Copyright
Israel C. (Israel Cook) Russell.

Glaciers of North America; a reading lesson for students of geography and geology online

. (page 21 of 24)
Online LibraryIsrael C. (Israel Cook) RussellGlaciers of North America; a reading lesson for students of geography and geology → online text (page 21 of 24)
Font size
QR-code for this ebook


occur at lower temperatures.

4. Water expands on freezing, the increase in volume being about
one-tenth. The converse is also true.

5. Water held in fissures or in the interstices of glaciers exerts a
hydraulic pressure, and obeys the laws governing capillary attraction.

6. Ice like other solids expands and contracts with changes of
temperature.



HOW AND WHY GLACIERS MOVE. 187

7. Heat can be made to pass through ice, as shown by Tyndall, and
portions of the ice may be melted by heat that has passed through other
portions without producing visible changes.

8. The melting-point of ice is lowered by pressure.

9. A mixture of ice and water under a pressure not exceeding one
atmosphere has a temperature of 32 F.

10. Ice 011 melting absorbs heat. Water on freezing gives out heat.

11. The temperature of ice is raised by compression and lowered by
tension.

Other laws may be added to this list, but at present those enumerated
seem to be the principal ones that can be applied in solving the question
of the causes of glacier motion.

The phenomena exhibited by glaciers for which explanation is sought
may also be briefly enumerated :

1. Glaciers exhibit a well-defined flowing movement, analogous to
the flow of plastic substances.

2. The flow of a glacier, best illustrated by one of the alpine type,
is greatest in the center and at the surface, and decreases toward the sides
and bottom. That is, it is analogous to the flow of a river.

3. The flow is greater in summer than in winter, and greater by day
than by night. That is, it varies in harmony with changes in atmospheric
temperature, and is greatest when the temperature is highest. More than
this, observations have shown that changes in the rate of flow respond
with considerable promptness to changes of temperature.

4. The movements of any given point in a glacier are not uniform in
any one direction, but vary from hour to hour. In the case of an alpine
glacier so far as has been observed, the algebraic sum of the movements
by day are in the direction of descent, while at night there may be a
resultant displacement toward the mountain from which the glacier flows.

5. Motion occurs both in neve regions and in the glacier proper, and
so far as known is of the same nature in each instance ; but more extended
studies in this connection are desired.

6. The mean rate at which a glacier flows is not the same in different
portions of its course. That is, for example, the average rate of move-
ment of all points in a given cross section may vary widely from a similar
average in another cross section.

7. When the grade of a valley through which a glacier flows changes
abruptly, or when its bottom or sides are markedly irregular, the ice
becomes broken and crevassed. Tension is also produced under other



188 GLACIERS OF NORTH AMERICA.

conditions, as when a glacier expands 011 a plain, and fissures are again
formed.

8. Glacial ice abounds in fissures and interstices which are usually
filled with water. Near the surface the water held in this manner fre-
quently freezes at night. The effect of winter temperatures must be felt
to a still greater depth, but how deep has not been determined. Water
flows from beneath the extremity of nearly every alpine glacier, even in
winter, and to a great extent represents the drainage of the ice. Evidently
the fall of temperature in winter is not sufficient, or not long enough con-
tinued, to congeal all the water that enters the ice during the summer season.

9. Glacial ice is granular. Neve snow is also granular. As shown
by Heim, however, the granules of the neve are distinct and of a different
nature from the granules of glacier ice. In the glacier proper the granules
increase in size from near the neve to its extremity. In restricted areas
the granules are of approximately the same size, large and small grains not
being intermingled.

10. When glacial ice is broken, as when crevasses are formed, and the
fragments brought in contact, they refreeze.

11. The rocks over which glaciers move become worn and striated.
Hard nodules in glaciated rocks are frequently left in relief. " Chattel-
marks," semi-lunar cracks, etc., also occur on surfaces but recently aban-
doned by a receding glacier.

12. Rock basins but recently vacated by glacier ice are smoothed and
striated within, showing that debris-charged ice descended into them so
as to wear their surfaces.

13. Debris contained in a glacier tends to decrease its rate of flow.
If we conceive of a glacier compound of clear ice moving at a given rate
and introduce debris earth, sand, stones, boulders, etc. into it, with-
out altering other conditions, the effect will be to decrease the rate of
flow, since rigid substances are added to one having properties that are
at least analogous to those of a plastic solid. If we gradually increase
the percentage of debris, the mass will become less and less mobile, and
finally acquire such rigidity that under the conditions normally influen-
cing the movements of glaciers it will cease to flow. If the debris, instead
of being uniformly commingled with the ice, is introduced irregularly,
local changes in the rate of flow and even local stagnation will result. 1

1 The influence of debris on the flow of glaciers, based on the assumption that ice is plastic
and when in sufficiently large masses will flow under the influence of its own weight, has been
discussed by the author in The Journal of Geology [Chicago], vol. 3, 1895, pp. 823-832.



HOW AND WHY GLACIERS MOVE. 189

This list of facts, bearing more or less directly on the character of the
movements that take place in glaciers, and thus furnishing data for test-
ing proposed explanations of their movements, might be extended, but I
believe that the most suggestive observations now in hand have been
enumerated.

By codifying the laws governing the behavior of ice under various
conditions, and grouping the phenomena related more or less directly with
glacial flow, in the manner just attempted, it will appear, I think, that
the movements of glacial ice are more complex than has commonly been
stated, and are due at different times and under different conditions to
different agencies, or to the interaction of various agencies.

Of the forces to which glaciers are exposed which tend to change
their shapes, gravity is the only one that acts continually and always in
the same direction. The fact that ice, as shown by careful experiments,
will change its shape under the influence of its own weight at all temper-
atures from a maximum rate at 32 F. as far below as tests have been
carried, and yields continuously to tension as well as to pressure, is strong
evidence favoring the assumption that glaciers descend or flow in a
manner analogous to the flow of plastic bodies. Supplementing this
cause of glacier motion, although apparently in most instances of minor
importance, is the hydrostatic pressure of water enclosed in glacial ice ;
dilatation of water in freezing in fissures ; expansion and contraction of
the ice with changes of temperature ; melting and refreezing, due to
changes in pressure ; regulation ; and, less clearly, molecular changes
caused by the transmission of heat, and the melting, refreezing, and
growth of granules.

The authors of this eclectic hypothesis may be considered to be De
Saussure, Charpentier, Agassiz, Forbes, Rendu, Guyot, Tyndall, Thompson,
Croll, Geikie, Heim, Helmholtz, Moseley, McConnel, Chamberlin, and in
fact all physicists and glacialists who either directly or indirectly have
contributed to the study of glacial dynamics. More than this, the study
of the physical properties of ice and the application of principles already
known or to be discovered in explanation of glacier movements, is not
yet completed. To the list of distinguished names given above, as the
authors of the " eclectic hypothesis," are to be added the names of those
who in the future make contributions to our knowledge of the properties
of ice and of its behavior under various conditions. The new facts and
new principles discovered are to be included in this hypothesis, which
will thus continue to be an illustration of the evolution of ideas.



CHAPTER X.

THE LIFE HISTORY OP A GLACIER.

GLACIERS, like streams and lakes, valleys and mountains, have their
periods of youth, adolescence, maturity, and old age, leading to extinction.
Like the snows of winter they come and go in obedience to unseen forces.
Their growth and decline may embrace thousands and even tens of
thousands of years, but even the longest-lived witness but a portion of
the changes in topographical development to which they lend their aid.
The study of existing ice bodies leads backward step by step to the far
greater ice sheets of the glacial epoch. Although the causes that pro-
duced vast continental glaciers in comparatively recent geological times
are not well understood, and have been a fruitful source of controversy,
yet when one has in mind the life history of a single existing glacier, it
becomes evident that former periods of extensive glaciation were but
greater steps in the same direction. Methods of study are thus indicated,
and suggestions obtained for attacking unsolved problems in the history
of the earth.

As a beginning in this broad field of exploration, let us endeavor to
obtain a graphic idea of the changes made manifest in the birth, growth,
decline, and death of a single alpine glacier.

The snow line the lowest limit of perennial snow may be said to
have its position determined by the intersection of the earth's surface with
an invisible, hollow spheroid of temperature. This invisible spheroid
may for present purposes be fancied to pass through all points having a
mean annual temperature of 32 F. In the tropics it is some 18,000
feet above the sea, but decreases in elevation toward either pole. In
high latitudes, it may pass below the earth's surface. Its size and form
change in obedience to many far-reaching and frequently antagonistic
agencies, and is never the same for two consecutive years or for any
two terms of years that may be selected. It is modified from within
by changes in the inherent heat of the earth, in movements producing
elevations and depressions, in the distribution of land and water, in the
direction and character of ocean currents; in the movements of the atmos-
phere, in the distribution of vegetation, in topographic relief, and in other



THE LIFE HISTORY OF A GLACIER. 191

ways. It is modified from without, principally by annual and secular
changes in the amount of heat that reaches the earth from the sun due to
changes in the position of the earth, the inclination of the earth's axis,
and perhaps other causes.

When one endeavors to marshal in fancy the interaction of the various
conditions on which the fluctuations of the snow line depend, the wonderful
complexity of glacial problems is suggested. The difficulties to be over-
come are still farther increased when one recalls the fact that while
glaciers do not originate when the mean annual temperature is above 32,
they may not form when that limit is reached, unless still other conditions,
as an abundance of snow, alternations of warm and cold seasons, etc., are
fulfilled.

Could we tint the ever-changing surface of the spheroid of 32 as the
student who uses the microscope sometimes tints the walls of the cells he
examines, and view the earth from a distance, its pulsations in obedience
to the many forces on which its size and form depend would be manifest.
Under those conditions, were time allowed, the various steps in the
gathering of perennial snows, the birth and growth of glaciers, and the
coming and going of geological winters could be followed.

This fancied view of the working of a single part of the complicate
machinery we term climate, is not intended to lead to a discussion of the
ultimate causes of glacial conditions, but merely to invite the reader to
cut loose from ideas of days and years, and view the growth and decline of
a glacier which numbers centuries in its life-span.

The histories of the three main classes of glaciers usually recognized are
not the same but have many features in common. Individual examples
of each class require such a length of time to run their appointed courses,
that but a faint idea of the changes they undergo can be gained from the
study of a single example, even if one spent an average lifetime in the
task. But by combining observations, made in various regions, on glaciers
that have reached different stages in their development or decline, the
chief episodes in the life history of a typical example may be outlined.
Let us climb to a station on a mountain-side, overlooking a deep valley
that leads from white peaks above to a dark, forest-covered plain below,
and watch in fancy the birth, growth, and retreat of a single glacier of the
alpine type.

The life of an alpine glacier usually begins when a mountain summit
pierces the spheroid of 32. Whether this happens on account of
changes in the lithosphere or in the spheroid of temperature, or by



192 GLACIERS OF NORTH AMERICA.

a mutual adjustment of the two, is beyond our present theme. As
the mountain peak reaches higher and higher above the spheroid of
32, the mantle of snow drawn about its summit descends lower and
lower. Above the snow line the winter's snow is not completely melted
during the succeeding summer, and accumulates from year to year. If
the mountain was sculptured by streams before the postulated change
occurred, or is irregular for other reasons, the snow will be blown from
the peaks and ridges and accumulate to a great depth in the depressions.
The head of a valley becomes filled in this manner with a broad snow
field, and in summer the mountain seems to be tipped with silver. The
snow toward the bottom of the accumulation becomes consolidated by
pressure. Water formed by surface melting percolates through it and is
frozen. The lower layers are thus changed to ice, and a neve is born.
The surface of the snow field is softened and partially melted during days
of sunshine or when warm winds blow over it, and freezes at night or
during storms, and a thin crust of ice is formed. This hard, glittering
layer is buried beneath the next succeeding snowfall, and remains as a
well-defined strata in the growing neve. In the walls of crevasses, the
thin sheets of ice formed in this way, as may be learned from a near
inspection, appear as narrow blue bands separating layers of snow, perhaps
many feet thick. Dust blown from adjacent peaks and cliffs that rise above
the nve, stains its surface. The discolored layer is buried beneath subse-
quent snowfalls, and again accents the stratification of the deposit.

If we could plant a row of signals across the neve at right angles to the
trend of the valley down which its surface inclines, we would find in the
course of a few days, or even in a few hours, if our observations were
sufficiently refined, that there is a slow surface movement, greater in the
central and lower portion and tending down the valley. Could we make
similar measurements in a vertical direction where the surface movement
is greatest, we would find that the maximum flow is below the surface,
and probably near the bottom of the deposit. That the rate of movement
increases with the depth and reaches a maximum near the bottom, is
only an inference from the study of superficial phenomena, and has
never been proven by direct measurements. In the glacier proper
the threads of most rapid flow are known to be at a higher level in
reference to the basement layer than is supposed to be the case in the
neve ; accompanying this apparent change in the position of the line
of greatest movement are important modifications in the behavior of
the flowing body.



THE LIFE HISTORY OF A GLACIER. 193

The surface snow of the neve is carried along by the more rapid move-
ment of the consolidated portion deep below, and great breaks are formed
at the base of the encircling cliffs owing to the surface snow being moved
away from them. These breaks, in which the rocks beneath are frequently
exposed, are conspicuous from a distance. As we watch the slow growth
of the glacier, we note in the course of centuries, that the amphitheatre
from which it flows becomes gradually enlarged, its walls at the same time
increasing in steepness. The great crevasses on the upper border of the
neve are filled each winter, but reopen each spring in about the same
places. Observations at intervals of centuries, however, would show that
their positions do not remain the same, but owing to the waste of the
rocks exposed each summer within them, a slow migration toward the
crest of the mountains takes place, that is, the cliffs recede.

As the neve increases in thickness, the motion of the deeper layers be-
comes greater, and at length, in late summer or early autumn, a protrusion
of solid ice is seen extending out from beneath its lower margin. The
flow of the young glacier in some instances is so energetic that the neVe
field from which it is fed is seemingly in danger of exhaustion. At times,
comparatively insignificant neves supply ice bodies that are disproportion-
ately large. This occurrence seems to accompany climatic conditions that
are unfavorable to surface melting. Possibly the workings of natural
laws in this connection are better illustrated by young glaciers than by
more aged examples, and more perfect snow drainage is secured than when
the ice stream below becomes congested and is clogged with morainal
material.

The young glacier advances its extremity by reason of the more rapid
rate of flow of the ice near the surface and in the center of the stream.
In this way, what was the expanded margin of the ice foot at any indi-
cated time, becomes covered by the ice thrust forward during the next
period of marked advance. The movement being greatest in summer
and least in winter, there is an annual pulsation of the slowly advancing
extremity. There are, besides, periodic changes of similar character but of
greater magnitude. When the advance of the extremity of the glacier is
rapid, the onward surface flow each summer buries the portion remaining
from the previous summer advance. Debris carried on the surface of the
glacial stream thus becomes transferred to the bottom layer. Moraines
deposited in front of the advancing glacier during one summer become
buried by the advancing terminus during the next succeeding summer,
and are added to the ground moraine. Like results follow also from



194 GLACIERS <3F NORTH AMERICA.

similar periodic changes of greater magnitude. Owing to the manner of
its advance, the terminus of a young glacier is steep. The frontal slope
is generally higher and bolder than during old age, when the terminus is
receding, but this is not always the case.

From our fancied station overlooking a valley down which a young
glacier has begun its journey, and where also in fancy centuries are but
as hours, we see other similar streams of ice descending tributary valleys
and entering the main avenue of drainage.

The roar of avalanches, especially after heavy snowfalls, as they plow
their way down the mountain-side, awakens the echoes as if heavy guns
had been discharged. The rushing snow masses carry with them dirt,
stones, and occasionally large rock masses, and assist in the formation of
lateral moraines along the borders of the glaciers. When an interval of
sunshine loosens the icy bands with which the shattered cliffs are bound,
stones break away and join the rubbish piles below. Again, when the
shadows of evening fall on the cliffs and the temperature is lowered below
the freezing-point, additional blocks of stone pried off from the faces of
precipices are shot downward with a shrill, whistling sound, and bury
themselves in the soft snow below or strike with a dull thud on the hard
ice. We can see many localities where the ice and snow adjacent to the
cliffs has been melted back by the heat reflected from the rocks, and a
deep gulf formed, into which stones falling from above are precipitated,
and injected, as it were, into the body of the glaciers. There forbidding
recesses, black with accumulated debris, are filled and buried by subse-
quent snowfalls. The waste from the cliffs is thus at the start sealed
up in the borders of the flowing ice stream. Much of the morainal
material, when it begins its slow, downward journey is thus inclosed in
the snow and ice, or is englacial. It becomes superglacial far down the
glacier, when the matrix melts and the foreign bodies contained in it are
concentrated at the surface.

The ice streams advancing down lateral valleys, each in its youth a
separate and independent glacier, unite on reaching the main channel and
form a single compound stream. The various branches do not lose their
identity and mingle as do the waters of confluent rivers, but with some
change of form flow on side by side. This is plainly shown by the
behavior of the marginal moraines on the adjacent borders of two glaciers
after they unite. The two lateral streams below their point of union
form a single medial moraine in the central part of the compound stream.
While the lateral moraines are apparently united, they still retain some-



THE LIFE HISTORY OF A GLACIER. 195

thing of their individuality. The material of which they are composed is
not at first commingled,* but continues as separate streams, flowing side by
side. If one glacier is fringed with fragments of white quartzite, for ex-
ample, and its neighbor with blocks of black basalt, the compound medial
moraine below their place of union will be white on one side and black on
the other. Such a division of a medial moraine may sometimes be noted
many miles below the place where two tributaries unite. On highly com-
pound glaciers the medial bands sometimes exhibit a score or more of
individual threads.

At the extremity of each of the younger glaciers we have watched
advancing there has been an arch in the ice, from beneath which a stream
of muddy water flowed out. Each glacier is the source of a stream, and
in some instances discharges a swift, roaring torrent having the volume
of a river. These streams pulsate with the change of seasons. Their
volume increases in summer and diminishes in winter. Winter and
summer they are heavily charged with silt and mud, while the stream
with which they unite, lower down the mountain, where there are no
glaciers, are clear except after rains. Evidently the glaciers are wearing
away the rocks over which they move, and the streams flowing beneath
them are carrying away the finer products produced by the ceaseless
grinding. The young glaciers conceal their work, and we must wait until
they grow old and melt away before we can discover what changes are
taking place in the channels through which they flow.

Our glacier now receiving the tribute of many lateral ice streams has
passed from the youthful stage to maturity. It fills the valley at our feet
from side to side, and is prolonged for many miles below the shining snow
fields from beneath which we watched it emerge. The vast river of ice
has a depth of a thousand feet or more and a breadth of perhaps one, two,
or three miles. Its width is less than the united breadth of its many
branches. Its great thickness is due to the lateral compression of the
tributaries that have contributed to its growth.

The distinction is well marked between the clear white neve* where the
surface is renewed each year by fresh falls of snow, and the black and
dirt-stained ice of the glacier proper, where waste exceeds supply and
previously englacial material is being concentrated at the surface. The
glacier proper, as well as the neve, is snow-covered each winter and the
details of its surface blotted out, but with the return of summer the snow
on the lower portion is entirely melted. The fringe of the snow mantle
with which the mountains are covered is withdrawn higher and higher as



196 GLACIERS OF NORTH AMERICA.

the warm season advances, until in late summer or early autumn, just
before the first storms of the next succeeding winter begin, it reaches its
maximum elevation. The true limit between the surface of the neve and
of the glacier proper is then revealed. The snow line is higher on rock
surfaces than on glaciers, showing that the dark rocks absorb more heat
and melt the snow resting on them more thoroughly than does the
brilliant ice. In our watch of centuries we note that the snow line
experiences many fluctuations. As the glaciers increase in number and


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 21 23 24

Online LibraryIsrael C. (Israel Cook) RussellGlaciers of North America; a reading lesson for students of geography and geology → online text (page 21 of 24)