GLACIERS.
Glaciers are rivers of ice, and, like other rivers, some of them are
small and some very large. They flow down the gorges from high
mountains, whose peaks are always covered with a blanket of eternal
snow. Summer and winter the snow is precipitated upon these mountains,
and from time to time the heat of the sun's rays softens the snow, when
by its great weight it packs more closely together until it is, in many
cases, formed into solid ice-cakes. If we take a quantity of snow or a
quantity of granulated ice and put it under a sufficient pressure we can
produce clear solid ice, and it is by this process that ice is formed
out of the snow and hail that falls continually upon the tops of these
glacial mountains. We have seen that ice possesses certain viscous or
semi-fluidic properties and that it will yield to pressure, but if we
put it under sufficient tensional strain it snaps like glass or any
other brittle substance. As the snows upon these mountains pile up
higher and higher the pressure becomes greater and greater until it
reaches a point where the mass begins to move gradually down the
mountain side, following the gulches and defiles that furnish a path of
least resistance to its flow.
At the sides and bottom, where there is contact with the earth, the
movement is slower than it is at the surface and in the middle of the
ice stream. If there were no curves in the ravine or gulch through which
it flows the point of greatest movement would be confined to the middle
of its width. But in flowing through a winding gulch the most rapid flow
follow the lines of greatest pressure, and this line is deflected from
side to side, so that the line of greatest flow is more winding than is
the bottom of the valley through which it flows. (The movement is called
a "flow," but it is very sluggish, only a few inches in a day, as will
appear later.)
If the bottom and sides of the valley were straight the surface of the
ice would be comparatively even; I say comparatively, for as compared
with a smooth surface it would be very rough; but there would be none of
the great crevasses or openings now to be found in the ice, which
sometimes are very large and extend to a great depth. If in its downward
course the bottom of the ravine suddenly becomes steeper, the top of the
ice is put under a tensional strain which causes it to break, thus
forming the crevasses.
If at the bottom of the descent the valley curves upward or preserves
the straight line for a considerable distance, these crevasses will
close at the top and perhaps open at the bottom, and the blocks of ice
will freeze together to such an extent that the water caused by the
melting ice will flow on top until it comes to another crevasse, where
it runs through to the bottom or underflow, which is always an attendant
of a glacier.
The glacier continues its flow down the mountain side till in some cases
it reaches quite to the valley below, and in others it stops short, as
the action of the sun is so great that it melts entirely away at this
point as fast as it moves down. In the winter time, however, the glacier
may flow far down into the valley and will accumulate greatly in bulk,
owing to the fact that the ice forms from the precipitation of snow on
top faster than it melts away underneath. If it were not for the fact
that in summer the glaciers melt faster than they form, the whole valley
would in time become a great river of ice. It is the case in Switzerland
that some years the accumulation is greater from snowfall than
diminution from melting. If this condition should continue it would
become a serious matter.
In the downward flow of a glacier--slow as it is--there is an exhibition
of wonderful power; great bowlders are torn from their beds and either
ground to powder or carried down to the end of the glacier, to be
dropped with the other debris that has been carried there by the same
force, forming an accumulation that geologists call the "moraine." Of
these moraines we will speak more fully later on.
It was the privilege of the writer some years since to visit the great
glaciers of Switzerland and to some extent study their action. Some
rivers have their origin chiefly in melting glaciers. They start as ice
rivers and end in rivers of water. The effects during the great ice age
of some of these glacial rivers, which are now extinct, are very
remarkable; we shall have occasion to refer to them when we come to
treat of the glacial period.
There is a glacial river flowing which is fed largely by the great Rhone
glacier in Switzerland. The water from this river is almost as white as
milk, which is occasioned by the grinding action of the great ice blocks
on the rock as it flows down the sides of the mountain. These glacial
rivers are much higher in summer, of course, than in winter, some of
them having not only an annual fluctuation, but a diurnal one. The
former is caused by the cold of winter, and the latter because it
freezes to some extent at night and checks the flow of water. The
difference between day and night in these high altitudes is very marked.
While it is extremely hot in the sun, it is cool the moment we step into
the shade.
I remember walking across one of the glaciers in the Alps, called the
Mer de Glace, one clear day in summer, when I suffered so much from the
heat, although standing upon a sea of ice, that it was necessary to
carry an umbrella. In fact, during my stay there was a case of sunstroke
that occurred upon this same glacier. This intense heat during the day
melts the surface of the ice, which forms streams that run along on the
top of a glacier until they come to a crevasse or riffle in the ice
river, where they plunge down and become a part of the glacial stream
that is flowing underneath the ice.
The speed at which these ice streams flow varies greatly with the size
of the glacier as to width and depth and the steepness of the grade, and
many other conditions. In its movement the glacier is constantly bending
and freezing and being torn asunder by tensional strain, yielding and
liquefying at other points by pressure, only to freeze again when that
pressure is removed. This, taken in connection with the friction of the
great ice bowlders, produces a movement that is exceedingly complicated
in its actions and interactions.
According to Professor Tyndall's investigations, the most rapid movement
observed in the glaciers of Switzerland is thirty-seven inches per day
at the point of greatest movement. From this point each way the motion
gradually diminishes until it reaches the sides of the glacier, where
the motion is not more than two or three inches.
The great North American glaciers move at a much higher rate of speed.
We are indebted to Dr. G. Frederick Wright, author of "The Ice Age in
North America," who spent a month studying the Muir glacier in Alaska,
for many details concerning that great ice river. This glacier empties
into Muir Inlet, which is an offshoot of Glacier Bay. It is situated in
latitude 58 degrees 50 minutes and longitude 136 degrees 40 minutes west
of Greenwich. The bay into which this glacier empties is about thirty
miles long and from eight to twelve miles wide. This bay, with its great
glacier, has a setting of grand mountain peaks. I cannot do better than
to quote the words of Dr. Wright when he describes the location of this
glacier. Dr. Wright lived for a month in a tent on the edge of this bay,
a short distance below the face of the great glacier, where the icebergs
fell off every few minutes into the deep water.
He says: "To the south the calm surface of the bay opened outward into
Cross Sound twenty-five miles away. The islands dotting the smooth
surface of the waters below us seemed but specks, and the grand vista of
snowclad mountains guarding either side of Chatham Strait seemed
gradually to come to a point on the southern horizon. Westward toward
the Pacific was the marvelous outline of the southern portion of the St.
Elias Alps. The lofty peaks of Crillon, 15,900 feet high, and Fair
Weather, 15,500 feet high, about twenty-five miles away and about the
same distance apart, stood as sentinels over the lesser peaks."
The Muir glacier might be likened to a great inland sea of ice fed by
many tributaries or ice rivers. It narrows up at the point where it
empties into Muir Inlet to 10,664 feet, or a little over two miles. An
enormous pressure is exerted at this point, which causes the ice to flow
in the central portion at the rate of about seventy feet per day. There
is a continual booming, like the firing of a cannon, going on, caused by
the bursting of some great iceberg either before it takes its final leap
into the water or at the moment of its fall. At the point where these
great icebergs drop off into the water they stand like a solid wall 300
feet above its surface. Dr. Wright says: "From this point there is a
constant succession of falls of ice into the water, accompanied by loud
reports. Scarcely ten minutes, either night or day, passed during the
whole month without our being startled with such reports; and frequently
they were like thunder claps or the booming of cannon at the
bombardment of a besieged city, and this though our camp was two and
one-half miles below the ice front.... Repeatedly I have seen vast
columns of ice extending up to the full height of the front topple over
and fall into the water. How far these columns extended below the water
could not be told accurately, but I have seen bergs floating away which
were certainly 500 feet in length."
It is estimated that the cubical contents of some of these icebergs are
equal to 40,000,000 feet. This great glacier is fed by the constant
precipitation of snow upon the sides and peaks of the high mountains
that surround its vast amphitheater, which is floored with icebergs.
Wonderful as this seems to us to-day, it is scarcely a microscopic speck
of what existed during the ice age all over the northern part of North
America.
There are many other great glaciers in the mountains of the Pacific
coast. Some years ago I saw one of these immense glaciers in British
Columbia, from a point called Glacier Station, in the Selkirk Mountains,
on the Canadian Pacific Railroad. It was during the month of August,
when all of the region was pervaded by a dense smoke occasioned by
burning forests. This glacier is a very showy one, owing to the
steepness of the side of the mountain and its great breadth. All the
glaciers that exist to-day are gradually receding, and are destined
eventually to entirely disappear, unless there is a change in
meteorological conditions, which some scientists claim will be the case
if we only wait long enough, when again all this northern country will
be covered with a great ice sheet. There is no doubt in regard to the
facts concerning a glacial period that must have existed in the ages
past. To anyone who has made a study of the subject there is not wanting
abundant evidence to prove that this northern country was at one time
enveloped with a great ice sheet of enormous thickness. The conditions
that existed to bring about such a state of things have been the subject
of much speculation by philosophers, but no one, as yet, has arrived at
any very satisfactory conclusion. Many theories have been advanced, some
of them not worth considering, while others have many things that give
them a show of plausibility. But all of them have what is said of the
Darwinian theory, "a missing link." It will be interesting, however, and
also instructive, to know what can be said in favor of a set of
conditions that would produce such momentous results.