declared his belief that a great ice-sheet once covered the northern
half of both countries, rounding the hills, scooping out the valleys and
lake basins, and scattering the boulders, gravel, and clay, as it
gradually melted away.
The belief of Professor Agassiz was not accepted at once, but further
studies prove that he guessed the riddle of the boulders. The rich soil
of the Northern States is the glacial drift--the mixture of rock
fragments of all sizes with fine boulder clay, left by the gradual
melting of the great ice-sheet as it retreated northward at the end of
the "Glacial Epoch."
GLACIERS AMONG THE ALPS
Switzerland is a little country without any seacoast, mountainous, with
steep, lofty peaks, and narrow valleys. The climate is cool and moist,
and snow falls the year round on the mountain slopes. A snow-cap covers
the lower peaks and ridges. Above the level of nine thousand feet the
bare peaks rise into a dry atmosphere; but below this altitude, and
above the six thousand-foot mark, lies the belt of greatest snowfall.
Peaks between six and nine thousand feet high are buried under the
Alpine snow-field, which adds thickness with each storm, and is drained
away to feed the rushing mountain streams in the lower valleys.
The snow that falls on the steep, smooth slope clings at first; but as
the thickness and the weight of these snow banks increase, their hold on
the slope weakens. They may slip off, at any moment. The village at the
foot of the slope is in danger of being buried under a snow-slide, which
people call an avalanche. "Challanche" is another name for it. The
hunter on the snow-clad mountains dares not shout for fear that his
voice, reechoing among the silent mountains, may start an avalanche on
its deadly plunge into the valley.
On the surface of the snow-field, light snow-flakes rest. Under them the
snow is packed closer. Deeper down, the snow is granular, like pellets
of ice; and still under this is ice, made of snow under pressure. The
weight of the accumulated snow presses the underlying ice out into the
valleys. These streams are the glaciers--rivers of ice.
The glaciers of the Alps vary in length from five to fifteen miles, from
one to three miles in width, and from two hundred to six hundred feet in
thickness. They flow at the rate of from one to three feet a day, going
faster on the steeper slopes.
It is hard to believe that any substance as solid and brittle as ice can
flow. Its movement is like that of stiff molasses, or wax, or pitch. The
tremendous pressure of the snow-field pushes the mass of ice out into
the valleys, and its own weight, combined with the constant pressure
from behind, keeps it moving.
The glacier's progress is hindered by the uneven walls and bed of the
valley, and by any decrease in the slope of the bed. When a flat, broad
area is reached, a lake of ice may be formed. These are not frequent in
the Alps. The water near the banks and at the bottom of a river does not
flow as swiftly as in the middle and at the surface of the stream. The
flow of ice in a glacier is just so. Friction with the banks and bottom
retards the ice while the middle parts go forward, melting under the
strain, and freezing again. There is a constant readjusting of
particles, which does not affect the solidity of the mass.
The ice moulds itself over any unevenness in its bed if it cannot remove
the obstruction. The drop which would cause a small waterfall in a
river, makes a bend in the thick body of the ice river. Great cracks,
called _crevasses_, are made at the surface, along the line of the bend.
The width of the V-shaped openings depends upon the depth of the glacier
and the sharpness of the bend that causes the breaks.
Rocky ridges in the bed of the ice-stream may cause crevasses that run
lengthwise of the glacier. Snow may fill these chasms or bridge them
over. The hunter or the tourist who ventures on the glacier is in
constant danger, unless he sees solid ice under him. Men rope themselves
together in climbing over perilous places, so that if one slips into a
crevasse his mates can save him.
A glacier tears away and carries away quantities of rock and earth that
form the walls of its bed. As the valley narrows, tremendous pressure
crowds the ice against the sides, tearing trees out by the roots and
causing rock masses to fall on the top of the glacier, or to be dragged
along frozen solidly into its sides. The weight of the ice bears on the
bed of the glacier, and its progress crowds irresistibly against all
loose rock material. The glacier's tools are the rocks it carries frozen
into its icy walls and bottom. These rocks rub against the walls,
grinding off debris which is pushed or carried along. No matter how
heavy the boulders are that fall in the way of the ice river, the ice
carries them along. It cannot drop them as a river of water would do.
Slowly they travel, and finally stop where the nose of the glacier melts
and leaves all debris that the mountain stream, fed by the melting of
the ice, cannot carry away.
The bedrock under a glacier is scraped and ground and scored by the
glacier's tools--the rock fragments frozen into the bottom of the ice.
These rocks are worn away by constant grinding, just as a steel knife
becomes thin and narrow by use. Scratches and scorings and polished
surfaces are found in all rocks that pass one another in close contact.
Its worn-out tools the glacier drops at the point where its ice melts.
This great, unsorted mass of rock meal and coarser debris the stream is
gradually scattering down the valley.
The name "moraine" has been given to the earth rubbish a glacier
collects and finally dumps. The _top moraine_ is at the surface of the
ice. The _lateral moraines_, one at each side, are the debris gathered
from the sides of the valley. The _ground moraine_ is what debris the
ice pushes and drags along on the bottom. The _terminal moraine_ is the
dumping-ground of this mass of material, where the ice river melts.
Glaciers, like other rivers, often have tributary streams. A _median
moraine_, seen as a dark streak running lengthwise on the surface of a
glacier, means that two branch glaciers have united to form this one. Go
back far enough and you will reach the place where the two streams come
together. The two lateral moraines that join form the middle line of
debris, the median moraine. Three ice-streams joined produce two top
moraines. They locate the lateral moraines of the middle glacier.
The surface of a glacier is often a mass of broken and rough ice,
forming a series of pits and pinnacles that make crossing impossible.
The sun melts the surface, forming pools and percolating streams of
water, that honeycomb the mass. Underneath, the ice is tunnelled, and a
rushing stream flows out under the end of the glacier. It is not clear,
but black with mud, called _boulder clay_, or _till_, made of ground
rock, and mixed with fragments of all shapes and sizes. This is the meal
from the glacier's mill, dumped where the water can sift it.
"Balanced rocks" are boulders, one upon another, that once lay on a
glacier, and were left in this strange, unstable position when the
supporting ice walls melted away from them. In Bronx Park in New York
the "rocking stone" always attracts attention. The glacier that lodged
it there, also rounded the granite dome in Central Park and scattered
the rock-strewn boulder clay on Long Island. Doubtless in an earlier day
the edges of this glacier were thrust out into the Atlantic, not far
from the Great South Bay, and icebergs broke off and floated away.
[Illustration: Potsdam sandstone showing ripple marks]
[Illustration: _By permission of the American Museum of Natural History_
Glacial striae on Lower Helderberg limestone]
[Illustration: Glacial grooves in the South Meadow, Central Park, New
York]
[Illustration: _By permission of the American Museum of Natural History_
Mt. Tom, West 83d St., New York]
Glaciers are small to-day compared with what they were long ago, in
Europe and in America. The climate became warmer, and the ice-cap
retreated. Old moraines show that the ice rivers of the Alps once came
much farther down the valleys than they do now. Smooth, deeply scored
domes of rock, the one in Central Park and the bald head of Mount Tom,
are just like those that lie in Alpine valleys from which the glaciers
have long ago retreated. There are old moraines far up the sides of
valleys, showing that once the glaciers were far deeper than now. No
other power could have brought rocks from strata higher up the
mountains, and lodged them thus.
Nearer home, Mt. Shasta and Mt. Rainier still have glaciers that have
dwindled in size, until they bear little comparison to the gigantic
ice-streams that once filled the smooth beds their puny successors flow
into. Remnants of glaciers lie in the hollows of the Sierras. We must go
north to find the snow-fields of Alaska and glaciers worthy to be
compared with those ancient ice rivers whose work is plainly to be seen,
though they are gone.
THE GREAT ICE-SHEET
Greenland is green only along its southern edge, and only in summer, so
its name is misleading. It is a frozen continent lying under a great
ice-cap, which covers 500,000 square miles and is several thousand feet
in thickness. The top of this icy table-land rises from five thousand to
ten thousand feet above the sea-level. The long, cold winters are marked
by great snowfall, and the drifts do not have time to melt during the
short summer; and so they keep getting deeper and deeper. Streams of ice
flow down the steeps into the sea, and break off by their weight when
they are pushed out into the water. These are the icebergs which float
off into the North Atlantic, and are often seen by passengers on
transatlantic steamers.
Long ago Greenland better deserved its name. Explorers who have climbed
the mountain steeps that guard the unknown ice-fields of the interior
have discovered, a thousand feet above the sea-level, an ancient beach,
strewn with shells of molluscs like those which now inhabit salt water,
and skeletons of fishes lie buried in the sand. It is impossible to
think that the ocean has subsided. The only explanation that accounts
for the ancient beach, high and dry on the side of Greenland's icy
mountain is that the continent has been lifted a thousand feet above its
former level. This is an accepted fact.
We know that climate changes with changed altitude as well as latitude.
Going up the side of a mountain, even in tropical regions, we may reach
the snow-line in the middle of summer. Magnolia trees and tree ferns
once grew luxuriantly in Greenland forests. Their fossil remains have
been found in the rocks. This was long before the continent was lifted
into the altitude of ice and snow. And it is believed that the climate
of northern latitudes has become more severe than formerly from other
causes. It is possible that the earth's orbit has gradually changed in
form and position.
If Greenland should ever subside until the ancient beach rests again at
sea-level, the secrets of that unknown land would be revealed by the
melting of the glacial sheet that overspreads it. Possibly it would turn
out to be a mere flock of islands. We can only guess. North America had,
not so long ago, two-thirds of its area covered with an ice-sheet like
that of Greenland, and a climate as cold as Greenland's. At this time
the land was lifted two to three thousand feet higher than its present
level. All of the rain fell as snow, and the ice accumulated and became
thicker year by year. Instead of glaciers filling the gorges, a great
ice flood covered all the land, and pushed southward as far as the Ohio
River on the east and Yellowstone Park in the west. The Rocky Mountains
and some parts of the Appalachian system accumulated snow and formed
local glaciers, separated from the vast ice-sheet.
The unstable crust of the earth began to sink at length, and gradually
the ice-sheet's progress southward was checked, and it began to recede
by melting. All along the borders of this great fan-shaped ice-field
water accumulated from the melting, and flooded the streams which
drained it to the Atlantic and the Gulf. Icebergs broken off of the edge
of the retiring ice-sheet floated in a great inland sea. The land sank
lower and lower until the general level was five hundred to one thousand
feet lower than it now is. The climate became correspondingly warm, and
the icebergs melted away. Then the land rose again, and in time the
inland sea was drained away into the ocean, except for the waters that
remained in thousands of lakes great and small that now occupy the
region covered by the ice.
Ancient sea beaches mark the level of high water at the time that the
flood followed the melting ice. On the shores of Lake Champlain, but
nearly five hundred feet higher than the present level of the lake,
curious geologists have found many kinds of marine shells on a
well-marked old sea beach. The members of one exploring party in the
same region were surprised and delighted to come by digging upon the
skeleton of a whale that had drifted ashore in the ancient days when
the inland sea joined the Atlantic.
Lake Ontario's ancient beach is five hundred feet above the present
water-level; Lake Erie's is two hundred fifty feet above it; Lake
Superior's three hundred thirty feet higher than the present beach. No
doubt when the water stood at the highest level, the Great Lakes formed
one single sheet of water which settled to a lower level as the rivers
flowing south cut their channels deep enough to draw off the water
toward the Gulf. Lake Winnipeg is now the small remnant of a vast lake
the shores of which have been traced. The Minnesota River finally made
its way into the Mississippi and drained this great area the stranded
beaches of which still remain. The name of Agassiz has been given to the
ancient lake formed by the glacial flood and drained away thousands of
years ago but not until it had built the terraced beach which locates it
on the geological map of the region.
When the ice-sheet came down from the north it dragged along all of the
soil and loose rock material that lay in its path. With the boulders
frozen into its lower surface it scratched and grooved the firm bedrock
over which it slid, and rounded it to a smooth and billowy surface. The
progress of the ice-sheet was southward, but it spread like a fan so
that its widening border turned to east and west.
When it reached its southernmost limit and began to melt, it laid down a
great ridge of unsorted rock material, remnants of which remain to this
day,--the terminal moraine of the ancient ice-sheet. The line of this
ancient deposit starts on Long Island, crosses New Jersey and
Pennsylvania, then dips southward, following the general course of the
Ohio River to its mouth, forming bluffs in southern Ohio, Indiana, and
Illinois. The line bends upward as it crosses central Missouri, a corner
of Kansas, and eastern Nebraska, parallel with the course of the
Missouri.
As the ice-sheet melted, boulders were dropped all over the Northern
States and Canada. These were both angular and rounded. In some places
they are scattered thickly over the surface and are so numerous as to be
a great hindrance to agriculture. In many places great boulders of
thousands of tons weight are perched on very slight foundations, just
where they lodged when the ice went off and left them, after carrying
them hundreds of miles. Around them are scattered quantities of loose
rock material, not scored or ground as are those which were carried on
the under-surface of the glacial ice. These unscarred fragments rode on
the top of the ice. They were a part of the top moraine of the glacial
sheet.
The finest material deposited is rock meal, ground by the great glacial
mill, and called "boulder clay." It is a stiff, dense, stony paste in
which boulders of all sizes, gravel, pebbles, and cobblestones are
cemented.
The "drift" of the ice-sheet is the rubbish, coarse and fine, it left
behind as it retreated. Below the Ohio River there is a deep soil
produced by the decay of rocks that lie under it. North of Ohio is
spread that peculiar mixture of earth and rock fragments which was
transported from the north and spread over the land which the ice-sheet
swept bare and ground smooth and polished.
The drift has been washed away in places by the floods that followed the
ice. Granite domes are thus exposed, the grooves and scratches of which
tell in what direction the ice flood was travelling. Miles away from
that scored granite, but in the same direction as the scratches,
scattered fragments of the same foundation rock cover fields and
meadows. Thus, much of the drift material can be traced to its original
home, and the course of the ice-sheet can be determined. Many immense
boulders the home of which was in the northern highlands of Canada rode
southward, frozen into icebergs that floated in the great inland sea.
Great quantities of debris were added to the original glacial drift
through the agency of these floating ice masses, which melted by slow
degrees.
FOLLOWING SOME LOST RIVERS
What would you think if the boat in which you were floating down a
pleasant river should suddenly grate upon sand, and you should look over
the gunwale and find that here the waters sank out of sight, the river
ended? I believe you would rub your eyes, and feel sure that you were
dreaming. Do not all rivers flow along their beds, growing larger with
every mile, and finally empty their waters into a sea, or bay, or lake,
or flow into some larger stream? This is the way of most rivers, but
there are exceptions. In the Far West there are some great rivers that
absolutely disappear before they reach a larger body of water. They
simply sink away into the sand, and sometimes reappear to finish their
courses after flowing underground for miles. Do you know the name of one
great western river of which I am thinking? Is there any stream in your
neighbourhood which has such peculiar ways?
Down in Kentucky there is a region where, it is said, one may walk fifty
miles without crossing running water. In the middle of our country, in
the region of plentiful rainfall, and in a state covered with beautiful
woodlands and famous for blue grass and other grain crops, it is
amazing that, over a large area, brooks and larger streams are lacking.
In most of the state there is plenty of water flowing in streams like
those in other parts of the eastern half of the United States. In the
near neighbourhood of this peculiar section of the state the streams
come to an end suddenly, pouring their water into funnel-shaped
depressions of the ground called sink-holes. After a heavy rain the
surface water, accumulating in rivulets, may also be traced to small
depressions which seem like leaks in the earth's crust, into which the
water trickles and disappears.
It must have been noticed by the early settlers who came over the
mountains from the eastern colonies, and settled in the new, wild, hilly
country, which they called Kentucky. The first settlers built their log
cabins along the streams they found, and shot deer and wild turkey and
other game that was plentiful in the woods. The deer showed them where
salt was to be found in earthy deposits near the streams; for salt is
necessary to every creature. Deer trails led from many directions to the
"salt licks" which the wild animals visited frequently.
Perhaps the same pioneers who dug the salt out of the earth found
likewise deposits of _nitre_, called also _saltpetre_, a very precious
mineral, for it is one of the elements necessary in the manufacture of
gunpowder. With the Indians all about him, and often showing themselves
unfriendly, the pioneer counted gunpowder a necessity of life. He relied
on his gun to defend and to feed his family. There were men among those
first settlers who knew how to make gunpowder, and saltpetre was one of
the things that had to be carried across the mountains into Kentucky,
until they found it in the hills. No wonder that prospectors went about
looking for nitre beds in the overhanging ledges of rocks along
stream-beds. In such situations the deposits of nitre were found. The
earth was washed in troughs of running water to remove the clayey
impurity. After a filtering through wood-ashes, the water which held the
nitre in solution was boiled down, and left to evaporate, after which
the crystals of saltpetre remained.
Solid masses of saltpetre weighing hundreds of pounds were sometimes
found in protected corners under shelving rocks. It was no doubt in the
fascinating hunt for lumps of this pure nitre that the early prospectors
discovered that the streams which disappeared into the sink-holes made
their way into caverns underground. Digging in the sides of ravines
often made the earthy wall cave in, and the surprised prospector stood
at the door of a cavern. The discoverer of a cave had hopes that by
entering he might find nitre beds richer than those he could uncover on
the surface, and this often turned out to be true. The hope of finding
precious metals and beds of iron ore also encouraged the exploration of
these caves. By the time the war of 1812 was declared, the mining of
saltpetre was a good-sized industry in Kentucky. Most of the mineral was
taken out of small caves, and shipped, when purified, over the
mountains, on mule-back by trails, and in carts over good roads that
were built on purpose to bring this mineral product to market. As long
as war threatened the country, the Government was ready to buy all the
saltpetre the Kentucky frontiersmen could produce. And the miners were
constantly in search of richer beds that promised better returns for
their labour.
It was this search that led to the exploration of the caves discovered,
although the explorer took his life in his hands when he left the
daylight behind him and plunged into the under-world.
Not all lost rivers tell as interesting stories or reveal as valuable
secrets as did those the neighbours of Daniel Boone traced along their
dark passages underground, and finally saw emerge as hillside springs,
in many cases, to feed Kentucky rivers. But it is plain that no river
sinks from sight unless it finds porous or honeycombed rocks that let it
through. The water seeks the nearest and easiest route to the sea. Its
weight presses toward the lowest level, always. The more water absorbs
of acid, the more powerfully does it attack and carry away the substance
of lime rocks through which it passes.
THE MAMMOTH CAVE OF KENTUCKY
There is no more fertile soil in the country than that of the famous
blue grass region of Kentucky. The surface soil rests upon a deep
foundation of limestone rocks, and very gradually the plant food locked
up in these underlying strata is pulled up to the surface by the soil
water, and greedily appropriated by the roots of the plants.
Part of the water of the abundant rainfall of this region soaks into the
layers of the lime rock, carrying various acids in solution which give
it power to dissolve the limestone particles, and thus to make its way
easily through comparatively porous rock to the very depths of the
earth. So it has come about that the surface of the earth is undermined.
Vast empty chambers have been carved by the patient work of trickling
water, which has carried away the lime that once formed solid and
continuous layers of the earth's crust. We must believe that the work
has taken thousands of years, at least, for no perceptible change has
come to these wonderful caves since the discovery and exploration of
them a century and more ago.
The streams that flow into the region of these caves disappear suddenly
into sink-holes and flow through caverns. After wearing away their
subterranean channels, leaping down from one level to another, forming
waterfalls and lakes, some emerge finally through hillsides in the form
of springs.