HOW THE MOUNTAINS WERE CARVED OUT.
And surely the mountain fadeth away,
And the rock is removed out of its place,
The waters wear away the stones:
The overflowings thereof wash away the dust of the earth.
_Job xiv. 18._
The mighty fortresses of the earth, which seem so imperishable, so
majestic in their strength, and have from time immemorial received
their title of "the everlasting hills," are nevertheless undergoing
constant change and decay. They cannot abide for ever. Those waste
leagues around their feet are loaded with the wrecks of what once
belonged to them; they are witnesses to the victory of the hostile
forces that are for ever contending with them, and pledges of a final
triumph. To those who will read their story, mountains stand like old
dismantled castles, mere wrecks of ruined masonry, that have nearly
crumbled away, telling us of a time when all their separate peaks
and crags were one solid mass, perhaps an elevated smooth plateau
untouched by the rude hand of time.
Let us now inquire how the work of destruction is accomplished.
Referring back to our illustration of the cathedral, given in chap.
v., pp. 143-147, the question we have now to consider is, how the
mountains were carved out into all these wonderful features of crag
and precipice, peak and pass, which are such a source of delight
to all who care for scenery. This work we included in the one word
"ornamentation." What, then, are the tools which Nature uses in this
work of carving out the hills? What are her axes and hammers, her
chisels and saws?
This question, like many others, must be answered by observing what
takes place at the present day. It is scarcely necessary to say that
mountains and mountain-ranges are not simply the result of upheaval,
though they have been upheaved. If that were so, they would probably
appear as long smooth, monotonous ridges, with no separate mountain
masses, no peaks, no glens or valleys; in some cases they might
appear as simply elevated and smooth plateaux. Such mountains, if
we may so call them, would be almost as uninteresting as the roof
of a gabled house down which the rain finds its way in one smooth
continuous sheet.
Mountains, reaching as they do into the higher regions of the
atmosphere, where the winds blow more fiercely than on the plains
below, storms rage more violently, and the extremes of heat and cold
are more severe,--in fact, where every process of change and decay
seems quickened,--suffer continually at the hands of the elements.
"Death must be upon the hills, and the cruelty of the tempests
smite them, and the thorn and the briar spring up upon them;
but they so smite as to bring their rocks into the fairest
forms, and so spring as to make the very desert blossom as the
rose."[23]
[23] Modern Painters.
Nature never leaves them alone, never gives them a brief armistice in
the long war that she wages against them. She is a relentless enemy,
ever on the move, and ever varying her methods of attack. Now she
assails them openly with her storm-clouds, and pelts them furiously
with driving rain; now we hear the thunder of her artillery, as she
pierces their crests with strange electric darts of fire; now she
secretly undermines their sides with her hidden sources of water,
till whole villages are destroyed by some fearful fall of overhanging
rocks (see chapter iii., pages 96-101). Her winds and gentle breezes
are for ever at work on their surfaces, causing them to crumble into
dust much in the same way as iron turns to rust.
Again, she heats them by day and then chills them suddenly at night,
under the cold starry sky, so that they crack under the strain of
expanding and contracting. Now she splits them with her ice-wedges;
now she furrows their sides with the dashing torrents and running
streams; and yet again she wears them gently down with her glaciers,
and carries away their débris--the token of her triumph--on those icy
streams, as conquering armies carry the spoils in procession.
This is, briefly, her mode of warfare; these are some of her tools,
_wind_, _rain_, _frost_, _snow_, _heat_ and _cold_, _streams_,
_rivers_, and _glaciers_. Lightning does occasionally break off
portions of a cliff or a mountain-peak; but compared to the others,
this agent is not very important.
Let us first inquire into the effects produced by the atmosphere.
The air around us is composed mainly of two well-known gases;
namely, oxygen and nitrogen. There is also a small proportion (about
one in ten thousand) of carbonic acid gas; a variable quantity of
water-vapour, and in the neighbourhood of towns, traces of other
noxious gases, such as sulphurous acid and chlorine.
Now, the nitrogen plays a very unimportant part, as it merely
serves to dilute the powerful gas, oxygen, which has such important
life-sustaining properties. We live by breathing oxygen; so do all
animals; and the more pure air we can contrive to get into our
lungs, the better. But undiluted oxygen would be too strong for us,
and so its strength is diminished by being mixed with four parts of
nitrogen; that is to say, the air only contains about one fifth by
volume, or bulk, of oxygen and four fifths of nitrogen.
Now, oxygen, being always ready to combine chemically with some other
element, is a great agent of change and decay. It attacks all the
metals except gold and platinum. Iron, we all know, oxidises, or
rusts, only too quickly; but copper, lead, silver, and other metals
are more or less attacked by it. So it is with all the rocks exposed
at or near the surface of the earth. Oxygen will, if it can, pick
out something to combine with and so bring about chemical changes
which lead to decay. But a much more powerful agent is the carbonic
acid gas in the atmosphere; although there is so little of it, there
is enough to play a very important part in causing rocks to crumble
away, and in some cases to dissolve them entirely. The supply of this
gas is continually being renewed, for all living animals breathe out
carbonic acid, and plants give it out by night. Under the influence
of sunlight plants give out oxygen, so that gas is supplied to the
air by day.
Both oxygen and carbonic acid gas are dissolved by rain as it falls
through the air; and so we cannot separate the effects of the dry
air by itself from those of rain and mist, which are more important
agents. The action of rain is partly mechanical, partly chemical, for
it not only beats against them, but it dissolves out certain mineral
substances that they contain.
All rocks are mixtures of two or more kinds of minerals, the
particles of each being often invisible to the naked eye. Thus
granites are essentially mixtures of felspar, quartz, and mica;
ordinary volcanic rocks ("trap-rocks") of felspar and augite;
sandstones consist mainly of particles of silica; limestones of
carbonate of lime; shales and slates of silicate of alumina, the
principal substance in clay. These grains are usually joined together
by a cement of some mineral differing more or less from the other
particles. Lime is found in many of the rocks as the cement that
binds their particles together; while oxide of iron and silica serve
this purpose in many other instances. Now, if the lime or iron or
silica is dissolved by water, the rock must tend to crumble away. Any
old building shows more or less manifold signs of such decay, and
this process is called "weathering." All this applies merely to the
surfaces of rocks; and if there were no other forces at work, their
rate of decay would be very slow.
But there are other forces at work. In the first place, sudden
changes of temperature have a destructive influence. If the sun
shines brightly by day, the rocks--especially in higher mountain
regions--are considerably expanded by the heat they receive; and if
a hot day is followed by a clear sky at night, the free radiation of
heat into space (see chap. ii., p. 39) causes them to become very
cold, and in cooling down they contract. In this way an internal
strain is set up which is often greater than they can bear, and so
they split and crack. Thus small pieces of rock are detached from a
mountain-side. An Alpine traveller told the writer that one night
when sleeping on a mountain-side, he heard stones rattling down at
frequent intervals. Livingstone records in his journal that when
in the desert he frequently heard stones splitting at night with a
report like that of a pistol. But sometimes the expansion by day is
sufficient to cause fragments of rock to be broken off.
Frost, however, is responsible for a vast amount of destruction among
rocks. When water freezes, it expands with tremendous force; and this
is the reason why water-pipes so frequently burst during a frost,
though we don't find it out until the thaw comes,--followed by long
plumbers' bills. Rocks, being traversed in several directions by
cracks, allow the water to get into them, and this in freezing acts
like a very powerful wedge; and so the rocks on the higher parts of
the mountains are continually being split up by Nature's ice-wedge.
The amount of rock broken up in this way every year is enormous.
Stone walls and buildings often suffer greatly from this cause during
a long frost, especially if the stone be of a more than usually
porous kind, that can take up a good deal of rain water.
Where trees, shrubs, etc., grow on rocks, the roots find their way
into its natural divisions, widened by the action of rain soaking
down into them; and as they grow, they slowly widen them, and in time
portions are actually detached in this manner. Moreover, the roots
and rootlets guide the rain water down into the cracks, or joints, as
they are called. Even the ivy that creeps over old ruined walls has a
decidedly destructive effect.
At the base of every steep mountain may be seen heaps of loose
angular stones; sometimes these are covered with soil, and form long
slopes on which trees and shrubs grow. Every one of the numerous
little gullies that furrow the mountain-sides has at its lower
end a similar little heap of stones. Sometimes a valley among the
mountains seems half choked with rocky fragments; and if these were
all removed, the valley would be deeper than it is. In some hot
countries, where the streams only flow in winter, this is especially
the case; for example, every valley, or "wady," in the region of
Mount Sinai and Mount Horeb is more or less choked up with boulders
and stones of every size, because the stones come down faster than
they can be carried away.
But the main work of carving out the hills and mountains of the
world is done by streams, rivers, and glaciers; and so we now pass
on to consider how they perform their tasks. Water by itself, even
when flowing fast, would be powerless to carve gorges and valleys
in the solid rock; but the stones which torrents and streams carry
along give them a marvellous grinding power, for with such material
a stream continually wears away its rocky bed. Moreover, the stones
themselves are all the while being rubbed down by each other, until
finally they are ground down to fine sand and mud, which help in the
work of erosion.
Every mountain stream or torrent runs in a ravine or valley of some
sort; and any traveller who will take the trouble to watch what goes
on there may easily convince himself that the ravine, gorge, or
valley has been carved out by the stream, aided by the atmospheric
influences to which we have already alluded.
But perhaps some may be inclined to look upon the ravine as a chasm
produced by some violent disturbance from below, whereby the rocks
were rent asunder, and that the stream somehow found its way into the
rent. A little inquiry will dispel this idea. In the first place,
such catastrophes are quite unknown at the present day; and as we
have more than once pointed out, the geologist's method is to apply
a knowledge of processes now in operation to the phenomena of the
rocks, in order to read their history. Secondly, no conclusion can be
accepted which is not supported strongly by evidence.
If such a rending of the rocks had taken place, there would assuredly
be some evidence of the fact. We should expect to find a great crack
running all along the bed of the stream; but of this there is no
sign. Go down in any weather when the stream is low, and look at
the rocks over which it flows, and you will search in vain for such
evidence. Instead of being broken, the rocks extend continually
across. You would also expect to find the strata "dipping," or
sloping away from the stream on each side, if they had been rent
by such an upheaval; but here again we are met by a total want of
evidence. Thirdly, a crack might be expected to run along more or
less evenly in one direction. But look at the ravine, follow it
up for some miles, and you will see that it winds along in a very
devious course, not in a straight line.
For these reasons, then, we must conclude that the ravine or
valley has been carved out by the stream; but perhaps the most
convincing arguments are afforded by the furrows and miniature
ravines so frequently met with on the sides of all mountains; and
it is impossible to examine these without concluding that they have
in every case been cut out of the solid rock by the little rapid
torrents that run along them after heavy rain. If we are fortunate
enough to see them on a thoroughly rainy day, we may derive much
instruction from watching the little torrents at work as they run
down the mountain-side, here and there dashing over the rocks in
little cascades, and bringing down to the base of the hill much of
the débris that forms higher up. In this way Nature gives us an
"object lesson," and seems to say: "Watch me at work here, and learn
from such little operations how I work on a larger scale, and carve
out my ravines and big valleys. Only give me plenty of time, and I
can accomplish much greater feats than this."
The question of time is no longer disputed; and all geologists are
willing to grant almost unlimited time, at least periods of time that
seem to us unlimited. Most streams have been flowing for thousands
of years; and when once we grant that, we find no difficulty in
believing that all valleys are the work of rain and rivers. Surely
no one would argue that the furrows on a mountain-side are all rents
which have been widened by the action of water; for if they were
rents, each must have been caused by some disturbance of the rocks
composing the mountain, and we should of course be able to see the
cracks for ourselves, and to find that the rocks had in some way been
disturbed and rent open.
Even the rain which falls on the road in a heavy shower teaches the
same simple but important lesson, as it runs off into the gutters
on each side; and we may often find the road furrowed by little
miniature rivers, that carve out for themselves tiny valleys as they
run off into the gutter, bringing with them much débris in the form
of mud and sand.
Sometimes a stream encounters in its course a layer of rock that is
harder than the rock underlying it. In this case the softer rock
is worn away faster, and the hard layer forms a kind of ridge at a
higher level; the result is a waterfall. Waterfalls are frequently
found in mountain streams. In this case, it is easy to trace the
ridge of harder rock running unbroken across the path of the stream,
showing clearly that it has not been rent in any way. First it showed
merely as a kind of step, but gradually the force of the falling
water told with greater effect on the softer rock below, wearing it
away more rapidly than that above, and so the depth of the waterfall
went on increasing year by year; and at the same time the hard layer
was slowly worn away until the stream sawed its way through.
Some river valleys are steep and narrow; others are broad, with
gently sloping sides. A careful study of the different valleys in
any large country such as Great Britain, shows that their forms vary
according to the nature of the rocks through which rivers flow. Where
hard rocks abound, the valleys are steep and narrow; where soft rocks
occur, the valleys are broad and low. This is only what might be
expected, for hard rocks are not easily worn away; a river must cut
its way through them, leaving cliffs on either side that cannot be
wasted away by rain. But in a district where clay or soft sandstone
occurs, the rain, as it finds its way to the valley, will wash them
away and give a smooth gentle slope to the sides of the valley.
It is very instructive to notice how the scenery of any district
depends on the nature of its prevailing rocks. Hard rocks give bold
scenery with steep hills and rocky defiles; while soft rocks make the
landscape comparatively flat and tame, though often very beautiful
in its way, especially where a rich soil abounds, so that we see
pleasant woods, rich pasture-land, and heavy crops in the fields.
Compare, for instance, the scenery of Kent or Surrey with that
of the Lake District or the west of Yorkshire. The difference
is due chiefly to the fact that in Kent and Surrey we have rocks
that succumb more easily to the action of rain and rivers, and
consequently are worn away more rapidly than the harder rocks in the
north country. Geologists have a word to express the effects of this
wear and tear; namely, "denudation," which means a stripping off, or
laying bare.
In Kent and Surrey the agents of denudation (rain and rivers, aided
by the effects of the air, of heat and cold, and so on) wear away the
whole surface of the county in a tolerably even and uniform manner,
because there are no hard rocks for them to contend with. In this
case rain washes away the sides of the valleys faster than the river
can carve its bed, consequently the valleys are shallow compared to
their width. And so the streams have broad valleys, while the hills
are smooth and gently rounded. Chalk, clay, and soft sandstone abound
there. The two latter rocks are washed away with comparative ease,
and the chalk is dissolved; whereas in the Lake District we have very
much harder and older rocks, that require to be split up and broken
by the action of frost, while every stream carves out for itself a
steep valley, and great masses of hard rock stand out as bold hills
or mountains, that seem to defy all the agents of denudation. Here
the opposite is the case, and the valleys are deepened faster than
they are widened. But for all that, a vast amount of solid rock has
been removed from the surface there, of which the mountains are, as
it were, but fragments that have escaped the general destruction.
Moreover, the rocks in this region have been greatly disturbed and
crumpled since they were first formed, and thereby thrown into
various shapes that give certain peculiar structures more or less
capable of resisting denudation.
Very effective illustrations of the power of rain by itself are
afforded by the "earth pillars" of the Tyrol, and "cañons" of
Colorado. The material of which they consist is called conglomerate,
because it is composed of stones and large blocks of rock with stiff
earth or clay between. All the taller ones have a big stone on the
top which protects the softer material below from being washed away
by heavy rains; and it is easily perceived that each pillar owes its
existence to the stone on the top, which prevents the soft materials
below it from being washed away. When, after a time, the weathering
of the soft strata diminishes the support of the capping boulders,
these at last topple over, and the pillar, thus left unprotected,
becomes an easy prey to the rain, and is rapidly washed away. Some
of the pillars are over a hundred feet in height. But it is only in
places where heavy rains fall that these interesting monuments of
denudation are to be seen.
By way of contrast we may turn now to a district in which very little
rain falls, but where the streams have a considerable slope, and so
can wear away, or erode, their valleys much faster than rain and
frost, etc., can bring down the rocks of which the sides are composed.
The river Colorado of the West, which runs from the Rocky Mountains
to the Gulf of California, flows for nearly three hundred miles at
the bottom of a profound chasm, or cañon, being hemmed in by vertical
walls which in some places are more than a mile in depth. The
tributary streams flowing into the river run through smaller ravines
forming side cañons; and there is no doubt that these wonderful
chasms have been, in the course of ages, slowly carved out by the
river Colorado and its numerous tributary streams. Sometimes the
walls of the cañon are not more than fifty yards apart, and in height
they vary from three thousand to six thousand feet.
Far above the level of the highest floods patches of gravel are found
here and there on the sides, which must have been left there by the
river when it had not cut its way so far down. These cañons afford
striking testimony to the erosive power of running water, of which
they are the most wonderful illustration in the world.
But water, even when in the form of ice, has more or less power to
wear away solid rock; and the glaciers that we see in Switzerland,
Norway, and other countries must slightly deepen the rocky valleys
down which they flow. Let us see how this can be accomplished.
The snow that falls in the High Alps, impelled by the weight of fresh
layers of snow overlying it, and by the slope of the mountain-sides,
gradually creeps down into the valleys. Owing to the pressure thus
put upon it, and partly to the melting power of the sun's rays, it
assumes the form of ice; and glaciers are composed of solid ice. The
downward motion is so slow that a glacier appears quite stationary;
and it is only by putting in stakes and watching them change their
positions that it can be shown to be moving.
In all respects except speed, glaciers flow like rivers, for ice is
a viscous body, behaving partly like a fluid and yet partly like
a solid substance; but it will not endure a sharp bend without
snapping. Hence, a glacier in traversing a valley frequently gets
split. The cracks thus formed widen by degrees until they expand
into chasms, or "crevasses." Like rivers, glaciers transport a large
amount of rocky matter to lower levels, and at the same time wear
away and deepen their rocky channels.
Let us see how they do this twofold work of transportation and
erosion. In the first place, a large amount of débris falls onto the
sides of a glacier from the peaks, precipices, and mountain-side
along which it flows. Some stones, however, fall down crevasses, and
so reach the bottom, where they become cemented in the ice. In this
way they are slowly carried down over the rocky floor of the valley,
until at last they reach the end of the glacier, where in the warmer
air the ice melts just as fast as it creeps down; and there they will
be left to form a heap of stones, sand, and mud.
Large blocks of stone, quite different from the rocks on which they
lie, are very numerous, and are called "erratics," since they are
evidently wanderers from a distance. Sometimes such blocks can be
proved to have been brought many miles from their home among the
higher peaks. The long lines of stones and mud seen on the sides of
a glacier are called "moraines," and at the end of every glacier we
find a big heap known as a "terminal moraine." But the stones of
which they are composed are probably not to be entirely accounted
for in this way. Can we not conceive that the weight and pressure of
a descending glacier may be sufficient to break off many protruding
portions of the rocky bed over which it flows, and then to drag them
along with it? This seems reasonable. Let us therefore consider
the materials of which moraines are composed to be derived partly
from the rocks beneath and partly from those above the glacier. But
whatever their origin, such materials must inevitably find their way
to the end of the glacier and be added to the big heap there. The
work of transportation is then taken up by the stream which always
flows from the end of a glacier. Such streams are in summer-time
laden with fine sediment, which gives them a milky and turbid
appearance.
Thus a glacier wears away the rocks over which it flows; rock
fragments become embedded in the ice, and these are the tools with
which a glacier does its work. It must be granted that the downward
movement of a great mass of ice is irresistible, and consequently
that as the moving glacier slowly creeps along, it must inevitably
cause the stones which it thus holds to grind over the surface of
the rock. It is easy to imagine the effects of this grinding action.
If sand-paper, rubbed for a minute or two over wood, wears down and
smooths its surface, what must be the result of all these stones,
together with sand and mud, grinding over the rocky bed?
The answer to this question is found in examining the rocks over
which glaciers once flowed. Now, the Swiss glaciers once extended
far beyond their present limits; and the rocks in the lower parts of
their present valleys, now free from ice, show unmistakable signs
of having been considerably worn down. The corners and angles of
projecting pieces of rock have been worn away until the once rugged
outline has become wavy and round, so much so as to produce more or
less resemblance to the backs of sheep lying down. Hence the name
_roches moutonnées_, by which rocks of this shape are known. They
frequently retain on their surface peculiar markings, such as long
scratches and grooves which must have been made as the old glacier,
with its embedded angular fragments of rock, slowly ground over
their surfaces. Such markings are called "striæ." But besides these
glacial records graven on the rocks, we have other evidence, in the
form of great moraines in some of the valleys of Switzerland, and
especially at those places where side valleys open out into a main
valley. Any one may learn by a little observation to recognise these
peculiar heaps of stones, mud, and sand, deposited long ago by the
old glaciers of Switzerland.
It will be perceived that the evidence for the erosive power of
glaciers is of two kinds,--first, there is the testimony of the
smoothed and striated rocks, which is very convincing; secondly,
the equally strong proofs from the moraines, both great and small.
These old rubbish heaps give us a very fair idea of the amount of
wear and tear that goes on under a glacier, for there we see the rock
fragments that tumbled down the mountain-side onto the surface of the
glacier (together with those which the glacier tore off its rocky
bed), all considerably smoothed, worn down, and striated. But a still
better idea of the work done is afforded by the gravel, mud, and sand
in which these stones are embedded. All this finer material must have
been the result of wear and tear. This kind of action may well be
compared to what takes place on a grindstone as one sharpens an axe
on it. The water poured on the stone soon becomes muddy, owing to the
presence of countless little grains of sand worn off the grindstone.
But a good deal of the mud thus formed is carried away by the little
stream that runs out from the end of every glacier; so that there is
more formed than we see in the moraine.
[Illustration: THE MER DE GLACE AND MONT BUET. FROM A PHOTOGRAPH
BY MR. DONKIN.]
We have already alluded in former chapters to the "Ice Age" in
Britain, when great glaciers covered all our high mountains, and
descended far and wide over the plains. Now, the evidence for the
former existence of these glaciers is of the same kind as that which
we have just described. In Wales and Scotland we may soon learn
to recognise the _roches moutonnées_, the old moraine heaps, and
the erratic boulders brought down by these old glaciers. Besides
these proofs, there is also the evidence of the arctic plants now
flourishing in the highlands (see chapter iv., pages 123-124).
There can be no doubt, then, that glaciers have an erosive action,
and therefore must be regarded as agents of denudation. But it is
important to bear in mind that their powers in this direction are
limited; for it is manifest that a mountain stream is a much more
powerful agent, and will deepen its little valley much more rapidly,
than a cumbrous, slow-moving glacier, advancing at the rate of a
few inches a day. It has been found by careful measurements that
the Mer de Glace of Chamouni moves during summer and autumn at the
average daily rate of twenty to twenty-seven inches in the centre,
and thirteen to nineteen and one half inches near the side, where
friction somewhat impedes its course. This seems very slow compared
to the rapid movement of a mountain stream; but then, a glacier
partly makes up for this by its great weight.
In considering a glacier as an agent of erosion, we must not forget
that probably a good deal of water circulates beneath glaciers. If
this is so, the water must have a considerable share in producing the
effects to which we have already alluded. It would be extremely rash
to conclude, as some students of glaciers have done, that valleys
can be carved out _entirely_ by glaciers; and we must be content
with believing that they have been somewhat deepened by ice-action,
and their features more or less altered, but no more. The valleys of
Switzerland, of Wales, and Scotland, were probably all in existence
before the period of the "Ice Age," having been carved out by streams
in the usual way; but the glaciers, as it were, put the final touches
and smoothed their surfaces.
Having learned how the three agents of denudation--namely, rain,
rivers, and glaciers--accomplish their work, let us now take a wider
view of the subject and consider the results of their united efforts
both in the present and in the past.
We have already alluded to the enormous amount of solid matter
brought down to the sea every year by rivers (see chap. v., pp.
166-168), and we pointed out that all this represents so much débris
swept off the land through which the rivers flow; also that it comes
down in three ways, one part being suspended in the water as fine
mud, another part being pushed along the river-bed as gravel, etc.,
while a third part is the carbonate of lime and other mineral matter
in a dissolved state, and therefore invisible.
Now, it is quite plain that rain and rivers, in sweeping away so much
solid matter from the surface of the land, must tend in the course
of time to lower its general level; and it therefore seems to follow
that after the lapse of ages any given continent or large island
might be entirely washed away, or in other words, reduced to the
level of the sea. This would certainly happen were it not that the
lands of the world seem to be slowly rising, so that the denudation
going on at the surface appears to be counterbalanced by continued
upheaval.
But, supposing no upheaval took place, how long would it take for
rain and rivers to wear away a whole continent? Let us see if there
is any way of answering this difficult question, for if it can be
even partially solved, it will help us to realise the enormous length
of time that must have been required to bring about the results of
denudation that we see all around us.
Although the calculations that have been made on this subject are
very complicated, yet the principle on which they are based is quite
simple. For an answer to our question we must go to the rivers again,
and measure the work they do in transporting solid matter down to
the sea. Let us take the Mississippi as a typical big river, for
it has been more carefully studied than any other, and it drains a
very extensive area, embracing many varieties of climate, rock, and
soil. As the result of many observations carried on continuously at
different parts of the river for months together, the engineers who
conducted the investigation found that the annual discharge of water
by this river is about nineteen thousand millions of cubic feet,
and that on the average the amount of sediment it contains is about
a 1/1500th part by weight. But besides the matter in suspension,
they observed that a large amount of sand, gravel, and stones is
being constantly pushed along the bottom of the river. This they
estimated at over seven hundred and fifty millions of cubic feet.
They also calculated that the Mississippi brings down every year
more than eight hundred thousand million pounds of mud. Putting the
two together, they found (as before stated) that the amount of solid
matter thus transported down to the Gulf of Mexico may be represented
by a layer 268 feet high, covering a space of one square mile; that
is, without allowing for what is brought down dissolved in the water,
which may be neglected in order to prevent any exaggeration.
Now, it is quite clear that all this débris must have come from the
immense area that is drained by the Mississippi. It could not have
been supplied by any rivers except those that are its tributaries.
And so if we can find out what is the extent of this area, it is not
difficult to calculate how much its general surface must have been
lowered, or in other words, how much must have been worn away from
it in order to supply all the material. This area is reckoned at
1,147,000 square miles; and a very simple calculation tells us that
the general surface would thus be lowered to the extent of 1/6000th
part of a foot. That of course means that one foot would be worn away
in six thousand years. On high ground and among mountains the rate of
denudation would of course be much greater; but we are now dealing
with an average for the whole surface.
The next thing we require to finish this calculation is the average
or mean height of the American continent. This was reckoned by the
celebrated Humboldt at 748 feet. Now if we may assume that all this
continent is being worn down at the same rate of one foot in six
thousand years (which is a reasonable assumption), we find, by a
simple process of multiplication, that it would require about four
and a half millions of years for rain and rivers to wash it all away
until its surface was all at the sea-level (with perhaps a few little
islands projecting here and there as relics of its vast denudation).
This is a very interesting result; and if the above measurements are
reliable, they afford us some idea of the rate at which denudation
takes place at the present time.
By a similar process it has been calculated the British Isles might
be levelled in about five and a half millions of years. Geologists
do not pretend to have solved this problem accurately; that is
impossible with our present knowledge. But even as rough estimates
these results are very valuable, especially when we come to study
the structure of the land in different countries, and to find out
therefrom, by actual measurement, how much solid rock has been
removed. We will now give some examples of this; but perhaps a simple
illustration will make our meaning clearer.
Suppose we picked up an old pair of boots, and found the soles worn
away in the centre. It would be easy to find out how much had been
worn away over the holes by simply measuring the thickness of leather
at the sides, where we will suppose that they were protected by
strong nails. Geologists apply a very similar kind of method in order
to find out how much rock has been removed from a certain region of
the earth. One of the simplest cases of this kind is that of the area
known as the Weald of Kent, Surrey, and Sussex (see illustration,
Fig. 1). A great deal of denudation has taken place here, because
there is ample evidence to prove that the great "formation" known
as the Chalk (now seen in the North and South Downs) once stretched
right across; and below this came the lower greensand and Weald clay.
They spread over this area in a low arch of which we now only see the
ruins.
[Illustration: Fig. 1. SECTION ACROSS THE WEALD OF KENT AND
SURREY.]
[Illustration: Fig. 2. THE HIGHLANDS OF SCOTLAND ON A TRUE SCALE
(after GEIKIE).]
The dotted lines in the figure show us their former extent; but
the vertical height is exaggerated, for otherwise the hills would
scarcely be seen.
These lines simply follow out the curves taken by the strata at each
end of the denuded arch, and therefore rightly indicate its former
height. By making such a drawing on a true scale, geologists can
easily measure the former height of the surface of this old arch,
or "anticline," of chalk, greensand, and other strata, just as an
architect might restore the outlines of an old traceried window from
a few portions left at the sides.
This very useful and instructive method is much employed in drawing
sections through mountain-chains, in order to gain some idea of the
amount of denudation which they have suffered.
Let us see how much has been removed from the present surface of
the Weald. First there is the chalk, which we may put down at six
hundred feet at least; then there is the lower greensand, say, eight
hundred feet; and below that, and forming the lowest ground in the
Weald, is the Weald clay, which is one thousand feet thick, and being
softer, was more rapidly borne away. Along the centre runs a ridge
of Hastings sand, forming higher ground on account of its greater
hardness, but this formation is not much denuded. However, adding
together the thicknesses of the others, we arrive at the conclusion
that about twenty-four hundred feet of chalk and other strata has
been removed from the present surface of the Weald. And all this
denudation has probably been effected by rain and rivers, for it is
very doubtful whether the sea had any share in this work.
But in other parts of our own country we find proofs of denudation
on a much grander scale than this; for example, in North Wales there
are rocks now lying exposed at the surface which are of a very much
greater antiquity than any that may be seen in the Wealden area,
belonging to the very ancient periods known as the Cambrian and
Silurian. These have evidently been exposed for a much longer time
to the action of denuding forces; and the Welsh hills, as we now
see them, are but fragments of what they once were. After carefully
mapping out the rocks in the neighbourhood of Snowdon, noting their
thickness, the directions in which they slope, or "dip," so that
the structure of this region might be ascertained, as in the case
of the Weald, it was found, on drawing sections of the rocks there,
and putting in dotted lines to continue the curves and slopes of the
strata as known at or near the surface, that from fifteen thousand
to twenty thousand feet of solid rock must have been removed (see
diagrams, chapter ix., p. 307). Applying the same method to the Lake
District, it has been calculated that the amount of denudation which
that beautiful country has suffered may be represented by twenty-six
thousand feet. Turning to the other side of the Atlantic, we find
the American geologists estimate that a thickness of five miles has
been removed from a large part of the Appalachian chain of mountains
(near their east coast), and that at least one mile has been eroded
from the entire region between the Rocky and Wahsatch Mountains (see
chapter ix.).
In conclusion, we must bear in mind that mountains, in spite of the
enormous erosion they have suffered, are more capable of resisting
the ever active agents of denudation than the softer rocks that form
the plains and lowlands, and consequently stand out in bold relief
from other features of the earth's surface. This truth has been
beautifully expressed in the following passage:--
" ... In order to bring the world into the form which it now
bears, it was not mere sculpture that was needed; the mountains
could not stand for a day unless they were formed of materials
altogether different from those which constitute the lower
hills and the surfaces of the valleys. A harder substance had
to be prepared for every mountain-chain, yet not so hard but
that it might be capable of crumbling down into earth, fit to
nourish the Alpine forest and the Alpine flowers; not so hard
but that in the midst of the utmost majesty of its enthroned
strength there should be seen on it the seal of death, and the
writing of the same sentence that had gone forth against the
human frame, 'Dust thou art and unto dust thou shalt return.'
And with this perishable substance the most majestic forms were
to be framed that were consistent with the safety of man, and
the peak was to be lifted and the cliff rent as high and as
steeply as was possible, in order yet to permit the shepherd
to feed his flocks upon the slope, and the cottage to nestle
beneath their shadow."[24]
[24] Modern Painters.