer extant, were ere long introduced; and eventually, as lord of the
whole, man took his place upon the scene.[7]
[Footnote 7: The reader who wishes to enter more fully into the
geological effects of icebergs, should consult the suggestive section
on that subject in De la Beche's _Geological Observer_; also the
_Principles_ and _Visit to the United Stales_ of Sir Charles Lyell,
with the various authorities referred to by these writers.]
It is pleasant to mark, when once the true solution of a difficulty is
obtained, how all the discordant elements fall one by one into order,
and how every new fact elicited tends to corroborate the conclusion. In
some parts of the glacial beds, there occur regular deposits of shells
which must have lived and died in the places where we find them. From
ten to fifteen per cent, of them belong to species which are extinct,
that is to say, have not been detected living in any sea. Some of them
are still inhabitants of the waters around our coasts, but the large
majority occur in the northern seas. They are emphatically northern
shells, and get smaller in size and fewer in number as they proceed
southward, till they disappear altogether. In like manner, the palm,
on the other hand, is characteristically a tropical plant. It attains
its fullest development in intertropical countries, getting stunted
in its progress towards either pole, and ceasing to grow in the open
air beyond the thirty-eighth parallel of latitude in the southern
hemisphere, and the forty-fifth in the northern. So, too, the ivy,
which in our country hangs out its glossy festoons in every woodland,
and around the crumbling walls of abbey, and castle, and tower, is
nursed in the drawing-rooms of St. Petersburg as a delicate and
favourite exotic. In short, the laws which regulate the habitat of a
plant or an animal are about as constant as those which determine its
form. There are, indeed, exceptions to both. We may sometimes find a
stray vulture from the shores of the Mediterranean gorging itself on
sheep or lambs among the wolds of England,[8] just as we often see
"A double cherry seeming parted,
But yet an union in partition;"
or as we hear of a sheep with five legs, and a kid with two heads.
But these exceptions, from their comparative rarity, only make the
laws more evident. When, therefore, we find, in various parts of our
country, beds of shells in such a state of preservation as to lead
us to believe that the animals must have lived and died where their
remains are now to be seen, we justly infer that the districts where
they occur must at one period have been submerged. If the shells
belong to fresh-water species, it is plain that they occur on the site
of an old lake. If they are marine, we conclude that the localities
where they are found no matter how high above the sea must formerly
have stood greatly lower, so as to form the ocean bed. To proceed one
step further. If the shells are of a southern type, that is, if they
belong to species[9] which are known to exist only in wanner seas than
our own, we pronounce that at a former period the latitudes of Great
Britain must have enjoyed a more temperate and genial climate, so as
to allow southern shells to have a wider range northwards. If, on the
other hand, they are of an arctic or boreal type, we in the same way
infer that our latitudes were once marked by a severer temperature than
they now possess, so as to permit northern shells to range farther
southwards. This reasoning is strictly correct, and the truth involved
forms the basis of all inquiries into the former condition of the earth
and its inhabitants.
[Footnote 8: Two of these birds (_Neopron pecnopterus_) are stated to
have been seen near Kilve, in Somersetshire, in October 1825. One was
shot, the other escaped.]
[Footnote 9: There is not a little difficulty in reasoning
satisfactorily as to climatal conditions, from the distribution of
kindred forms. Even in a single genus there may be a wide range of
geographical distribution, so that mere generic identity is not always
a safe guide. Thus, the elephant now flourishes in tropical countries,
but in the glacial period a long-haired species was abundant in the
frozen north. I have above restricted myself entirely to _species_
whose habits and geographical distribution are already sufficiently
known.]
The evidence furnished by the northern shells in the boulder-clay
series is, accordingly, of the most unmistakable kind. These organisms
tell us that at the time they lived our country lay sunk beneath a sea,
such as that of Iceland and the North Cape, over which many an iceberg
must have journeyed, and thus they corroborate our conclusions, derived
independently from the deep clay and boulder beds and the striated
rock-surfaces, as to the glacial origin of the boulder-clay.