You all know the history of the Nile; how, when the rains fall
very heavily in March and April in the mountains of Abyssinia,
the river comes rushing down and brings with it a load of mud
which it spreads out over the Nile valley in Egypt. This annual
layer of mud is so thin that it takes a thousand years for it to
become 2 or 3 feet thick; but besides that which falls in the
valley a great deal is taken to the mouth of the river and there
forms new land, making what is called the "Delta" of the Nile.
Alexandria, Rosetta, and Damietta, are towns which are all built
on land made of Nile mud which was carried down ages and ages ago,
and which has now become firm and hard like the rest of the
country. You will easily remember other deltas mentioned in books,
and all these are made of the mud carried down from the land to
the sea. The delta of the Ganges and Brahmapootra in India, is
actually as large as the whole of England and Wales, (58,311
square miles.) and the River Mississippi in America drains such a
large tract of country that its delta grows, Mr. Geikie tells us,
at the rate of 86 yards in year.
All this new land laid down in Egypt, in India, in America, and
in other places, is the work of water. Even on the Thames you
may see mud-banks, as at Gravesend, which are made of earth
brought from the interior of England. But at the mouth of the
Thames the sea washes up very strongly every tide, and so it
carries most of the mud away and prevents a delta growing up
there. If you will look about when you are at the seaside, and
notice wherever a stream flows down into the sea, you may even
see little miniature deltas being formed there, though the sea
generally washes them away again in a few hours, unless the
place is well sheltered.
This, then, is what becomes of the earth carried down by rivers.
Either on plains, or in lakes, or in the sea, it falls down to
form new land. But what becomes of the dissolved chalk and other
substances? We have seen that a great deal of it is used by river
and sea animals to build their shells and skeletons, and some of
it is left on the surface of the ground by springs when the water
evaporates. It is this carbonate of lime which forms a hard crust
over anything upon which it may happen to be deposited, and then
these things are called "petrified."
But it is in the caves and hollows of the earth that this
dissolved matter is built up into the most beautiful forms. If
you have ever been to Buxton in Derbyshire, you will probably
have visited a cavern called Poole's Cavern, not far from there,
which when you enter it looks as if it were built up entirely of
rods of beautiful transparent white glass, hanging from the
ceiling, from the walls, or rising up from the floor. In this
cavern, and many others like it,*(See the picture at the head of
the lecture.) water comes dripping through the roof, and as it
falls carbonate of lime forms itself into a thin, white film on
the roof, often making a complete circle, and then, as the water
drips from it day by day, it goes on growing and growing till it
forms a long needle-shaped or tube-shaped rod, hanging like an
icicle. These rods are called stalactites, and they are so
beautiful, as their minute crystals glisten when a light is
taken into the cavern, that one of them near Tenby is called the
"Fairy Chamber." Meanwhile, the water which drips on to the
floor also leaves some carbonate of lime where it falls, and this
forms a pillar, growing up towards the roof, and often the hanging
stalactites and the rising pillars (called stalagmites) meet in
the middle and form one column. And thus we see that underground,
as well as aboveground, water moulds beautiful forms in the crust
of the earth. At Adelsberg, near Trieste, there is a magnificent
stalactite grotto made of a number of chambers one following
another, with a river flowing through them; and the famous Mammoth
Cave of Kentucky, more than ten miles long, is another example of
these wonderful limestone caverns.
But we have not yet spoken of the sea, and this surely is not
idle in altering the shape of the land. Even the waves
themselves in a storm wash against the cliffs and bring down
stones and pieces of rock on to the shore below. And they help
to make cracks and holes in the cliffs, for as they dash with
force against them they compress the air which lies in the joints
of the stone and cause it to force the rock apart, and so larger
cracks are made and the cliff is ready to crumble.
It is, however, the stones and sand and pieces of rock lying at
the foot of the cliff which are most active in wearing it away.
Have you never watched the waves breaking upon a beach in a
heavy storm? How they catch up the stones and hurl them down
again, grinding them against each other! At high tide in such a
storm these stones are thrown against the foot of the cliff, and
each blow does something towards knocking away part of the rock,
till at last, after many storms, the cliff is undermined and large
pieces fall down. These pieces are in their turn ground down to
pebbles which serve to batter against the remaining rock.
Professor Geikie tells us that the waves beat in a storm against
the Bell Rock Lighthouse with as much force as if you dashed a
weight of 3 tons against every square inch of the rock, and
Stevenson found stones of 2 tons' weight which had been thrown
during storms right over the ledge of the lighthouse. Think what
force there must be in waves which can lift up such a rock and
throw it, and such force as this beats upon our sea-coasts and
eats away the land.
Fig. 28 is a sketch on the shores of Arbroath which I made some
years ago. You will not find it difficult to picture to
yourselves how the sea has eaten away these cliffs till some of
the strongest pieces which have resisted the waves stand out by
themselves in the sea. That cave in the left-hand corner ends in a
narrow dark passage from which you come out on the other side of
the rocks into another bay. Such caves as these are made chiefly
by the force of the waves and the air, bringing down pieces of
rock from under the cliff and so making a cavity, and then as the
waves roll these pieces over and over and grind them against the
sides, the hole is made larger. There are many places on the
English coast where large pieces of the road are destroyed by the
crumbling down of cliffs when they have been undermined by caverns
such as these.
Thus, you see, the whole of the beautiful scenery of the sea -
the shores, the steep cliffs, the quiet bays, the creeks and
caverns - are all the work of the "sculptor" water; and he works
best where the rocks are hardest, for there they offer him a
good stout wall to batter, whereas in places where the ground is
soft it washes down into a gradual gentle slope, and so the
waves come flowing smoothly in and have no power to eat away the
shore.
And now, what has Ice got to do with the sculpturing of the land?
First, we must remember how much the frost does in breaking up
the ground. The farmers know this, and always plough after a
frost, because the moisture, freezing in the ground, has broken
up the clods, and done half their work for them.
But this is not the chief work of ice. You will remember how we
learnt in our last lecture that snow, when it falls on the
mountains, gradually slides down into the valleys, and is pressed
together by the gathering snow behind until it becomes moulded
into a solid river of ice (see Fig. 29, Frontispiece). In
Greenland and in Norway there are enormous ice-rivers or glaciers,
and even in Switzerland some of them are very large. The Aletsch
glacier, in the Alps, is fifteen miles long, and some are even
longer than this. They move very slowly - on an average about 20
to 27 inches in the centre, and 13 to 19 inches at the sides every
twenty-four hours, in the summer and autumn. How they move, we
cannot stop to discuss now; but if you will take a slab of thin
ice and rest it upon its two ends only, you can prove to yourself
that ice does bend, for in a few hours you will find that its own
weight has drawn it down in the centre, so as to form a curve.
This will help you to picture to yourselves how glaciers can adapt
themselves to the windings of the valley, creeping slowly onwards
until they come down to a point where the air is warm enough to
melt them, and then the ice flows away in a stream of water. It is
very curious to see the number of little rills running down the
great masses of ice at the glacier's mouth, bringing down with
them gravel, and every now and then a large stone, which falls
splashing into the stream below. If you look at the glacier in the
Frontispiece, you will see that these stones come from those long
lines of stones and boulders stretching along the sides and centre
of the glacier. It is easy to understand where the stones at the
side come from; for we have seen that damp and frost cause pieces
to break off the surface of the rocks, and it is natural that
these pieces should roll down the steep sides of the mountains on
to the glacier. But the middle row requires some explanation. Look
to the back of the picture, and you will see that this line of
stones is made of two side rows, which come from the valleys
above. Two glaciers, you see, have there joined into one, and so
made a heap of stones all along their line of junction.
These stones are being continually, though slowly, conveyed by
the glacier, from all the mountains along its sides, down to the
place where it melts. Here it lets them fall, and they are
gradually piled up till they form great walls of stone, which
are called moraines. Some of the moraines left by the larger
glaciers of olden time, in the country near Turin, form high
hills, rising up even to 1500 feet.
Therefore, if ice did no more than carry these stone blocks, it
would alter the face of the country; but it does much more than
this. As the glacier moves along, it often cracks for a
considerable way across its surface, and this crack widens and
widens, until at last it becomes a great gaping chasm, or
crevasse as it is called, so that you can look down it right to
the bottom of the glacier. Into these crevasses large blocks of
rock fall, and when the chasm is closed again as the ice presses
on, these masses are frozen firmly into the bottom of the
glacier, much in the same way as a steel cutter is fixed in the
bottom of a plane. And they do just the same kind of work; for
as the glacier slides down the valley, they scratch and grind the
rocks underneath them, rubbing themselves away, it is true, but
also scraping away the ground over which they move. In this way
the glacier becomes a cutting instrument, and carves out the
valleys deeper and deeper as it passes through them.
You may always know where a glacier has been, even if no trace of
ice remains; for you will see rocks with scratches along them
which have been cut by these stones; and even where the rocks
have not been ground away, you will find them rounded like those
in the left-hand of the Frontispiece, showing that the glacier-
plane has been over them. These rounded rocks are called "roches
moutonnees," because at the distance they look like sheep lying
down.
You have only to look at the stream flowing from the mouth of a
glacier to see what a quantity of soil it has ground off from
the bottom of the valley; for the water is thick, and coloured a
deep yellow by the mud it carries. This mud soon reaches the
rivers into which the streams run; and such rivers as the Rhone
and the Rhine are thick with matter brought down from the Alps.
The Rhone leaves this mud in the Lake of Geneva, flowing out at
the other end quite clear and pure. A mile and a half of land
has been formed at the head of the lake since the time of the
Romans by the mud thus brought down from the mountains.
Thus we see that ice, like water, is always busy carving out the
surface of the earth, and sending down material to make new land
elsewhere. We know that in past ages the glaciers were much
larger than they are in our time; for we find traces of them
over large parts of Switzerland where glaciers do not now exist,
and huge blocks which could only have been carried by ice, and
which are called "erratic blocks," some of them as big as
cottages, have been left scattered over all the northern part of
Europe. These blocks were a great puzzle to scientific men till,
in 1840, Professor Agassiz showed that they must have been brought
by ice all the way from Norway and Russia.
In those ancient days, there were even glaciers in England; for
in Cumberland and in Wales you may see their work, in scratched
and rounded rocks, and the moraines they have left. Llanberis
Pass, so famous for its beauty, is covered with ice-scratches,
and blocks are scattered all over the sides of the valley. There
is one block high up on the right-hand slope of the valley, as
you enter from the Beddgelert side, which is exactly poised upon
another block, so that it rocks to and fro. It must have been
left thus balanced when the ice melted round it. You may easily
see that these blocks were carried by ice, and not by water,
because their edges are sharp, whereas if they had been rolled
in water, they would have been smoothed down.
We cannot here go into the history of that great Glacial Period
long ago, when large fields of ice covered all the north of
England; but when you read it for yourselves and understand the
changes on the earth's surface which we can see being made by
ice now, then such grand scenery as the rugged valleys of Wales,
with large angular stone blocks scattered over them, will tell
you a wonderful story of the ice of bygone times.
And now we have touched lightly on the chief ways in which water
and ice carve out the surface of the earth. We have seen that
rain, rivers, springs, the waves of the sea, frost, and glaciers
all do their part in chiselling out ravines and valleys, and in
producing rugged peaks or undulating plains - here cutting through
rocks so as to form precipitous cliffs, there laying down new land
to add to the flat country - in one place grinding stones to
powder, in others piling them up in gigantic ridges. We cannot go
a step into the country without seeing the work of water around
us; every little gully and ravine tells us that the sculpture is
going on; every stream, with its burden of visible or invisible
matter, reminds us that some earth is being taken away and carried
to a new spot. In our little lives we see indeed but the very
small changes, but by these we learn how greater ones have been
brought about, and how we owe the outline of all our beautiful
scenery, with its hills and valleys, its mountains and plains, its
cliffs and caverns, its quiet nooks and its grand rugged
precipices, to the work of the "Two great sculptors, Water and
Ice."