The two cataclysms, of which we have spoken, surprised Europe at the
moment of the development of an important creation. The whole scope of
animated Nature, the evolution of animals, was suddenly arrested in that
part of our hemisphere over which these gigantic convulsions spread,
followed by the brief but sudden submersion of entire continents.
Organic life had scarcely recovered from the violent shock, when a
second, and perhaps severer blow assailed it. The northern and central
parts of Europe, the vast countries which extend from Scandinavia to the
Mediterranean and the Danube, were visited by a period of sudden and
severe cold: the temperature of the polar regions seized them. The
plains of Europe, but now ornamented by the luxurious vegetation
developed by the heat of a burning climate, the boundless pastures on
which herds of great Elephants, the active Horse, the robust
Hippopotamus, and great Carnivorous animals grazed and roamed, became
covered with a mantle of ice and snow.
To what cause are we to attribute a phenomenon so unforeseen, and
exercising itself with such intensity? In the present state of our
knowledge no certain explanation of the event can be given. Did the
central planet, the sun, which was long supposed to distribute light and
heat to the earth, lose during this period its calorific powers? This
explanation is insufficient, since at this period the solar heat is not
supposed to have greatly influenced the earth’s temperature. Were the
marine currents, such as the _Gulf Stream_, which carries the Atlantic
Ocean towards the north and west of Europe, warming and raising its
temperature, suddenly turned in the contrary direction? No such
hypothesis is sufficient to explain either the cataclysms or the glacial
phenomena; and we need not hesitate to confess our ignorance of this
strange, this mysterious, episode in the history of the globe.
There have been attempts, and very ingenious ones too, to explain these
phenomena, of which we shall give a brief summary, without committing
ourselves to any further opinion, using for that purpose the information
contained in M. Ch. Martins’ excellent work. “The most violent
convulsions of the solid and liquid elements,” says this able writer,
“appear to have been themselves only the effects due to a cause much
more powerful than the mere expansion of the pyrosphere; and it is
necessary to recur, in order to explain them, to some new and bolder
hypothesis than has yet been hazarded. Some philosophers have belief in
an astronomical revolution which may have overtaken our globe in the
first age of its formation, and have modified its position in relation
to the sun. They admit that the poles have not always been as they are
now, and that some terrible shock displaced them, changing at the same
time the inclination of the axis of the rotation of the earth.” This
hypothesis, which is nearly the same as that propounded by the Danish
geologist, Klee, has been ably developed by M. de Boucheporn. According
to this writer, many multiplied shocks, caused by the violent contact of
the earth with comets, produced the elevation of mountains, the
displacement of seas, and perturbations of climate--phenomena which he
ascribes to the sudden disturbance of the parallelism of the axis of
rotation. The antediluvian equator, according to him, makes a right
angle with the existing equator.
“Quite recently,” adds M. Martins, “a learned French mathematician, M.
J. Adhémar, has taken up the same idea; but, dismissing the more
problematical elements of the concussion with comets as untenable, he
seeks to explain the deluges by the laws of gravitation and celestial
mechanics, and his theory has been supported by very competent writers.
It is this: We know that our planet is influenced by two essential
movements--one of rotation on its axis, which it accomplishes in
twenty-four hours; the other of translation, which it accomplishes in a
little more than 365¼ days. But besides these great and perceptible
movements, the earth has a third, and even a fourth movement, with one
of which we need not occupy ourselves; it is that designated _nutation_
by astronomers. It changes periodically, but within very restricted
limits, the inclination of the terrestrial axis to the plane of the
ecliptic by a slight oscillation, the duration of which is only eighteen
hours, and its influence upon the relative length of day and night
almost inappreciable. The other movement is that on which M. Adhémar’s
theory is founded.
“We know that the curve described by the earth in its annual revolution
round the sun is not a circle, but an ellipse; that is, a slightly
elongated circle, sometimes called a circle of two centres, one of which
is occupied by the sun. This curve is called the ecliptic. We know,
also, that, in its movement of translation, the earth preserves such a
position that its axis of rotation is intercepted, at its centre, by the
plane of the ecliptic. But in place of being perpendicular, or at right
angles with this plane, it crosses it obliquely in such a manner as to
form on one side an angle of one-fourth, and on the other an angle of
three-fourths of a right angle. This inclination is only altered in an
insignificant degree by the movement of _nutation_. I need scarcely add
that the earth, in its annual revolution, occupies periodically four
principal positions on the ecliptic, which mark the limits of the four
seasons. When its centre is at the extremity most remote from the sun,
or _aphelion_, it is the summer solstice for the northern hemisphere.
When its centre is at the other extremity, or _perihelion_, the same
hemisphere is at the winter solstice. The two intermediate points mark
the equinoxes of spring and autumn. The great circle of separation of
light and shade passes, then, precisely through the poles, the day and
night are equal, and the line of intersection of the plane of the
equator and that of the ecliptic make part of the vector ray from the
centre of the sun to the centre of the earth--what we call the
_equinoctial line_.
“Thus placed, it is evident that if the terrestrial axis remained always
parallel to itself, the equinoctial line would always pass through the
same point on the surface of the globe. But it is not absolutely thus.
The parallelism of the axis of the earth is changed slowly, very slowly,
by a movement which Arago ingeniously compares to the varying
inclination of a top when about to cease spinning. This movement has the
effect of making the equinoctial points on the surface of the earth
retrograde towards the east from year to year, in such a manner that at
the end of 25,800 years according to some astronomers, but 21,000 years
according to Adhémar, the equinoctial point has literally made a circuit
of the globe, and has returned to the same position which it occupied at
the beginning of this immense period, which has been called the ‘_great
year_.’ It is this retrograde evolution, in which the terrestrial axis
describes round its own centre that revolution round a double conic
surface, which is known as the _precession of the equinoxes_. It was
observed 2,000 years ago by Hipparchus; its cause was discovered by
Newton; and its complete evolution explained by D’Alembert and Laplace.
“Now, we know that the consequence of the inclination of the terrestrial
axis with the plane of the ecliptic is--
“1. That the seasons are inverse to the two hemispheres--that is to
say, the northern hemisphere enjoys its spring and summer, while the
southern hemisphere passes through autumn and winter.
“2. When the earth approaches nearest to the sun, our hemisphere has its
autumn and winter; and the regions near the pole, receiving none of the
solar rays, are plunged into darkness, approaching that of night, during
six months of the year.
“3. When the earth is most distant from the sun, when much the greater
half of the ecliptic intervenes between it and the focus of light and
heat, the pole, being then turned towards this focus, constantly
receives its rays, and the rest of the northern hemisphere enjoys its
long days of spring and summer.
“Bearing in mind that, in going from the equinox of spring to the
autumnal equinox of our hemisphere, the earth traverses a much longer
curve than it does on its return; bearing in mind, also, the accelerated
movement it experiences in its approach to the sun from the attraction,
which increases in inverse proportion to the square of its distance, we
arrive at the conclusion that our summer should be longer and our winter
shorter than the summer and winter of our antipodes; and this is
_actually_ the case by about eight days.
“I say _actually_, because, if we now look at the effects of the
precession of the equinoxes, we shall see that in a time equal to half
of the _grand year_, whether it be 12,900 or 10,500 years, the
conditions will be reversed; the terrestrial axis, and consequently the
poles, will have accomplished the half of their bi-conical revolution
round the centre of the earth. It will then be the northern hemisphere
which will have the summers shorter and the winters longer, and the
southern hemisphere exactly the reverse. In the year 1248 before the
Christian era, according to M. Adhémar, the north pole attained its
maximum summer duration. Since then--that is to say for the last 3,112
years--it has begun to decrease, and this will continue to the year 7388
of our era before it attains its maximum winter duration.
“But the reader may ask, fatigued perhaps by these abstract
considerations, What is there here in common with the deluges?
“The _grand year_ is here divided, for each hemisphere, into two great
seasons, which De Jouvencel calls the great summer and winter, which
will each, according to M. Adhémar, be 10,500 years.
“During the whole of this period one of the poles has constantly had
shorter winters and longer summers than the other. It follows that the
pole which experiences the long winter undergoes a gradual and
continuous cooling, in consequence of which the quantities of ice and
snow, which melt during the summer, are more than compensated by those
which are again produced in the winter. The ice and snow go on
accumulating from year to year, and finish at the end of the period by
forming, at the coldest pole, a sort of crust or cap, vast, thick, and
heavy enough to modify the spheroidal form of the earth. This
modification, as a necessary consequence, produces a notable
displacement of the centre of gravity, or--for it amounts to the same
thing--of the centre of attraction, round which all the watery masses
tend to restore it. The south pole, as we have seen, finished its _great
winter_ in 1248 B.C. The accumulated ice then added itself to the snow,
and the snow to the ice, at the south pole, towards which the watery
masses all tended until they covered nearly the whole of the southern
hemisphere. But since that date of 1248, our _great winter_ has been in
progress. Our pole, in its turn, goes on getting cooler continually; ice
is being heaped upon snow, and snow upon ice, and in 7,388 years the
centre of gravity of the earth will return to its normal position, which
is the geometrical centre of the spheroid. Following the immutable laws
of central attraction, the southern waters accruing from the melted ice
and snow of the south pole will return to invade and overwhelm once more
the continents of the northern hemisphere, giving rise to new
continents, in all probability, in the southern hemisphere.”
Such is a brief statement of the hypothesis which Adhémar has very
ingeniously worked out. How far it explains the mysterious phenomena
which we have under consideration we shall not attempt to say, our
concern being with the effects. Does the evidence of upward and downward
movements of the surface in Tertiary times explain the great change? For
if the cooling which preceded and succeeded the two European deluges
still remains an unsolved problem, its effects are perfectly
appreciable. The intense cold which visited the northern and central
parts of Europe resulted in the annihilation of organic life in those
countries. All the watercourses, the rivers and streams, the seas and
lakes, were frozen. As Agassiz says in his first work on “Glaciers”: “A
vast mantle of ice and snow covered the plains, the valleys, and the
seas. All the springs were dried up; the rivers ceased to flow. To the
movements of a numerous and animated creation succeeded the silence of
death.” Great numbers of animals perished from cold. The Elephant and
Rhinoceros perished by thousands in the midst of their grazing grounds,
which became transformed into fields of ice and snow. It is then that
these two species disappeared, and seem to have been effaced from
creation. Other animals were overwhelmed, without their race having been
always entirely annihilated. The sun, which lately lighted up the
verdant plains, as it dawned upon these frozen steppes, was only saluted
by the whistling of the north winds, and the horrible rending of the
crevasses, which opened up on all sides under the heat of its rays,
acting upon the immense glacier which formed the sepulchre of many
animated beings.
How can we accept the idea that the plains, but yesterday smiling and
fertile, were formerly covered, and that for a very long period, with an
immense sheet of ice and snow? To satisfy the reader that the proof of
this can be established on sufficient evidence, it is necessary to
direct his attention to certain parts of Europe. It is essential to
visit, at least in idea, a country where _glacial phenomena_ still
exist, and to prove that the phenomena, now confined to those countries,
were spread, during geological times, over spaces infinitely vaster. We
shall choose for our illustration, and as an example, the glaciers of
the Alps. We shall show that the glaciers of Switzerland and Savoy have
not always been restricted to their present limits; that they are, so to
speak, only miniature resemblances of the gigantic glaciers of times
past; and that they formerly extended over all the great plains which
extend from the foot of the chain of the Alps.
To establish these proofs we must enter upon some consideration of
existing glaciers, upon their mode of formation, and their peculiar
phenomena.
The snow which, during the whole year, falls upon the mountains, does
not melt, but maintains its solid state, when the elevation exceeds the
height of 9,000 feet or thereabouts. Where the snow accumulates to a
great thickness, in the valleys, or in the deep fissures in the ground,
it hardens under the influence of the pressure resulting from the
incumbent weight. But it always happens that a certain quantity of
water, resulting from the momentary thawing of the superficial portions,
traverses its substance, and this forms a crystalline mass of ice, with
a granular structure, which the Swiss naturalists designate _névé_. From
the successive melting and freezing caused by the heat by day and the
cold by night, and the infiltration of air and water into its
interstices, the _névé_ is slowly transformed into a homogeneous azure
mass of ice, full of an infinite number of little air-bubbles--this was
what was formerly called _glace bulleuse_ (bubble-ice). Finally, these
masses, becoming completely frozen, water replaces the bubbles of air.
Then the transformation is complete; the ice is homogeneous, and
presents those beautiful azure tints so much admired by the tourist who
traverses the magnificent glaciers of Switzerland and Savoy.
Such is the origin of, and such is the mode in which the glaciers of
the Alps are formed. An important property of glaciers remains to be
pointed out. They have a general movement of translation in the
direction of their slope, under the influence of which they make a
certain yearly progress downward, according to the angle of the slope.
The glacier of the Aar, for example, advances at the rate of about 250
feet each year.
Under the joint influence of the slope, the weight of the frozen mass,
and the melting of the parts which touch the earth, the glacier thus
always tends downwards; but from the effects of a more genial
temperature, the lower extremity melting rapidly, has a tendency to
recede. It is the difference between these two actions which constitutes
the real progressive movement of the glacier.
The friction exercised by the glacier upon the bottom and sides of the
valley, ought necessarily to leave its traces on the rocks with which it
may happen to be in contact. Over all the places where a glacier has
passed, in fact, we remark that the rocks are polished, levelled,
rounded, and, as it is termed, _moutonnées_. These rocks present,
besides, striations or scratches, running in the direction of the motion
of the glacier, which have been produced by hard and angular fragments
of stones imbedded in the ice, and which leave their marks on the
hardest rocks under the irresistible pressure of the heavy-descending
mass of ice. In a work of great merit, which we have before quoted, M.
Charles Martins explains the physical mechanism by which granite rocks
borne onwards in the progressive movements of a glacier, have scratched,
scored, and rounded the softer rocks which the glacier has encountered
in its descent. “The friction,” says M. Martins, “which the glacier
exercises upon the bottom and upon the walls, is too considerable not to
leave its traces upon the rocks with which it may be in contact; but its
action varies according to the mineralogical nature of the rocks, and
the configuration of the ground they cover. If we penetrate between the
soil and the bottom of the glacier, taking advantage of the ice-caverns
which sometimes open at its edge or extremity, we creep over a bed of
pebbles and fine sand saturated with water. If we remove this bed, we
soon perceive that the underlying rock is levelled, polished, ground
down by friction, and covered with rectilinear striæ, resembling
sometimes small grooves, more frequently perfectly straight scratches,
as though they had been produced by means of a graver, or even a very
fine needle. The mechanism by which these striæ have been produced is
that which industry employs to polish stones and metals. We rub the
metallic surface with a fine powder called _emery_, until we give it a
brilliancy which proceeds from the reflection of the light from an
infinity of minute striæ. The bed of pebbles and mud, interposed between
the glacier and the subjacent rock, here represents the emery. The rock
is the metallic surface, and the mass of the glacier which presses on
and displaces the mud in its descent towards the plain, represents the
hand of the polisher. These striæ always follow the direction of the
glacier; but as it is sometimes subject to small lateral deviations, the
striæ sometimes cross, forming very small angles with one another. If we
examine the rocks by the side of a glacier, we find similar striæ
engraved on them where they have been in contact with the frozen mass. I
have often broken the ice where it thus pressed upon the rock, and have
found under it polished surfaces, covered with striations. The pebbles
and grains of sand which had engraved them were still encased in the
ice, fixed like the diamond of the glazier at the end of the instrument
with which he marks his glass.
“The sharpness and depth of the striæ or scratches depend on many
circumstances: if the rock acted upon is calcareous, and the emery is
represented by pebbles and sand derived from harder rocks, such as
gneiss, granite, or protogine, the scratches are very marked. This we
can verify at the foot of the glaciers of Rosenlaui, and of the
Grindenwald in the Canton of Berne. On the contrary, if the rock is
gneissic, granitic, or serpentinous, that is to say, very hard, the
scratches will be less deep and less marked, as may be seen in the
glaciers of the Aar, of Zermatt, and Chamounix. The polish will be the
same in both cases, and it is often as perfect as in marble polished for
architectural purposes.
“The scratches engraved upon the rocks which confine these glaciers are
generally horizontal or parallel to the surface. Sometimes, owing to the
contractions of the valley, these striæ are nearly vertical. This,
however, need not surprise us. Forced onwards by the superincumbent
weight, the glacier squeezes itself through the narrow part, its bulk
expanding upwards, in which case the flanks of the mountain which barred
its passage are marked vertically. This is admirably seen near the
Châlets of Stieregg, a narrow defile which the lower glacier of the
Grindenwald has to clear before it discharges itself into the valley of
the same name. Upon the right bank of the glacier the scratches are
inclined at an angle of 45° to the horizon. Upon the left bank the
glacier rises sometimes quite up to the neighbouring forest, carrying
with it great clods of earth charged with rhododendrons and clumps of
alder, birches, and firs. The more tender or foliated rocks were broken
up and demolished by the prodigious force of the glacier; the harder
rocks offered more resistance, but their surface is planed down,
polished, and striated, testifying to the enormous pressure which they
had to undergo. In the same manner the glacier of the Aar, at the foot
of the promontory on which M. Agassiz’ tent was erected, is polished to
a great height, and on the face, turned towards the upper part of the
valley, I have observed scratches inclined 64°. The ice, erect against
this escarpment, seemed to wish to scale it, but the granite rock held
fast, and the glacier was compelled to pass round it slowly.
“In recapitulation, the considerable pressure of a glacier, joined to
its movement of progression, acts at once upon the bottom and flanks of
the valley which it traverses: it polishes all the rocks which may be
too hard to be demolished by it, and frequently impresses upon them a
peculiar and characteristic form. In destroying all the asperities and
inequalities of these rocks, it levels their surfaces and rounds them on
the sides pointing up the stream, whilst in the opposite direction, or
down the stream, they sometimes preserve their abrupt, unequal, and
rugged surface. We must comprehend, in short, that the force of the
glacier acts principally on the side which is towards the circle whence
it descends, in the same way that the piles of a bridge are more damaged
up-stream, than down, by the icebergs which the river brings down during
the winter. Seen from a distance, a group of rocks thus rounded and
polished reminds us of the appearance of a flock of sheep: hence the
name _roches moutonnées_ given them by the Swiss naturalists.”
Another phenomenon which plays an important part in existing glaciers,
and in those, also, which formerly covered Switzerland, is found in the
fragments of rock, often of enormous size, which have been transported
and deposited during their movement of progression.
The peaks of the Alps are exposed to continual degradations. Formed of
granitic rocks--rocks eminently alterable under the action of air and
water, they become disintegrated and often fall in fragments more or
less voluminous. “The masses of snow,” continues Martins, “which hang
upon the Alps during winter, the rain which infiltrates between their
beds during summer, the sudden action of torrents of water, and more
slowly, but yet more powerfully, the chemical affinities, degrade,
disintegrate, and decompose the hardest rocks. The débris thus produced
falls from the summits into the circles occupied by the glaciers with a
great crash, accompanied by frightful noises and great clouds of dust.
Even in the middle of summer I have seen these avalanches of stone
precipitated from the highest ridges of the Schreckhorn, forming upon
the immaculate snow a long black train, consisting of enormous blocks
and an immense number of smaller fragments. In the spring a rapid
thawing of the winter snows often causes accidental torrents of extreme
violence. If the melting is slow, water insinuates itself into the
smallest fissures of the rocks, freezes there, and rends asunder the
most refractory masses. The blocks detached from the mountains are
sometimes of gigantic dimensions: we have found them sixty feet in
length, and those measuring thirty feet each way are by no means rare in
the Alps.”[106]
[106] _Revue des Deux Mondes_, p. 925; March 1, 1847.
Thus, the action of aqueous infiltrations followed by frost, the
chemical decomposition which granite undergoes under the influence of a
moist atmosphere, degrade and disintegrate the rocks which constitute
the mountains enclosing the glacier. Blocks, sometimes of very
considerable dimensions, often fall at the foot of these mountains on to
the surface of the glacier. Were it immovable the débris would
accumulate at its base, and would form there a mass of ruins heaped up
without order. But the slow progression, the continuous displacement of
the glacier, lead, in the distribution of these blocks, to a certain
kind of arrangement: the blocks falling upon its surface participate in
its movement, and advance with it. But other downfalls take place daily,
and the new débris following the first, the whole form a line along the
outer edge of the glacier. These regular trains of rocks bear the name
of “_moraines_.” When the rocks fall from two mountains, and on each
edge of the glacier, and two parallel lines of débris are formed, they
are called _lateral moraines_. There are also _median moraines_, which
are formed when two glaciers are confluent, in such a manner that the
_lateral moraine_, on the right of the one, trends towards the left-hand
one of the other. Finally, those moraines are _frontal_, or _terminal_,
which repose, not upon the glacier, but at its point of termination in
the valleys, and which are due to the accumulation of blocks fallen from
the terminal escarpments of glaciers there arrested by some obstacle. In
PLATE XXXI. we have represented an actual Swiss glacier, in which are
united the physical and geological peculiarities belonging to these
enormous masses of frozen water: the moraines here are _lateral_, that
is to say, formed of a double line of débris.
[Illustration: XXXI.--Glaciers of Switzerland.]
Transported slowly on the surface of the glacier, all the blocks from
the mountain preserve their original forms unaltered; the sharpness of
their edges is never altered by their gentle transport and almost
imperceptible motion. Atmospheric agency only can affect or destroy
these rocks when formed of hard resisting material. They then remain
nearly of the same form and volume they had when they fell on the
surface of the glacier; but it is otherwise with blocks and fragments
enclosed between the rock and the glacier, whether it be at the bottom
or between the glacier and its lateral walls. Some of these, under the
powerful and continuous action of this gigantic grinding process, will
be reduced to an impalpable mud, others are worn into facets, while
others are rounded, presenting a multitude of scratches crossing each
other in all directions. These scratched pebbles are of great importance
in studying the extent of ancient glaciers; they testify, on the spot,
to the existence of pre-existing glaciers which shaped, ground, and
striated the pebbles, which water does not; on the contrary, in the
latter, they become polished and rounded, and even natural striations
are effaced.
Thus, huge blocks transported to great distances from their true
geological beds, that is, _erratic blocks_, to use the proper technical
term, rounded (_moutonnées_), polished, and scratched surfaces,
_moraines_; finally, pebbles, ground, polished, rounded, or worn into
smooth surfaces, are all physical effects of glaciers in motion, and
their presence alone affords sufficient proof to the naturalist that a
glacier formerly existed in the locality where he finds them. The reader
will now comprehend how it is possible to recognise, in our days, the
existence of ancient glaciers in different parts of the world. Above
all, wherever we may find both _erratic blocks_ and _moraines_, and
observe, at the same time, indications of rocks having been polished and
striated in the same direction, we may pronounce with certainty as to
the existence of a glacier during geological times. Let us take some
instances.
At Pravolta, in the Alps, going towards _Monte Santo-Primo_, upon a
calcareous rock, we find the mass of granite represented in Fig. 196.
This erratic block exists, with thousands of others, on the slopes of
the mountain. It is about fifty feet long, nearly forty feet broad, and
five-and-twenty in height; and all its edges and angles are perfect.
Some parallel striæ occur along the neighbouring rocks. All this clearly
demonstrates that a glacier existed, in former times, in this part of
the Alps, where none appear at the present time. It is a glacier, then,
which has transported and deposited here this enormous block, weighing
nearly 2,000 tons.
In the Jura Mountains, on the hill of Fourvières, a limestone eminence
at Lyons, blocks of granite are found, evidently derived from the Alps,
and transported there by the Swiss glaciers. The particular mode of
transport is represented theoretically in Fig. 197. A represents, for
example, the summit of the Alps, B the Jura Mountains, or the hill of
Fourvières, at Lyons. At the glacial period, the glacier A B C extended
from the Alps to the mountain B. The granitic débris, which was detached
from the summit of the Alpine mountains, fell on the surface of the
glacier. The movement of progression of this glacier transported these
blocks as far as the summit B. At a later period the temperature of the
globe was raised, and when the ice had melted, the blocks, D E, were
quietly deposited on the spots where they are now found, without having
sustained the slightest shock or injury in this singular mode of
transport.
[Illustration: Fig. 196.--Erratic Blocks in the Alps.]
[Illustration: Fig. 197.--Transported blocks.]
Every day traces, more or less recognisable, are found on the Alps of
ancient glaciers far distant from their existing limits. Heaps of
débris, of all sizes, comprehending blocks with sharp-pointed angles,
are found in the Swiss plains and valleys. _Blocs perchés_ (Perched
blocks), as in PL. XXXI., are often seen perched upon points of the Alps
situated far above existing glaciers, or dispersed over the plain which
separates the Alps from the Jura, or even preserving an incredible
equilibrium, when their great mass is taken into consideration, at
considerable heights on the eastern flank of this chain of mountains. It
is by the aid of these indications that the geologist has been able to
trace to extremely remote distances signs of the former existence of the
ancient glaciers of the Alps, to follow them in their course, and fix
their point of origin, and where they terminated. Thus the humble Mount
Sion, a gently-swelling hill situated to the north of Geneva, was the
point at which three great ancient glaciers had their confluence--the
glacier of the Rhône, which filled all the basin of Lake Leman, or Lake
of Geneva; that of the Isère, which issued from the Annecy and Bourget
Lakes; and that of the Arve, which had its source in the valley of
Chamounix, all converged at this point. According to M. G. de Mortillet,
who has carefully studied this geological question, the extent and
situation of these ancient glaciers of the Alps were as follows:--Upon
its northern flank the _glacier of the Rhine_ occupied all the basin of
Lake Constance, and extended to the borders of Germany; that of the
_Linth_, which was arrested at the extremity of the Lake of Zurich--this
city is built upon its terminal moraine--that of the _Reus_, which
covered the lake of the four cantons with blocks torn from the peaks of
Saint-Gothard;--that of the _Aar_, the last moraines of which crown the
hills in the environs of Berne;--those of the _Arve_ and the _Isère_,
which, as we have said, debouched from Lake Annecy and Lake Bourget
respectively;--that of the _Rhône_, the most important of all. It is
this glacier which has deposited upon the flanks of the Jura, at the
height of 3,400 feet above the level of the sea, the great _erratic
blocks_ already described. This mighty glacier of the Rhône had its
origin in all the lateral valleys formed by the two parallel chains of
the Valais. It filled all the Valais, and extended into the plain, lying
between the Alps and the Jura, from Fort de L’Écluse, near the fall of
the Rhône, up to the neighbourhood of Aarau.
The fragments of rocks transported by the ice-sea which occupied all the
Swiss plain follow, in northerly direction, the course of the valley of
the Rhine. On the other hand, the glacier of the Rhône, after reaching
the plain of Switzerland, turned off obliquely towards the south,
received the glacier of the Arve, then that of the Isère, passed between
the Jura and the mountains of the Grande-Chartreuse, spread over La
Bresse, then nearly all Dauphiny, and terminated in the neighbourhood of
Lyons.
Upon the southern flank of the Alps, the ancient glaciers, according to
M. de Mortillet’s map, occupied all the great valleys from that of the
Dora, on the west, to that of the Tagliamento, on the east. “The glacier
of the _Dora_” says de Mortillet, whose text we greatly abridge,
“debouched into the valley of the Po, close to Turin. That of the
_Dora-Baltéa_ entered the plain of Ivréa, where it has left a
magnificent semicircle of hills, which formed its terminal moraine. That
of the _Toce_ discharged itself into Lake Maggiore, against the glacier
of the Tessin, and then threw itself into the valley of Lake Orta, at
the southern extremity of which its terminal moraines were situated.
That of the Tessin filled the basin of Lake Maggiore, and established
itself between Lugano and Varèse. That of the _Adda_ filled the basin of
Lake Como, and established itself between Mendrizio and Lecco, thus
describing a vast semicircle. That of the _Oglio_ terminated a little
beyond Lake Iseo. That of the _Adige_, finding no passage through the
narrow valley of Roveredo, where the valley became very narrow, took
another course, and filled the immense valley of the Lake of Garda. At
Novi it has left a magnificent moraine, of which Dante speaks in his
‘Inferno.’ That of the _Brenta_ extended over the plain of that commune.
The _Drave_ and the _Tagliamento_ had also their glaciers. Finally,
glaciers occupied all the valleys of the Austrian and Bavarian
Alps.”[107]
[107] “Carte des Anciens Glaciers des Alpes,” pp. 8-10. (1860.)
Similar traces of the existence of ancient glaciers occur in many other
European countries. In the Pyrenees, in Corsica, the Vosges, the Jura,
&c., extensive ranges of country have been covered, in geological
times, by these vast plains of ice. The glacier of the Moselle was the
most considerable of the glaciers of the Vosges, receiving numerous
affluents; its lowest frontal moraine, which is situated below
Remiremont, could not be less than a mile and a quarter in length.
But the phenomenon of the glacial extension which we have examined in
the Alps was not confined to Central Europe. The same traces of their
ancient existence are observed in all the north of Europe, in Russia,
Iceland, Norway, Prussia, the British Islands, part of Germany, in the
north, and even in some parts of the south, of Spain. In England,
_erratic_ blocks of granite are found which were derived from the
mountains of Norway. It is evident that these blocks were borne by a
glacier which extended from the north pole to England. In this manner
they crossed the Baltic and the North Seas. In Prussia similar traces
are observable.
Thus, during the Quaternary epoch, glaciers which are now limited to the
Polar regions, or to mountainous countries of considerable altitude,
extended very far beyond their present known limits; and, taken in
connection with the deluge of the north, and the vast amount of organic
life which they destroyed, they form, perhaps, the most striking and
mysterious of all geological phenomena.
M. Edouard Collomb, to whom we owe much of our knowledge of ancient
glaciers, furnishes the following note explanatory of a map of Ancient
Glaciers which he has prepared:--
“The area occupied by the ancient Quaternary glaciers may be divided
into two orographical regions:--1. The region of the north, from lat.
52° or 55° up to the North Pole. 2. The region of Central Europe and
part of the south.
“The region of the north which has been covered by the ancient glaciers
comprehends all the Scandinavian peninsula, Sweden, Norway, and a part
of Western Russia, extending from the Niemen on the north in a curve
which passed near the sources of the Dnieper and the Volga, and thence
took a direction towards the shores of the glacial ocean. This region
comprehends Iceland, Scotland, Ireland, the isles dependent on them,
and, finally, a great part of England.
“This region is bounded, on all its sides, by a wide zone from 2° to 5°
in breadth, over which is recognised the existence of erratic blocks of
the north: it includes the middle region of Russia in Europe, Poland, a
part of Prussia, and Denmark; losing itself in Holland on the Zuider
Zee, it cut into the northern part of England, and we find a shred of it
in France, upon the borders of the Cotentin.
“The ancient glaciers of Central Europe consisted, first, of the grand
masses of the Alps. Stretching to the west and to the north, they
extended to the valley of the Rhône as far as Lyons, then crossing the
summit-level of the Jura, they passed near Basle, covering Lake
Constance, and stretching beyond into Bavaria and Austria. Upon the
southern slopes of the Alps, they turned round the summit of the
Adriatic, passed near to Udinet, covered Peschiera, Solferino, Como,
Varèse, and Ivréa, extended to near Turin, and terminated in the valley
of the Stura, near the Col de Tenda.
“In the Pyrenees, the ancient glaciers have occupied all the principal
valleys of this chain, both on the French and Spanish sides, especially
the valleys of the centre, which comprehend those of Luchon, Aude,
Baréges, Cauterets, and Ossun. In the Cantabrian chain, an extension of
the Pyrenees, the existence of ancient glaciers has also been
recognised.
“In the Vosges and the Black Forest they covered all the southern parts
of these mountains. In the Vosges, the principal traces are found in the
valleys of Saint-Amarin, Giromagny, Munster, the Moselle, &c.
“In the Carpathians and the Caucasus the existence of ancient glaciers
of great extent has also been observed.
“In the Sierra Nevada, in the south of Spain, mountains upwards of
11,000 feet high, the valleys which descend from the Picacho de Veleta
and Mulhacen have been covered with ancient glaciers during the
Quaternary epoch.”
There is no reason to doubt that at this epoch all the British islands,
at least all north of the Thames, were covered by glaciers in their
higher parts. “Those,” says Professor Ramsay, “who know the Highlands of
Scotland, will remember that, though the weather has had a powerful
influence upon them, rendering them in places rugged, jagged, and
cliffy, yet, notwithstanding, their general outlines are often
remarkably rounded and flowing; and when the valleys are examined in
detail, you find in their bottoms and on the sides of the hills that the
mammillated structure prevails. This rounded form is known, by those who
study glaciers, by the name of _roches moutonnées_, given to them by the
Swiss writers. These mammillated forms are exceedingly common in many
British valleys, and not only so, but the very same kind of grooving and
striation, so characteristic of the rocks in the Swiss valleys, also
marks those of the Highlands of Scotland, of Cumberland, and Wales.
Considering all these things, geologists, led by Agassiz some five or
six and twenty years ago, have by degrees come to the conclusion, that a
very large part of our island was, during the glacial period, covered,
or nearly covered, with a thick coating of ice in the same way that the
north of Greenland is at present; and that by the long-continued
grinding power of a great glacier, or set of glaciers nearly universal
over the northern half of our country, and the high ground of Wales, the
whole surface became moulded by ice.”
Whoever traverses England, observing its features with attention, will
remark in certain places traces of the action of ice in this era. Some
of the mountains present on one side a naked rock, and on the other a
gentle slope, smiling and verdant, giving a character more or less
abrupt, bold, and striking, to the landscape. Considerable portions of
dry land were formerly covered by a bluish clay, which contained many
fragments of rock or “boulders” torn from the old Cumbrian mountains;
from the Pennine chain; from the moraines of the north of England; and
from the Chalk hills--hence called “boulder” clay--present themselves
here and there, broken, worn, and ground up by the action of water and
ice. These erratic blocks or “boulders” have clearly been detached from
the parent rock by violence, and often transported to considerable
distances. They have been carried, not only across plains, but over the
tops of mountains; some of them being found 130 miles from the parent
rocks. We even find, as already hinted, some rocks of which no
prototypes have been found nearer than Norway. There is, then, little
room for doubting the fact of an extensive system of glaciers having
covered the land, although the proofs have only been gathered
laboriously and by slow degrees in a long series of years. In 1840
Agassiz visited Scotland, and his eye, accustomed to glaciers in his
native mountains, speedily detected their signs. Dr. Buckland became a
zealous advocate of the same views. North Wales was soon recognised as
an independent centre of a system which radiated from lofty Snowdon,
through seven valleys, carrying with them large stones and grooving the
rocks in their passage. In the pass of Llanberis there are all the
common proofs of the valley having been filled with glacier ice. “When
the country was under water,” says Professor Ramsay, “the drift was
deposited which more or less filled up many of the Welsh valleys. When
the land had risen again to a considerable height, the glaciers
increased in size: although they never reached the immense magnitude
which they attained in the earlier portion of the icy epoch. Still they
became so large that such a valley as the Pass of Llanberis was a second
time occupied by ice, which ploughed out the drift that more or less
covered the valley. By degrees, however, as we approach nearer our own
days, the climate slowly ameliorated, and the glaciers began to decline,
till, growing less and less, they crept up and up; and here and there,
as they died away, they left their terminal and lateral moraines still
as well defined in some cases as moraines in lands where glaciers now
exist. Frequently, too, masses of stone, that floated on the surface of
the ice, were left perched upon the rounded _roches moutonnées_, in a
manner somewhat puzzling to those who are not geologists.
“In short, they were let down upon the surface of these rocks so quietly
and so softly, that there they will lie, until an earthquake shakes them
down, or until the wasting of the rock on which they rest precipitates
them to a lower level.”
It was the opinion of Agassiz, after visiting Scotland, that the
Grampians had been covered by a vast thickness of ice, whence erratic
blocks had been dispersed in all directions as from a centre; other
geologists after a time adopted the opinion--Mr. Robert Chambers going
so far as to maintain, in 1848, that Scotland had been at one time
moulded by ice. Mr. T. F. Jamieson followed in the same track, adducing
many new facts to prove that the Grampians once sent down glaciers in
all directions towards the sea. “The glacial grooves,” he says, “radiate
outward from the central heights towards all points of the compass,
although they do not strictly conform to the actual shape and contour of
the minor valleys and ridges.” But the most interesting part of Mr.
Jamieson’s investigations is undoubtedly the ingenious manner in which
he has worked out Agassiz’ assertion that Glenroy, whose remarkable
“_Parallel Roads_” have puzzled so many investigators, was once the
basin of a frozen lake.
Glenroy is one of the many romantic glens of Lochaber, at the head of
the Spey, near to the Great Glen, or the valley of the Caledonian Canal,
which stretches obliquely across the country in a northwesterly
direction from Loch Linnhè to Loch Ness, leaving Loch Arkaig, Loch Aich,
Glen Garry, and many a highland loch besides, on the left, and Glen
Spean, in which Loch Treig, running due north and south, has its mouth,
on the south. Glenroy opens into it from the north, while Glen Gluoy
opens into the Great Glen opposite Loch Arkaig. Mr. Jamieson commenced
his investigations at the mouth of Loch Arkaig, which is about a mile
from the lake itself. Here he found the gneiss ground down as if by ice
coming from the east. On the hill, north of the lake, the gneiss, though
much worn and weathered, still exhibited well-marked striæ, directed up
and down the valley. Other markings showed that the Glen Arkaig glacier
not only blocked up Glen Gluoy, but the mouth of Glen Spean, which lies
two miles or so north of it on the opposite side.
At Brackletter, on the south side of Glen Spean, near its junction with
Glen Lochy, glacial scores pointing more nearly due west, but slightly
inclining to the north, were observed, as if caused by the pressure of
ice from Glen Lui. The south side of Glen Spean, from its mouth to Loch
Treig, is bounded by lofty hills--an extension of Ben Nevis, the highest
of these peaks exceeding 3,000 feet. Numerous gullies intersect their
flanks, and the largest of these, Corry N’Eoin, presents a series of
rocky amphitheatres, or rather large caldrons, whose walls have been
ground down by long-continued glacial action: the quartz-veins are all
shorn down to the level of the gneiss, and streaked with fine scratches,
pointing down the hollows and far up the rocks on either side. During
all these operations the great valley was probably filled up with ice,
which would close Glen Gluoy and Glen Spean, and might also close the
lowest of the lines in Glenroy. But how about the middle and upper
lines?
A glacier crossing from Loch Treig, and protruding across Glen Spean,
would cut off Glens Glaibu and Makoul, when the water in Glenroy could
only escape over the Col into Strathspey, when the first level would be
marked.
Now let the Glen Treig glacier shrink a little, so as to let out water
to the level of the second line by the outline at Makoul, and the theory
is complete. When the first and greatest glacier gave way, Glenroy would
be nearly in its present state.
The glacier, on issuing from the gorge at the end of Loch Treig, would
dilate immensely, the right flank spreading over a rough expanse of
syenite, the neighbouring hills being mica-schists, with veins of
porphyry. Now the syenite breaks into large cuboidal blocks of immense
size. These have been swept before the advancing glacier along with
other débris, and deposited in a semicircle of mounds having a sweep of
several miles, forming circular bands which mark the edges of the
glacier as it shrunk from time to time under the influence of a milder
climate.
This moraine, which was all that was wanting to complete the theory laid
down by Agassiz, is found on the pony-road leading from the mouth of
Loch Treig towards Badenoch. A mile or so brings the traveller to the
summit-level of the road, and beyond the hill a low moor stretches away
to the bottom of the plain. Here, slanting across the slope of the hill
towards Loch Treig, two lines of moraine stretch across the road. At
first they consist of mica-schists and bits of porphyry, but blocks of
syenite soon become intermingled. Outside these are older hillocks,
rising in some places sixty and seventy feet high, forming narrow
steep-sided mounds, with blocks fourteen feet in length sticking out of
the surface, mixed with fragments of mica-schist and gneiss. The inner
moraine consists, almost wholly, of large blocks of syenite, five, ten,
fifteen, and five-and-twenty feet long.
[Illustration: Fig. 198.--Parallel roads of Glenroy; from a sketch by
Professor J. Phillips.]
The present aspect of Glenroy is that of an upper and lower glen opening
up from the larger Glen Spean. The head-waters of Lochaber gather in a
wild mountain tract, near the source of the Spey. The upper glen is an
oval valley, four miles long, by about one broad, bounded on each side
by high mountains, which throw off two streams dividing the mica-schist
from the gneissic systems; the former predominating on the west side,
and the latter on the east. The united streams flow to the south-west
for two miles, when the valley contracts to a rocky gorge which
separates the upper from the lower glen. Passing from the upper to the
lower glen, a line is observed to pass from near the junction of the two
streams, on a level with a flat rock at the gorge, and also with the
uppermost of the three lines of terraces in the lower glen. This line
girdles the sides of the hills right and left, with a seemingly higher
sweep, and is followed by two other perfectly parallel and continuous
lines till Glenroy expands into Glen Spean, which crosses its mouth and
enters the great glen a little south of Loch Lochy. At the point,
however, where Glenroy enters Glen Spean, the two upper terraces cease,
while the lower of the three appears on the north and south side of Glen
Spean, as far as the pass of Glen Muckal, and southward a little way up
the Gubban river and round the head of Loch Treig.
In Scotland, and in Northern England and Wales, there is distinct
evidence that the Glacial Epoch commenced with an era of continental
ice, the land being but slightly lower than at present, and possibly at
the same level, during which period the Scottish hills received their
rounded outlines, and scratched and smoothed rock-surfaces; and the
plains and valleys became filled with the stiff clay, with angular
scratched stones, known as the “Till,” which deposit is believed by
Messrs. Geikie, Jamieson, and Croll to be a _moraine profonde_, the
product of a vast ice-sheet.
In Wales, Professor Ramsay has described the whole of the valleys of the
Snowdonian range as filled with enormous glaciers, the level of the
surface of the ice filling the Pass of Llanberis, rising 500 feet above
the present watershed at Gorphwysfa. In the Lake District of Cumberland
and Westmorland, Mr. De Rance has shown that a vast series of glaciers,
or small ice-sheets, filled all the valleys, radiating out in all
directions from the larger mountains, which formed centres of
dispersion, the ice actually pushing over many of the lesser watersheds,
and scooping out the great rock-basins in which lie the lakes
Windermere, Ullswater, Thirlmere, Coniston Water, and Wastwater, the
bottoms of which are nearly all below the sea-level. The whole of this
district, he has shown, experienced a second glaciation, after the
period of great submergence, in which valley-glaciers scooped out the
marine drift, and left their _moraines_ in the Liza, Langdale, and
other valleys, and high up in the hills, as at Harrison’s Stickle, where
a tarn has been formed by a little _moraine_, acting as a dam, as shown
by Professor Hull.
In Wales, also, valley-glaciers existed after the submergence beneath
the Glacial sea. Thus in Cwm-llafar, under the brow of Carnedd Dafydd,
and Carnedd Llewelyn, Professor Ramsay has shown that a narrow glacier,
about two miles in length, has ploughed out a long narrow hollow in the
drift (which “forms a succession of terraces, the result of marine
denudation, during pauses in the re-elevation of its submersion) to a
depth of more than 2,000 feet.”[108]
[108] Professor Ramsay, “The Old Glaciers of North Wales.” Longman,
1860.
The proofs of this great submergence, succeeding the era of “land-ice,”
are constantly accumulating. Since 1863, when Professor Hull first
divided the thick glacial deposits of Eastern Lancashire and Cheshire
into an Upper Boulder Clay, and Lower Boulder Clay divided by a Middle
Sand and Gravel, the whole of which are of marine origin, these
subdivisions have been found to hold good, by himself and Mr. A. H.
Green, over 600 square miles of country around Manchester, Bolton, and
Congleton; by Mr. De Rance over another 600 square miles, around
Liverpool, Preston, Blackpool, Blackburn, and Lancaster, and also in the
low country lying between the Cumberland and Welsh mountains and the
sea.
In Ireland, also, the same triplex arrangement appears to exist.
Professors Harkness and Hull have identified the “Limestone and Manure
Gravels” of the central plain, as referable to the “Middle Sand and
Gravel,” and the “Lower Boulder Clay” rests on a glaciated rock-surface
along the coasts of Antrim and Down, and is overlain by sand, which, in
1832, was discovered by Dr. Scouler to be shell-bearing. At Kingstown
the three deposits are seen resting on a moutonnéed surface of granite,
scored from the N.N.W.
In Lancashire and on the coast of North Wales, between Llandudno and
Rhyl, Mr. De Rance has shown that these deposits often lie upon the
denuded and eroded surface of another clay, of older date, which he
believes to be the product of land-ice, the remnant of the _moraine
profonde_, and the equivalent of the Scotch “Till.” He also shows that
the Lower Boulder Clay never rises above an elevation of fifty or eighty
feet above the sea-level; and that the Middle Sand and Shingle rests
directly upon the rock, or on the surface of this old Till.
Near Manchester the Lower Boulder Clay occasionally rests upon an old
bed of sand and gravel. It is extremely local, but its presence has been
recorded in several sections by Mr. Edward Binney, who was the first to
show, in 1842,[109] that the Lancashire Boulder Clays were formed in the
sea, and that the erratic pebbles and boulders, mainly derived from the
Cumberland Lake Districts, were brought south by means of floating ice.
[109] In 1840 Dr. Buckland described the occurrence of boulders of
Criffel Granite between Shalbeck and Carlisle, and attributed
their position to the agency of ice floating across the Solway
Firth.
Most of the erratic pebbles and boulders in the Lancashire clays are
more or less scratched and scored, many of them (though quite rounded)
in so many directions that Mr. De Rance believes the Cumberland and
Westmoreland hills to have been surrounded by an ice-belt, which,
occasionally thawing during summer or warm episodes, admitted “breaker
action” on the gradually subsiding coast, wearing the fragments of rocks
brought down by rivers or by glaciers into pebbles that, with the return
of the cold, became covered with the “ice-belt,” which, lifted by the
tides, rolled and dinted the pebbles one against another, and gradually
allowed them to be impressed into its mass, with which they eventually
floated away.
The Middle Sands and Shingles in England have also afforded a great
number of shells of mollusca. At Macclesfield they have been described
by Messrs. Prestwich and Darbishire as occurring at an elevation of
1,100 to 1,200 feet above the level of the sea.[110]
[110] Mr. Darbishire records seventy species from Macclesfield and
Moel Tryfaen, taken together, of which 6 are Arctic, and 18 are
not known in the Upper Crag.
Among other proofs of glacial action and submersion in Wales may be
mentioned the case of Moel Tryfaen, a hill 1,400 feet high, lying to the
westward of Caernarvon Bay, and six or seven miles from Caernarvon. Mr.
Joshua Trimmer had observed stratified drift near the summit of this
mountain, from which he obtained some marine shells; but doubts were
entertained as to their age until 1863, when a deep and extensive
cutting was made in search of slates. In this cutting a stratified mass
of loose sand and gravel was laid open near the summit, thirty-five feet
thick, containing shells, some entire, but mostly in fragments. Sir
Charles Lyell examined the cutting, and obtained twenty species of
shells, and in the lower beds of the drift, “large heavy boulders of
far-transported rocks, glacially polished and scratched on more than one
side:” underneath the whole, the edges of vertical slates were exposed
to view, exhibiting “unequivocal marks of prolonged glaciation.” The
shells belonged to species still living in British or more northern
seas.
From the gravels of the Severn Valley, described by Mr. Maw, thirty-five
forms of mollusca have been identified by Mr. Gwyn Jeffreys. In the
Shingle beds of Leyland, Euxton, Chorley, Preston, Lancaster, and
Blackpool,[111] Mr. De Rance has obtained nearly thirty species.
[111] The typical species in West Lancashire are _Tellina Balthica_,
_Cardium edule_, _C. aculeatum_, _C. rusticum_, _Psammobia
ferroensis_, _Turritella terebra_.
In Eastern Yorkshire, Mr. Searles V. Wood, Jun., has divided the glacial
deposits into “Purple Clay without Chalk,” “Purple Clay with Chalk,” and
“Chalky Clay,” the whole being later than his “Middle Glacial Sands and
Gravel,” which, in East Anglia, are overlain by the “Chalky Clay,” and
rest unconformably upon the “Contorted Drift” of Norfolk, the Cromer
Till, and the Forest Bed. His three Yorkshire clays are, however,
considered by most northern geologists to be the representatives of the
“Upper Boulder Clay” west of the Pennine Chain, the “Chalky Clay” having
been formed before the country had sufficiently subsided to allow the
sandstones and marls, furnishing the red colouring matter, to have
suffered denudation; while the “Purple Clay without Chalk, and with Shap
Granite,” was deposited when all the chalk was mainly beneath the sea,
and the granite from Shap Fell, which had been broken up by
breaker-action during the Middle Sand era, was floated across the passes
of the Pennine Chain and southwards and northwards. A solitary pebble of
Shap granite has been found by Mr. De Rance at Hoylake, in Cheshire; and
many of Criffel Granite, in that county, and on the coast of North
Wales, by Mr. Mackintosh, who has also traced the flow of this granite
in the low country lying north and south of the Cumberland mountains.
At Bridlington, in Yorkshire, occurs a deposit at the base of the
“Purple Clay,” with a truly Arctic fauna. Out of seventy forms of
mollusca recorded by Mr. S. V. Wood, Jun., nineteen are unknown to the
Crag--of these thirteen are purely arctic, and two not known as living.
Shells have been found in the Upper Boulder Clay of Lancashire, at
Hollingworth Reservoir, near Mottram, by Messrs. Binney, Bateman, and
Prestwich, at an elevation of 568 feet above the sea, consisting of
_Fusus Bamffius_, _Purpura lapillus_, _Turritilla terebra_, and _Cardium
edule_. The clay is described by Mr. Binney as sandy, and
brown-coloured, with pebbles of granite and greenstone, some rounded and
some angular. All the above shells, as well as _Tellina Balthica_, have
been found in the Upper Clay of Preston, Garstang, Blackpool, and
Llandudno, by Mr. De Rance, who has also found all the above species
(with the exception of _Fusus_), as well as _Psammobia ferroensis_, and
the siliceous spiculæ of marine sponges, in the Lower Boulder Clay of
West Lancashire. He has described the ordinary red Boulder Clay of
Lancashire as extending continuously through Cheshire and Staffordshire
into Warwickshire, gradually becoming less red and more chalky,
everywhere overlying intermittent sheets of “sands and shingle-beds,”
one of which is particularly well seen at Leamington and Warwick, where
it contains Pectens from the Crag, _Gryphæa_ from the Lias, and chalk
fossils and flints. The latter have also been found by Mr. Lucy in the
neighbourhood of Mount Sorrel, associated with bits of the Coral Rag of
Yorkshire. The gravels of Leicester, Market Harborough, and Lutterworth
were long ago described by the Rev. W. D. Conybeare as affording
“specimens of the organic remains of most of the Secondary Strata in
England.”
The Rev. O. Fisher, F.G.S., has paid much attention to the superficial
covering usually described as “heading,” or “drift,” as well as to the
contour of the surface, in districts composed of the softer strata, and
has published his views in various papers in the _Journal of the
Geological Society_ and in the _Geological Magazine_. He thinks that the
contour of the surface cannot be ascribed entirely to the action of rain
and rivers, but that the changes in the ancient contour since produced
by those changes can be easily distinguished. He finds the covering beds
to consist of two members--a lower one, entirely destitute of organic
remains, and generally unstratified, which has often been forcibly
indented into the bed beneath it, sometimes exhibiting slickenside at
the junction.
There is evidence of this lower member having been pushed or dragged
over the surface, from higher to lower levels, in a plastic condition;
on which account he has named it “The Trail.”
The upper member of the covering beds consists of soil derived from the
lower one, by weathering. It contains, here and there, the remains of
the land-shells which lived in the locality at a period antecedent to
cultivation. It is “The Warp” of Mr. Trimmer.
Neither of these accumulations occur on low flats, where the surface has
been modified since the recent period. They both alike pass below
high-water mark, and have been noticed beneath estuarine deposits.
Mr. Fisher is of opinion that land-ice has been instrumental in forming
the contour of the surface, and that the trail is the remnant of its
_moraine profonde_. And he has given reasons[112] for believing that the
climate of those latitudes may have been sufficiently rigorous for that
result about 100,000 years ago. He attributes the formation of the
superficial covering of Warp to a period of much rainfall and severe
winter-frosts, after the ice-sheet had disappeared.
[112] _Geological Magazine_, vol. iii., p. 483.
The phenomena which so powerfully affected our hemisphere present
themselves, in a much grander manner, in the New World. The
glacier-system appears to have taken in America the same gigantic
proportions which other objects assume there. Nor is it necessary, in
order to explain the permanent existence of this icy mantle in temperate
climates, to infer the prevalence of any very extraordinary degree of
cold. On this subject M. Ch. Martins thus expresses himself: “The mean
temperature of Geneva is 9° 5 Cent. Upon the surrounding mountains the
limit of perpetual snow is found at 8,800 feet above the level of the
sea. The great glaciers of the valley of Chamounix descend 5,000 feet
below this line. Thus situated, let us suppose that the mean temperature
of Geneva was lowered only 4°, and the average became 5° 5; the decrease
of temperature with the height being 1° c. for every 600 feet, the limit
of perpetual snow would be lowered by 2,437 feet, and would be 6,363
feet above the level of the sea. We can readily admit that the glaciers
of Chamounix would descend below this new limit, to an extent at least
equal to that which exists between their present limit and their lower
extremity. Now, in reality, the foot of these glaciers is 5,000 feet
above the ocean; with a climate 4° colder, it would be 2,437 feet lower;
that is to say, at the level of the Swiss plain. Thus, the lowering of
the line of perpetual snow to this extent would suffice to bring the
glacier of the Arve to the environs of Geneva.... Of the climate which
has favoured the prodigious development of glaciers we have a pretty
correct idea; it is that of Upsala, Stockholm, Christiana, and part of
North America, in the State of New York.... To diminish by four degrees
the mean temperature of a country in order to explain one of the
grandest revolutions of the globe, is to venture on an hypothesis not
bolder than geology has sometimes permitted to itself.”[113]
[113] _Revue des Deux Mondes._
In proving that glaciers covered part of Europe during a certain period,
that they extended from the North Pole to Northern Italy and the Danube,
we have sufficiently established the reality of this _glacial period_,
which we must consider as a curious episode, as well as certain, in the
history of the earth. Such masses of ice could only have covered the
earth when the temperature of the air was lowered at least some degrees
below zero. But organic life is incompatible with such a temperature;
and to this cause must we attribute the disappearance of certain species
of animals and plants--in particular, the Rhinoceros and the
Elephant--which, before this sudden and extraordinary cooling of the
globe, appear to have limited themselves, in immense herds, to Northern
Europe, and chiefly to Siberia, where their remains have been found in
such prodigious quantities. Cuvier says, speaking of the bodies of the
quadrupeds which the ice had seized, and in which they have been
preserved, with their hair, flesh, and skin, up to our own times: “If
they had not been frozen as soon as killed, putrefaction would have
decomposed them; and, on the other hand, this eternal frost could not
have previously prevailed in the place where they died; for they could
not have lived in such a temperature. It was, therefore, at the same
instant when these animals perished that the country they inhabited was
rendered glacial. These events must have been sudden, instantaneous, and
without any gradation.”[114]
[114] “Ossements fossiles. Discours sur les Révolutions du Globe.”
[Illustration: Fig. 199.--Fissurella nembosa.
(Living shell.)]
How can we explain the _glacial period_? We have explained M. Adhémar’s
hypothesis, to which it may be objected that the cold of the glacial
period was so general throughout the Polar and temperate regions on both
sides of the equator, that mere local changes in the external
configuration of our planet and displacement of the centre of gravity
scarcely afford adequate causes for so great a revolution in
temperature. Sir Charles Lyell, speculating upon the suggestion of
Ritter and the discovery of marine shells spread far and wide over the
Sahara Desert by Messrs. Escher von der Linth, Desor, and Martins--which
seem to prove that the African Desert has been under water at a very
recent period--infers that the Desert of Sahara constituted formerly a
wide marine area, stretching several hundred miles north and south, and
east and west. “From this area,” he adds, “the south wind must formerly
have absorbed moisture, and must have been still further cooled and
saturated with aqueous vapour as it passed over the Mediterranean. When
at length it reached the Alps, and, striking them, was driven into the
higher and more rarefied regions of the atmosphere, it would part with
its watery burthen in the form of snow; so that the same aërial current
which, under the name of the Föhn, or Sirocco, now plays a leading part
with its hot and dry breath, sometimes, even in the depth of winter, in
melting the snow and checking the growth of glaciers, must, at the
period alluded to, have been the principal feeder of Alpine snow and
ice.”[115] Nevertheless, we repeat, no explanation presents itself which
can be considered conclusive; and in science we should never be afraid
to say, _I do not know_.
[115] Lyell’s “Elements of Geology,” p. 175.