It is not my intention, in these articles, to discuss a general theory
of the glaciers upon physical and mechanical principles. My special
studies, always limited to Natural History, have but indifferently
fitted me for such a task, and quite recently the subject has been
admirably treated from this point of view by Dr. Tyndall, in his
charming volume entitled "Glaciers of the Alps." I have worked upon the
glaciers as an amateur, devoting my summer vacations, with friends
desirous of sharing my leisure, to excursions in the Alps, for the sake
of relaxation from the closer application of my professional studies,
and have considered them especially in their connection with geological
phenomena, with a view of obtaining, by means of a thorough acquaintance
with glaciers as they exist now, some insight into the glacial phenomena
of past times, the distribution of drift, the transportation of
boulders, etc. It was, however, impossible to treat one series of facts
without some reference to the other; but such explanations as I have
given of the mechanism of the glacier, in connection with its structure,
are presented in the language of the unprofessional observer, without
any attempt at the technicalities of the physicist. I do not wonder,
therefore, that those who have looked upon the glacier chiefly with
reference to the physical and mechanical principles involved in its
structure and movement should have found my Natural Philosophy
defective. I am satisfied with their agreement as to my correct
observation of the facts, and am the less inclined to quarrel with the
doubts thrown on my theory since I see that the most eminent physicists
of the day do not differ from me more sharply than they do from each
other. The facts will eventually test all our theories, and they form,
after all, the only impartial jury to which we can appeal. In the mean
while, I am not sorry that just at this moment, when recent
investigations and publications have aroused new interest in the
glaciers, the course of these articles brings me naturally to a
discussion of the subject in its bearing upon geological questions. I
shall, however, address myself especially, as I have done throughout
these papers, to my unprofessional readers, who, while they admire the
glaciers, may also wish to form a general idea of their structure and
mode of action, as well as to know something of the important part they
have played in the later geological history of our earth. It would,
indeed, be out of place, were I to undertake here a discussion of the
different views entertained by the various students who have
investigated the glacier itself, among whom Dr. Tyndall is especially
distinguished, or those of the more theoretical writers, among whom Mr.
Hopkins occupies a prominent position.
Removed, as I am, from all possibility of renewing my own observations,
begun in 1836 and ended in 1845, I will take this opportunity to call
the attention of those particularly interested in the matter to one
essential point with reference to which all other observers differ from
me. I mean the stratification of the glacier, which I do not believe to
be rightly understood, even at this moment. It may seem presumptuous to
dissent absolutely from the statements of one who has seen so much and
so well as Dr. Tyndall, on a question for the solution of which, from
the physicist's point of view, his special studies have been a far
better preparation than mine; and yet I feel confident that I was
correct in describing the stratification of the glacier as a fundamental
feature of its structure, and the so-called dirt-bands as the margins of
the snow-strata successively deposited, and in no way originating in the
ice-cascades. I shall endeavor to make this plain to my readers in the
course of the present article. I believe, also, that renewed
observations will satisfy dissenting observers that there really exists
a net-work of capillary fissures extending throughout the whole glacier,
constantly closing and reopening, and constituting the channels by means
of which water filtrates into its mass. This infiltration, also, has
been denied, in consequence of the failure of some experiments in which
an attempt was made to introduce colored fluids into the glacier. To
this I can only answer, that I succeeded completely, myself, in the
self-same experiments which a later investigator found impracticable,
and that I see no reason why the failure of the latter attempt should
cast a doubt upon the former. The explanation of the difference in the
result may, perhaps, be found in the fact, that, as a sponge gorged with
water can admit no more fluid than it already contains, so the glacier,
under certain circumstances, and especially at noonday in summer, may be
so soaked with water that all attempts to pour colored fluids into it
would necessarily fail. I have stated, in my work upon glaciers, that my
infiltration-experiments were chiefly made at night; and I chose that
time, because I knew the glacier would most readily admit an additional
supply of liquid from without when the water formed during the day at
its surface and rushing over it in myriad rills had ceased to flow.
While we admit a number of causes as affecting the motion of a glacier,
namely, the natural tendency of heavy bodies to slide down a sloping
surface, the pressure to which the mass is subjected forcing it onward,
the infiltration of moisture, its freezing and consequent expansion,--we
must also remember that these various causes, by which the accumulated
masses of snow and ice are brought down from higher to lower levels, are
not all acting at all times with the same intensity, nor is their action
always the same at every point of the moving mass. While the bulk of
snow and ice moves from higher to lower levels, the whole mass of the
snow, in consequence of its own downward tendency, is also under a
strong vertical pressure, arising from its own incumbent weight, and
that pressure is, of course, greater at its bottom than at its centre or
surface. It is therefore plain, that, inasmuch as the snow can be
compressed by its own weight, it will be more compact at the bottom of
such an accumulation than at its surface, this cause acting most
powerfully at the upper part of a glacier, where the snow has not yet
been transformed into a more solid icy mass. To these two agencies, the
downward tendency and the vertical pressure, must be added the pressure
from behind, which is most-effective where the mass is largest and the
amount of motion in a given time greatest. In the glacier, the mass is,
of course, largest in the centre, where the trough which holds it is
deepest, and least on the margins, where the trough slopes upward and
becomes more shallow. Consequently, the middle of a glacier always
advances more rapidly than the sides. Were the slope of the ground over
which it passes, combined with the pressure to which the mass is
subjected, the whole secret of the onward progress of a glacier, it is
evident that the rate of advance would be gradually accelerated,
reaching its maximum at its lower extremity, and losing its impetus by
degrees on the higher levels nearer the point where the descent begins.
This, however, is not the case. The glacier of the Aar, for instance, is
about ten miles in length; its rate of annual motion is greatest near
the point of junction of the two great branches by which it is formed,
diminishing farther down, and reaching a minimum at its lower extremity.
But in the upper regions, near their origin, the progress of these
branches is again gradually less. Let us see whether the next cause of
displacement, the infiltration of moisture, may not in some measure
explain this retardation, at least of the lower part of the glacier.
This agency, like that of the compression of the snow by its own weight
and the pressure from behind, is most effective where the accumulation
is largest. In the centre, where the body of the mass is greatest, it
will imbibe the most moisture. But here a modifying influence comes in,
not sufficiently considered by the investigators of glacial structure.
We have already seen that snow and ice at different degrees of
compactness are not equally permeable to moisture. Above the line at
which the annual winter snow melts, there is, of course, little
moisture; but below that point, as soon as the temperature rises in
summer sufficiently to melt the surface, the water easily penetrates the
mass, passing through it more readily where the snow is lightest and
least compact,--in short, where it has not begun its transformation into
ice. A summer's day sends countless rills of water trickling through
such a mass of snow. If the snow be loose and porous throughout, the
water will pass through its whole thickness, accumulating at the bottom,
so that the lower portion of the mass will be damper, more completely
soaked with water, than the upper part; if, on the contrary, in
consequence of the process previously described, alternate melting and
freezing combined with pressure, the mass has assumed the character of
icy snow, it does not admit moisture so readily, and still farther down,
where the snow is actually transformed into pure compact ice, the amount
of surface-water admitted into its structure will, of course, be greatly
diminished. There may, however, be conditions under which even the
looser snow is comparatively impervious to water; as, for instance, when
rain falls upon a snow-field which has been long under a low
temperature, and an ice-crust is formed upon its surface, preventing the
water from penetrating below. Admitting, as I believe we must, that the
water thus introduced into the snow and ice is one of the most powerful
agents to which its motion is due, we must suppose that it has a twofold
influence, since its action when fluid and when frozen would be
different. When fluid, it would contribute to the advance of the mass in
proportion to its quantity; but when frozen, its expansion would produce
a displacement corresponding to the greater volume of ice as compared
with water; add to this that while trickling through the mass it will
loosen and displace the particles of already consolidated ice. I have
already said that I did not intend to trespass on the ground of the
physicist, and I will not enter here upon any discussion as to the
probable action of the laws of hydrostatic pressure and dilatation in
this connection. I will only state, that, so far as my own observation
goes, the movement of the glacier is most rapid where the greatest
amount of moisture is introduced into the mass, and that I believe there
must be a direct relation between these two facts. If I am right in
this, then the motion, so far as it is connected with infiltrated
moisture or with the dilatation caused by the freezing of that moisture,
will, of course, be most rapid where the glacier is most easily
penetrated by water, namely, in the region of the _névé_ and in the
upper portion of the glacier-troughs, where the _névé_ begins to be
transformed into more or less porous ice. This cause also accounts, in
part at least, for another singular fact in the motion of the glacier:
that, in its higher levels, where its character is more porous and the
water entering at the surface sinks readily to the bottom, there the
bottom seems to move more rapidly than the superficial parts of the
mass, whereas at the lower end of the glacier, in the region of the
compact ice, where the infiltration of the water at the bottom is at its
minimum, while the disintegrating influences at the surface admit of
infiltration to a certain limited depth, there the motion is greater
near the surface than toward the bottom. But, under all circumstances,
it is plain that the various causes producing motion, gravitation,
pressure, infiltration of water, frost, will combine to propel the mass
at a greater rate along its axis than near its margins. For details
concerning the facts of the case, I would refer to my work entitled
"Système Glaciaire."
We will next consider the stratification of the glacier. I have stated
in my introductory remarks, that I consider this to be one of its
primary and fundamental features, and I confess, that, after a careful
examination of the results obtained by my successors in the field of
glacial phenomena, I still believe that the original stratification of
the mass of snow from which the glacier arises gives us the key to many
facts of its internal structure. The ultimate features resulting from
this connection are so exceedingly intricate and entangled that their
relation is not easily explained. Nevertheless, I trust my readers will
follow me in this Alpine excursion, where I shall try to smooth the
asperities of the road for them as much as possible.
Imparted to it, at the very beginning of its formation, by the manner in
which snow accumulates, and retained through all its transformations,
the stratification of a glacier, however distorted, and at times almost
obliterated, remains, notwithstanding, as distinct to one who is
acquainted with all its phases, as is the stratified character of
metamorphic rocks to the skilful geologist, even though they may be
readily mistaken for plutonic masses by the common observer. Indeed,
even those secondary features, as the dirt-bands, for instance, which we
shall see to be intimately connected with snow-strata, and which
eventually become so prominent as to be mistaken for the cause of the
lines of stratification, do nevertheless tend, when properly understood,
to make the evidence of stratification more permanent, and to point out
its primitive lines.
On the plains, in our latitude, we rarely have the accumulated layers of
several successive snow-storms preserved one above another. We can,
therefore, hardly imagine with what distinctness the sequence of such
beds is marked in the upper Alpine regions. The first cause of this
distinction between the layers is the quality of the snow when it falls,
then the immediate changes it undergoes after its deposit, then the
falling of mist or rain upon it, and lastly and most efficient of all,
the accumulation of dust upon its surface. One who has not felt the
violence of a storm in the high mountains, and seen the clouds of dust
and sand carried along with the gusts of wind passing over a
mountain-ridge and sweeping through the valley beyond, can hardly
conceive that not only the superficial aspect of a glacier, but its
internal structure also, can be materially affected by such a cause. Not
only are dust and sand thus transported in large quantities to the
higher mountain-regions, but leaves are frequently found strewn upon the
upper glacier, and even pine-cones, and maple-seeds flying upward on
their spread wings, are scattered thousands of feet above and many miles
beyond the forests where they grew.
This accumulation of sand and dust goes on all the year round, but the
amount accumulated over one and the same surface is greatest during the
summer, when the largest expanse of rocky wall is bare of snow and its
loose soil dried by the heat so as to be easily dislodged. This summer
deposit of loose inorganic materials, light enough to be transported by
the wind, forms the main line of division between the snow of one year
and the next, though only that of the last year is visible for its whole
extent. Those of the preceding years, as we shall see hereafter, exhibit
only their edges cropping out lower down one beyond another, being
brought successively to lower levels by the onward motion of the
glacier.
Other observers of the glacier, Professor Forbes and Dr. Tyndall, have
noticed only the edges of these seams, and called them dirt-bands.
Looking upon them as merely superficial phenomena, they have given
explanations of their appearance which I hold to be quite untenable.
Indeed, to consider these successive lines of dirt on the glacier as
limited only to its surface, and to explain them from that point of
view, is much as if a geologist were to consider the lines presented by
the strata on a cut through a sedimentary mass of rock as representing
their whole extent, and to explain them as a superficial deposit due to
external causes.
A few more details may help to make this statement clearer to my
readers. Let us imagine that a fresh layer of snow has fallen in these
mountain-regions, and that a deposit of dirt has been scattered over its
surface, which, if any moisture arises from the melting of the snow or
from the falling of rain or mist, will become more closely compacted
with it. The next snow-storm deposits a fresh bed of snow, separated
from the one below it by the sheet of dust just described, and this bed
may, in its turn, receive a like deposit. For greater ease and
simplicity of explanation, I speak here as if each successive snow-layer
were thus indicated; of course this is not literally true, because
snow-storms in the winter may follow each other so fast that there is no
time for such a collection of foreign materials upon each newly formed
surface. But whenever such a fresh snow-bed, or accumulation of beds,
remains with its surface exposed for some time, such a deposit of dirt
will inevitably be found upon it. This process may go on till we have a
number of successive snow-layers divided from each other by thin sheets
of dust. Of course, such seams, marking the stratification of snow, are
as permanent and indelible as the seams of coarser materials alternating
with the finest mud in a sedimentary rock.
The gradual progress of a glacier, which, though more rapid in summer
than in winter, is never intermitted, must, of course, change the
relation of these beds to each other. Their lower edge is annually cut
off at a certain level, because the snow deposited every winter melts
with the coming summer, up to a certain line, determined by the local
climate of the place. But although the snow does not melt above this
line, we have seen, in the preceding article, that it is prevented from
accumulating indefinitely in the higher regions by its own tendency to
move down to the lower valleys, and crowding itself between their walls,
thus to force its way toward the outlet below. Now, as this movement is
very gradual, it is evident that there must be a perceptible difference
in the progress of the successive layers, the lower and older ones
getting the advance of the upper and more recent ones: that is, when the
snow that has covered the face of the country during one winter melts
away from the glacier up to the so-called snow-line, there will be seen
cropping out below and beyond that line the layers of the preceding
years, which are already partially transformed into ice, and have become
a part of the frozen mass of the glacier with which they are moving
onward and downward. In the autumn, when the dust of a whole season has
been accumulated upon the surface of the preceding winter's snow, the
extent of the layer which year after year will henceforth crop out lower
down, as a dirt-band, may best be appreciated.
Beside the snow-layers and the sheets of dust alternating with them,
there is still another feature of the horizontal and parallel structure
of the mass in immediate connection with those above considered. I
allude to the layers of pure compact ice occurring at different
intervals between the snow-layers. In July, when the snow of the
preceding winter melts up to the line of perpetual snow, the masses
above, which are to withstand the summer heat and become part of the
glacier forever, or at least until they melt away at the lower end,
begin to undergo the changes through which all snow passes before it
acquires the character of glacial ice. It thaws at the surface, is
rained upon, or condenses moisture, thus becoming gradually soaked, and
after assuming the granular character of _névé_-ice, it ends in being
transformed into pure compact ice. Toward the end of August, or early
in September, when the nights are already very cold in the Alps, but
prior to the first permanent autumnal snow-falls, the surface of these
masses becomes frozen to a greater or less depth, varying, of course,
according to temperature. These layers of ice become numerous and are
parallel to each other, like the layers of ice formed from slosh. Such
crusts of ice I have myself observed again and again upon the glacier.
This stratified snowy ice is now the bottom on which the first autumnal
snow-falls accumulate. These sheets of ice may be formed not only
annually before the winter snows set in, but may recur at intervals
whenever water accumulating upon an extensive snow-surface, either in
consequence of melting or of rain, is frozen under a sharp frost before
another deposit of snow takes place. Or suppose a fresh layer of light
porous snow to have accumulated above one the surface of which has
already been slightly glazed with frost; rain or dew, falling upon the
upper one, will easily penetrate it; but when it reaches the lower one,
it will be stopped by the film of ice already formed, and under a
sufficiently low temperature, it will be frozen between the two. This
result may be frequently noticed in winter, on the plains, where sudden
changes of temperature take place.
There is still a third cause, to which the same result may possibly be
due, and to which I shall refer at greater length hereafter; but as it
has not, like the preceding ones, been the subject of direct
observation, it must be considered as hypothetical. The admirable
experiments of Dr. Tyndall have shown that water may be generated in ice
by pressure, and it is therefore possible that at a lower depth in the
glacier, where the incumbent weight of the mass above is sufficient to
produce water, the water thus accumulated may be frozen into ice-layers.
But this depends so much upon the internal temperature of the glacier,
about which we know little beyond a comparatively superficial depth,
that it cannot at present afford a sound basis even for conjecture.
There are, then, in the upper snow-fields three kinds of horizontal
deposits: the beds of snow, the sheets of dust, and the layers of ice,
alternating with each other. If, now, there were no modifying
circumstances to change the outline and surface of the glacier,--if it
moved on uninterruptedly through an open valley, the lower layers,
forming the mass, getting by degrees the advance of the upper ones, our
problem would be simple enough. We should then have a longitudinal mass
of snow, inclosed between rocky walls, its surface crossed by straight
transverse lines marking the annual additions to the glacier, as in the
adjoining figure.
[Illustration]
But that mass of snow, before it reaches the outlet of the valley, is to
be compressed, contorted, folded, rent in a thousand directions. The
beds of snow, which in the upper ranges of the mountain were spread out
over broad, open surfaces, are to be crowded into comparatively
circumscribed valleys, to force and press themselves through narrow
passes, alternately melting and freezing, till they pass from the
condition of snow into that of ice, to undergo, in short, constant
transformations, by which the primitive stratification will be
extensively modified. In the first place, the more rapid motion of the
centre of the glacier, as compared with the margins, will draw the lines
of stratification downward toward the middle faster than at the sides.
Accurate measurements have shown that the axis of a glacier may move
ten- or twenty-fold more rapidly than its margins. This is not the place
to introduce a detailed account of the experiments made to ascertain
this result; but I would refer those who are interested in the matter to
the measurements given in my "Système Glaciaire," where it will be seen
that the middle may move at a rate of two hundred feet a year, while the
margins may not advance more than ten or fifteen or twenty feet. These
observations of mine have the advantage over those of other observers,
that, while they embrace the whole extent of the glacier, transversely
as well as in its length, they cover a period of several successive
years, instead of being limited to summer campaigns and a few winter
observations. The consequence of this mode of progressing will be that
the straight lines drawn transversely across the surface of the glacier
above will be gradually changed to curved ones below. After a few years,
such a line will appear on the surface of the glacier like a crescent,
with the bow turned downward, within which, above, are other crescents,
less and less sharply arched up to the last year's line, which may be
again straight across the snow-field. (See the subjoined figure, which
represents a part of the glacier of the Lauter-Aar.)
[Illustration]
Thus the glacier records upon its surface its annual growth and
progress, and registers also the inequality in the rate of advance
between the axis and the sides.
But these are only surface-phenomena. Let us see what will be the effect
upon the internal structure. We must not forget, in considering the
changes taking place within glaciers, the shape of the valleys which
contain them. A glacier lies in a deep trough, and the tendency of the
mass will be to sink toward its deeper part, and to fold inward and
downward, if subjected to a strong lateral pressure,--that is, to dip
toward the centre and slope upward along the sides, following the scoop
of the trough. If, now, we examine the face of a transverse cut in the
glacier, we find it traversed by a number of lines, vertical in some
places, more or less oblique in others, and frequently these lines are
joined together at the lower ends, forming loops, some of which are
close and vertical, while others are quite open. These lines are due to
the folding of the strata in consequence of the lateral pressure they
are subjected to, when crowded into the lower course of the valleys, and
the difference in their dip is due to the greater or less force of that
pressure. The wood-cut on the next page represents a transverse cut
across the Lauter-Aar and the Finster-Aar, the two principal tributaries
to the great Aar glacier, and includes also a number of small lateral
glaciers which join them. The beds on the left, which dip least, and are
only folded gently downward, forming very open loops, are those of the
Lauter-Aar, where the lateral pressure is comparatively slight. Those
which are almost vertical belong in part to the several small tributary
glaciers, which have been crowded together and very strongly compressed,
and partly to the Finster-Aar. The close uniform vertical lines in this
wood-cut represent a different feature in the structure of the glacier,
called blue bands, to which I shall refer presently. These loops or
lines dipping into the internal mass of the glacier have been the
subject of much discussion, and various theories have been recently
proposed respecting them. I believe them to be caused, as I have said,
by the snow-layers, originally deposited horizontally, but afterward
folded into a more or less vertical position, in consequence of the
lateral pressure brought to bear upon them. The sheets of dust and of
ice alternating with the snow-strata are of course subjected to the same
action, and are contorted, bent, and folded by the same lateral
pressure.
[Illustration]
Dr. Tyndall has advanced the view that the lines of apparent
stratification, and especially the dirt-bands across the surface of the
glacier, are due to ice-cascades: that is, the glacier, passing over a
sharp angle, is cracked across transversely in consequence of the
tension, and these rents, where the back of the glacier has been
successively broken, when recompacted, cause the transverse lines, the
dirt being collected in the furrow formed between the successive ridges.
Unfortunately for his theory, the lines of stratification constantly
occur in glaciers where no such ice-falls are found. His principal
observations upon this subject were made on the Glacier du Géant, where
the ice-cascade is very remarkable. The lines may perhaps be rendered
more distinct on the Glacier du Géant by the cascade, and necessarily
must be so, if the rents coincide with the limit at which the annual
snow-line is nearly straight across the glacier. In the region of the
Aar glacier, however, where my own investigations were made, all the
tributaries entering into the larger glacier are ribbed across in this
way, and most of them join the main trunk over uniform slopes, without
the slightest cascade.
It must be remembered that these surface-phenomena of the glacier are
not to be seen at all times, nor under all conditions. During the first
year of my sojourn on the glacier of the Aar, I was not aware that the
stratification of its tributaries was so universal as I afterward found
it to be; the primitive lines of the strata are often so far erased that
they are not perceptible, except under the most favorable circumstances.
But when the glacier has been washed clean by rain, and the light
strikes upon it in the right direction, these lines become perfectly
distinct, where, under different conditions, they could not be discerned
at all. After passing many summers on the same glacier, renewing my
observations year after year over the same localities, I can confidently
state that not only do the lines of stratification exist throughout the
great glacier of the Aar, but in all its tributaries also. Of course,
they are greatly modified in the lower part of the glacier by the
intimate fusion of its tributaries, and by the circumstance that their
movement, primarily independent, is merged in the movement of the main
glacier embracing them all. We have seen that not only does the centre
of a glacier move more rapidly than its sides, but that the deeper mass
of the glacier also moves at a different rate from its more superficial
portion. My own observations (for the details of which I would again
refer the reader to my "Système Glaciaire ") show that in the higher
part of the glacier, especially in the region of the _névé_, the bottom
of the mass seems to move more rapidly than the surface, while lower
down, toward the terminus of the glacier, the surface, on the contrary,
moves faster than the bottom. The annexed wood-cut exhibits a
longitudinal section of the glacier, in which this difference in the
motion of the upper and lower portions of the mass is represented, the
beds being almost horizontal in the upper snow-fields, while their lower
portion slopes move rapidly downward in the _névé_ region, and toward
the lower end the upper portion takes the lead, and advances more
rapidly than the lower.
[Illustration]
I presented these results for the first time in two letters, dated
October 9th, 1842, which were published in a German periodical, the
Jahrbuch of Leonhard and Bronn. The last three wood-cuts introduced
above, the transverse and longitudinal sections of the glacier as well
as that representing the concentric lines of stratification on the
surface, are the identical ones contained in those communications. These
papers seem to have been overlooked by contemporary investigators, and I
may be permitted to translate here a passage from one of them, since it
sums up the results of the inequality of motion throughout the glacier
and its influence on the primitive stratification of the mass in as few
words and as correctly as I could give them to-day, twenty years
later:--"Combining these views, it appears that the glacier may be
represented as composed of concentric shells which arise from the
parallel strata of the upper region by the following process. The
primitively regular strata advance into gradually narrower and deeper
valleys, in consequence of which the margins are raised, while the
middle is bent not only downward, but, from its more rapid motion,
forward also, so that they assume a trough-like form in the interior of
the mass. Lower down, the glacier is worn by the surrounding air, and
assumes the peculiar form characteristic of its lower course." The last
clause alludes to another series of facts, which we shall examine in a
future article, when we shall see that the heat of the walls in the
lower part of its course melts the sides of the glacier, so that,
instead of following the trough-like shape of the valley, it becomes
convex, arching upward in the centre and sinking at the margins.
I have dwelt thus long, and perhaps my readers may think tediously, upon
this part of my subject, because the stratification of the glacier has
been constantly questioned by the more recent investigators of glacial
phenomena, and has indeed been set aside as an exploded theory. They
consider the lines of stratification, the dirt-bands, and the seams of
ice alternating with the more porous snow, as disconnected
surface-phenomena, while I believe them all to be intimately connected
together as primary essential features of the original mass.
* * * * *
There is another feature of glacial structure, intimately connected, by
similarity of position and aspect, with the stratification, which has
greatly perplexed the students of glacial phenomena. I allude to the
so-called blue bands, or bands of infiltration, also designated as
veined structure, ribboned or laminated structure, marginal structure,
and longitudinal structure. The difficulty lies, I believe, in the fact
that two very distinct structures, that of the stratification and the
blue bands, are frequently blended together in certain parts of the
glacier in such a manner as to seem identical, while elsewhere the one
is prominent and the other subordinate, and _vice versâ_. According to
their various opportunities of investigation, observers have either
confounded the two, believing them to be the same, or some have
overlooked the one and insisted upon the other as the prevailing
feature, while that very feature has been absolutely denied again by
others who have seen its fellow only, and taken that to be the only
prominent and important fact in this peculiar structural character of
the ice.
We have already seen how the stratification of the glacier arises,
accompanied by layers of dust and other material foreign to the glacier,
and how blue bands of compact ice may be formed parallel to the surface
of these strata. We have also seen how the horizontality of these strata
may be modified by pressure till they assume a position within the mass
of the glacier, varying from a slightly oblique inclination to a
vertical one. Now, while the position of the strata becomes thus altered
under pressure, other changes take place in the constitution of the ice
itself.
Before attempting to explain how these changes take place, let us
consider the facts themselves. The mass of the glacial ice is traversed
by thin bands of compact blue ice, these bands being very numerous along
the margins of the glacier, where they constitute what Dr. Tyndall calls
marginal structure, and still more crowded along the line upon which two
glaciers unite, where he has called it longitudinal structure. In the
latter case, where the extreme pressure resulting from the junction of
two glaciers has rendered the strata nearly vertical, these blue bands
follow their trend so closely that it is difficult to distinguish one
from the other. It will be seen, on referring to the wood-cut on page
758, where the close, uniform, vertical lines represent the true veined
structure, that at several points of that section the lines of
stratification run so nearly parallel with them, that, were the former
not drawn more strongly, they could not be easily distinguished from the
latter. Along the margins, also, in consequence of the retarded motion,
the blue bands and the lines of stratification run nearly parallel with
each other, both following the sides of the trough in which they move.
Undoubtedly, in both these instances, we have two kinds of blue bands,
namely: those formed primitively in a horizontal position, indicating
seams of stratification, and those which have arisen subsequently in
connection with the movement of the whole mass, which I have
occasionally called bands of infiltration, as they appeared to me to be
formed by the infiltration and freezing of water. The fact that these
blue bands are most numerous where two glaciers are crowded together
into a common bed naturally suggests pressure as their cause. And since
the beautiful experiments of Dr. Tyndall have illustrated the internal
liquefaction of ice by pressure, it becomes highly probable that his
theory of the origin of these secondary blue bands is the true one. He
suggests that layers of water may be formed in the glacier at right
angles with the pressure, and pass into a state of solid ice upon the
removal of that pressure, the pressure being of course relieved in
proportion to the diminution in the body of the ice by compression. The
number of blue bands diminishes as we recede from the source of the
pressure,--few only being formed, usually at right angles with the
surfaces of stratification, in the middle of a glacier, half-way between
its sides. If they are caused by pressure, this diminution of their
number toward the middle of the glacier would be inevitable, since the
intensity of the pressure naturally fades as we recede from the motive
power.
Dr. Tyndall also alludes to another structure of the same kind, which he
calls transverse structure, where the blue bands extend in
crescent-shaped curves, more or less arched, across the surface of the
glacier. Where these do not coincide with the stratification, they are
probably formed by vertical pressure in connection with the unequal
movement of the mass.
With these facts before us, it seems to me plain that the primitive blue
bands arise with the stratification of the snow in the very first
formation of the glacier, while the secondary blue bands are formed
subsequently, in consequence of the onward progress of the glacier and
the pressure to which it is subjected. The secondary blue bands
intersect the planes of stratification at every possible angle, and may
therefore seem identical with the stratification in some places, while
in others they cut it at right angles. It has been objected to my theory
of glacial structure, that I have considered the so-called blue bands as
a superficial feature when compared with the stratification. And in a
certain sense this is true; since, if my views are correct, the glacier
exists and is in full life and activity before the secondary blue bands
arise in it, whereas the stratification is a feature of its embryo
condition, already established in the accumulated snow before it begins
its transformation into glacier-ice. In other words, the veined
structure of the glacier is not a primary structural feature of its
whole mass, but the result of various local influences acting upon the
constitution of the ice: the marginal structure resulting from the
resistance of the sides of the valley to the onward movement of the
glacier, the longitudinal structure arising from the pressure caused by
two glaciers uniting in one common bed, the transverse structure being
produced by vertical pressure in consequence of the weight of the mass
itself and the increased rate of motion at the centre.
In the _névé_ fields, where the strata are still horizontal, the few
blue bands observed are perpendicular to the strata of snow, and
therefore also perpendicular to the blue seams of ice and the sheets of
dust alternating with them. Upon the sides of the glacier they are more
or less parallel to the slopes of the valley; along the line of junction
of two glaciers they follow the vertical trend of the axis of the mass;
while at intermediate positions they are more or less oblique. Along the
outcropping edges of the strata, on the surface of the glacier, they
follow more or less the dip of the strata themselves; that is to say,
they are more or less parallel with the dirt-bands. In conclusion, I
would recommend future investigators to examine the glaciers, with
reference to the distribution of the blue bands, after heavy rains and
during foggy days, when the surface is freed from the loose materials
and decomposed fragments of ice resulting from the prolonged action of
the sun.
* * * * *
The most important facts, then, to be considered with reference to the
motion of the glacier are as follows. First that the rate of advance
between the axis and the margins of a glacier differs in the ratio of
about ten to one and even less; that is to say; when the centre is
advancing at a rate of two hundred and fifty feet a year, the motion
toward the sides may be gradually diminished to two hundred, one hundred
and fifty, one hundred, fifty feet, and so on, till nearest the margin
it becomes almost inappreciable. Secondly, the rate of motion is not the
same throughout the length of the glacier, the advance being greatest
about half-way down in the region of the _névé_, and diminishing in
rapidity both above and below; thus the onward motion in the higher
portion of a glacier may not exceed twenty to fifty feet a year, while
it reaches its maximum of some two hundred and fifty feet annually in
the _névé_ region, and is retarded again toward the lower extremity,
where it is reduced to about one-fourth of its maximum rate. Thirdly,
the glacier moves at different rates throughout the thickness of its
mass; toward the lower extremity of the glacier the bottom is retarded,
and the surface portion moves faster, while in the upper region the
bottom seems to advance more rapidly. I say _seems_, because upon this
latter point there are no positive measurements, and it is only
inferred from general appearances, while the former statement has been
demonstrated by accurate experiments. Remembering the form of the
troughs in which the glaciers arise, that they have their source in
expansive, open fields of snow and _névé_, and that these immense
accumulations move gradually down into ever narrowing channels, though
at times widening again to contract anew, their surface wasting so
little from external influences that they advance far below the line of
perpetual snow without any sensible diminution in size, it is evident
that an enormous pressure must have been brought to bear upon them
before they could have been packed into the lower valleys through which
they descend.
Physicists seem now to agree that pressure is the chief agency in the
motion of glaciers. No doubt, all the facts point that way; but it now
becomes a matter of philosophical interest to determine in what
direction it acts most powerfully, and upon this point glacialists are
by no means agreed. The latest conclusion seems to be, that the weight
of the advancing mass is itself the efficient cause of the motion. But
while this is probably true in the main, other elements tending to the
same result, and generally overlooked by investigators, ought to be
taken into consideration; and before leaving the subject, I would add a
few words upon infiltration in this connection.
The weight of the glacier, as a whole, is about the same all the year
round. If, therefore, pressure, resulting from that weight, be the
all-controlling agency, its progress should be uniform daring the whole
year, or even greatest in winter, which is by no means the case. By a
series of experiments, I have ascertained that the onward movement,
whatever be its annual average, is accelerated in spring and early
summer. The average annual advance of the glacier being, at a given
point, at the rate of about two hundred feet, its average summer
advance, at the same point, will be at a rate of two hundred and fifty
feet, while its average rate of movement in winter will be about one
hundred and fifty feet. This can be accounted for only by the increased
pressure due to the large accession of water trickling in spring and
early summer into the interior through the net-work of capillary
fissures pervading the whole mass. The unusually large infiltration of
water at that season is owing to the melting of the winter snow. Careful
experiments made on the glacier of the Aar, respecting the water thus
accumulating on the surface, penetrating its mass, and finally
discharged in part at its lower extremity, fully confirm this view.
Here, then, is a powerful cause of pressure and consequent motion, quite
distinct from the permanent weight of the mass itself, since it operates
only at certain seasons of the year. In midwinter, when the infiltration
is reduced to a minimum, the motion is least. The water thus introduced
into the glacier acts, as we have seen above, in various ways: by its
weight, by loosening the particles of snow through which it trickles,
and by freezing and consequent expansion, at least within the limits and
during the season at which the temperature of the glacier sinks below
32° Fahrenheit. The simple fact, that in the spring the glacier swells
on an average to about five feet more than its usual level, shows how
important this infiltration must be. I can therefore only wonder that
other glacialists have given so little weight to this fact. It is
admitted by all, that the waste of a glacier at its surface, in
consequence of evaporation and melting, amounts to about nine or ten
feet in a year. At this rate of diminution, a glacier, even one thousand
feet in thickness, could not advance during a single century without
being exhausted. The water supplied by infiltration no doubt repairs the
loss to a great degree. Indeed, the lower part of the glacier must be
chiefly maintained from this source, since the annual increase from the
fresh accumulations of snow is felt only above the snow-line, below
which the yearly snow melts away and disappears. In a complete theory
of the glaciers, the effect of so great an accession of plastic material
cannot be overlooked.
I now come to some points in the structure of the glacier, the
consideration of which is likely to have a decided influence in settling
the conflicting views respecting their motion. The experiments of
Faraday concerning regelation, and the application of the facts made
known by the great English physicist to the theory of the glaciers, as
first presented by Dr. Tyndall in his admirable work, show that
fragments of ice with most surfaces are readily reunited under pressure
into a solid mass. It follows from these experiments, that glacier-ice,
at a temperature of 32° Fahrenheit, may change its form and preserve its
continuity during its motion, in virtue of the pressure to which it is
subjected. The statement is, that, when two pieces of ice with moistened
surfaces are placed in contact, they become cemented together by the
freezing of a film of water between them, while, when the ice is below
32° Fahrenheit, and therefore _dry_, no effect of the kind can be
produced. The freezing was also found to take place under water; and the
result was the same, even when the water into which the ice was plunged
was as hot as the hand can bear.
The fact that ice becomes cemented under these circumstances is fully
established, and my own experiments have confirmed it to the fullest
extent. I question, however, the statement, that regelation takes place
_by the freezing of a film of water between the fragments_. I never have
been able to detect any indication of the presence of such a film, and
am, therefore, inclined to consider this result as akin to what takes
place when fragments of moist clay or marl are pressed together and thus
reunited. When examining beds of clay and marl, or even of compact
limestone, especially in large mountain-masses, I have frequently
observed that the rock presents a net-work of minute fissures pervading
the whole, without producing a distinct solution of continuity, though
generally determining the lines according to which it breaks under
sudden shocks. The net-work of capillary fissures pervading the glacier
may fairly be compared to these rents in hard rocks,--with this
difference, however, that in ice they are more permeable to water than
in stone.
How this net-work of capillary fissures is formed has not been
ascertained by direct observation. Following, however, the
transformation of the snow and _névé_ into compact ice, it is easily
conceived that the porous mass of snow, as it falls in the upper regions
of the Alps, and in the broad caldrons in which the glaciers properly
originate, cannot pass into solid ice, by the process described in a
former article, without retaining within itself larger or smaller
quantities of air. This air is finally surrounded from all sides by the
cementation of the granules of _névé_, through the freezing of the water
that penetrates it. So inclosed, the bubbles of air are subject to the
same compression as the ice itself, and become more flattened in
proportion as the snow has been more fully transformed into compact ice.
As long as the transformation of snow into ice is not complete, a rise
of its temperature to 32° Fahrenheit, accompanied with thawing, reduces
it at once again to the condition of loose grains of _névé_; but when
more compact, it always presents the aspect of a mass composed of
angular fragments, wedged and dove-tailed together, and separated by
capillary fissures, the flattened air-bubbles trending in the same
direction in each fragment, but varying in their trend from one fragment
to another. There is, moreover, this important point to notice,--that,
the older the _névé_, the larger are its composing granules; and where
_névé_ passes into porous ice, small angular fragments are mixed with
rounded _névé_-granules, the angular fragments appearing larger and more
numerous, and the _névé_-granules fewer, in proportion as the _névé_-ice
has undergone most completely its transformation into compact
glacier-ice. These facts show conclusively that the dimensions and form
of the _névé_-granules, the size and shape of the angular fragments, the
porosity of the ice, the arrangement of its capillary fissures, and the
distribution and compression of the air-bubbles it contains, are all
connected features, mutually dependent. Whether the transformation of
snow into ice be the result of pressure only, or, as I believe, quite as
much the result of successive thawings and freezings, these structural
features can equally be produced, and exhibit these relations to one
another. It may be, moreover, that, when the glacier is at a temperature
below 32°, its motion produces extensive fissuration throughout the
mass.
Now that water pervades this net-work of fissures in the glacier to a
depth not yet ascertained, my experiments upon the glacier of the Aar
have abundantly proved; and that the fissures themselves exist at a
depth of two hundred and fifty feet I also know, from actual
observation. All this can, of course, take place, even if the internal
temperature of the glacier never should fall below 32° Fahrenheit; and
it has actually been assumed that the temperature within the glacier
does not fall below this point, and that, therefore, no phenomena,
dependent upon a greater degree of cold, can take place beyond a very
superficial depth, to which the cold outside may be supposed to
penetrate. I have, however, observed facts which seem to me
irreconcilable with this assumption. In the first place, a
thermometrograph indicating -2° Centigrade, (about 28° Fahrenheit,) at a
depth of a little over two metres, that is, about six feet and a half,
has been recovered from the interior of the glacier of the Aar, while
all my attempts to thaw out other instruments placed in the ice at a
greater depth utterly failed, owing to the circumstance, that, after
being left for some time in the glacier, they were invariably frozen up
in newly formed water-ice, entirely different in its structure from the
surrounding glacier-ice. This freezing could not have taken place, did
the mass of the glacier never fall below 32° Fahrenheit. And this is not
the only evidence of hard frost in the interior of the glaciers. The
innumerable large walls of water-ice, which may be seen intersecting
their mass in every direction and to any depth thus far reached, show
that water freezes in their interior. It cannot be objected, that this
is merely the result of pressure; since the thin fluid seams, exhibited
under pressure in the interesting experiments of Dr. Tyndall, and
described in his work under the head of Crystallization and Internal
Liquefaction, cannot be compared to the large, irregular masses of
water-ice found in the interior of the glacier, to which I here allude.
In the absence of direct thermometric observations, from which the
lowest internal temperature of the glacier could be determined with
precision in all its parts, we are certainly justified in assuming that
every particle of water-ice found in the glacier, the formation of which
cannot be ascribed to the mere fact of pressure, is due to the influence
of a temperature inferior to 32° Fahrenheit at the time of its
consolidation. The fact that the temperature in winter has been proved
by actual experimentation to fall as low as 28° Fahrenheit, that is,
four degrees below the freezing-point at a depth of six feet below a
thick covering of snow, though not absolutely conclusive as to the
temperature at a greater depth, is certainly very significant.
Under these circumstances, it is not out of place to consider through
what channels the low temperature of the air surrounding the glacier may
penetrate into the interior. The heavy cold air may of course sink from
the surface into every large open space, such as the crevasses, large
fissures, and _moulins_ or mill like holes to be described in a future
article; it may also penetrate with the currents which ingulf themselves
under the glacier, or it may enter through its terminal vault, or
through the lateral openings between the walls of the valley and the
ice. Indeed, if all the spaces in the mass of the glacier, not occupied
by continuous ice, could be graphically represented, I believe it would
be seen that cold air surrounds the glacier-ice itself in every
direction, so that probably no masses of a greater thickness than that
already known to be permeable to cold at the surface would escape this
contact with the external temperature. If this be the case, it is
evident that water may freeze in any part of the glacier.
To substantiate this position, which, if sustained, would prove that the
dilatation of the mass of the glacier is an essential element of its
motion, I may allude to several other well-known facts. The loose snow
of the upper regions is gradually transformed into compact ice. The
experiments of Dr. Tyndall prove that this may be the result of
pressure; but in the region of the _névé_ it is evidently owing to the
transformation of the snow-flakes into ice by repeated melting and
freezing, for it takes place in the uppermost layers of the snow, where
pressure can have no such effect, as well as in its deeper beds. I take
it for granted, also, that no one, familiar with the presence of the
numerous ice-seams parallel to the layers of snow in these upper regions
of the glacier, can doubt that they, as well as the _névé_, are the
result of frost. But be this as it may, the difference between the
porous ice of the upper region of the glacier and the compact blue ice
of its lower track seems to me evidence direct that at times the whole
mass must assume the rigidity imparted to it by a temperature inferior
to the freezing-point. We know that at 32° Fahrenheit, regelation
renders the mass continuous, and that it becomes brittle only at a
temperature below this. In other words, the ice can break up into a mass
of disconnected fragments, such as the capillary fissures and the
infiltration-experiments described in my "Système Glaciaire," show to
exist, only when it is below 32° Fahrenheit. If it be contended that ice
at 32° does break, and that therefore the whole mass of the glacier may
break at that temperature, setting aside the contradiction to the facts
of regelation which such an assumption involves, I would refer to Dr.
Tyndall's experiments concerning the vacuous spots in the ice.
[Illustration]
Those who have read his startling investigations will remember that by
sending a beam of sunlight through ice he brought to view the primitive
crystalline forms to which it owes its solidity, and that he insisted
that these star-shaped figures are always in the plane of
crystallization. Without knowing what might be their origin, I had
myself noticed these figures, and represented them in a diagram, part of
which is reproduced in the annexed wood-cut. I had considered them to be
compressed air-bubbles; and though I cannot, under my present
circumstances, repeat the experiment of Dr. Tyndall upon glacier-ice, I
conceive that the star-shaped figures represented upon Pl. VII. figs. 8
and 9, in my "Système Glaciaire," may refer to the same phenomenon as
that observed by him in pond-ice. Yet while I make this concession, I
still maintain, that besides these crystalline figures there exist
compressed air-bubbles in the angular fragments of the glacier-ice, as
shown in the above wood-cut; and that these bubbles are grouped in sets,
trending in the same direction in one and the same fragment, and
diverging under various angles in the different fragments. I have
explained this fact concerning the position of the compressed
air-bubbles, by assuming that ice, under various pressure, may take the
appearance it presents in each fragment with every compressed air-bubble
trending in the same direction, while their divergence in the different
fragments is owing to a change in the respective position of the
fragments resulting from the movement of the whole glacier. I have
further assumed, that throughout the glacier the change of the snow and
porous ice into compact ice is the result of successive freezing,
alternating with melting, or at least with the resumption of a
temperature of 32° Fahrenheit in consequence of the infiltration of
liquid water, to which the effects of pressure must be added, the
importance of which in this connection no one could have anticipated
prior to the experiments of Dr. Tyndall. Of course, if the interior
temperature of the glacier never falls below 32°, the changes here
alluded to could not take place. But if the _vacuous spaces_ observed by
Dr. Tyndall are really identical with the spaces I have described as
_extremely flattened air-bubbles_, I think the arrangement of these
spaces as above described proves that it freezes in the interior of the
glacier to the depth at which these crosswise fragments have been
observed: that is, at a depth of two hundred feet. For, since the
experiments of Dr. Tyndall show that the vacuous spaces are parallel to
the surface of crystallization, and as no crystallization of water can
take place unless the surrounding temperature fall below 32°, it follows
that these vacuous spaces could not exist in such large continuous
fragments, presenting throughout the fragments the same trend, if there
had been no frost within the mass, affecting the whole of such a
fragment while it remained in the same position.
The most striking evidence, in my opinion, that at times the whole mass
of the glacier actually freezes, is drawn from the fact, already alluded
to, that, while the surface of the glacier loses annually from nine to
ten feet of its thickness by evaporation and melting, it swells, on the
other hand, in the spring, to the amount of about five feet. Such a
dilatation can hardly be the result of pressure and the packing of the
snow and ice, since the difference in the bulk of the ice brought down,
during one year, from a point above to that under observation, would not
account for the swelling. It is more readily explained by the freezing
of the water of infiltration during spring and early summer, when the
infiltration is most copious and the winter cold has been accumulating
for the longest time. This view of the case is sustained by Élie de
Beaumont, who states his opinion upon this point as follows:--
"Pendant l'hiver, la température de la surface du glacier s'abaisse à un
grand nombre de degrés au-dessous de zéro, et cette basse température
pénètre, quoique avec un affaiblissement graduel, dans l'interieur de la
masse. Le glacier se fendille par l'effet de la contraction résultant de
ce refroidissement. Les fentes restent d'abord vides, et concourent an
refroidissement des glaciers en favorisant l'introduction de l'air froid
extérieur; mais an printemps, lorsque les rayons du soleil échaffent la
surface de la neige qui couvre le glacier, ils la remènent d'abord à
zéro, et ils produisent ensuite de l'eau à zéro qui tombe dans le
glacier refroidi et fendillé. Cette eau s'y congèle à l'instant, en
laissant dégager de la chaleur qui tend à ramener le glacier à zéro; et
la phénomène se continue jusqu'à ce que la masse entière du glacier
refroidi soit ramené à la température de zéro."[H]
But where direct observations are still so scanty, and the
interpretations of the facts so conflicting, it is the part of wisdom to
be circumspect in forming opinions. This much, however, I believe to be
already settled: that any theory which ascribes the very complicated
phenomena of the glacier to one cause must be defective and one-sided.
It seems to me most probable, that, while pressure has the larger share
in producing the onward movement of the glacier, as well as in the
transformation of the snow into ice, a careful analysis of all the facts
will show that this pressure is owing partly to the weight of the mass
itself, partly to the pushing on of the accumulated snow from behind,
partly to its sliding along the surface upon which it rests, partly to
the weight of water pervading the whole, partly to the softening of the
rigid ice by the infiltration of water, and partly, also, to the
dilatation of the mass, requiting from the freezing of this water. These
causes, of course, modify the ice itself, while they contribute to the
motion. Further investigations are required to ascertain in what
proportion these different influences contribute to the general result,
and at what time and under what circumstances they modify most directly
the motion of the glacier.
That a glacier cannot be altogether compared to a river, although there
is an unmistakable analogy between the flow of the one and the onward
movement of the other, seems to me plain,--since the river, by the
combination of its tributaries, goes on increasing in bulk in
consequence of the incompressibility of water, while a glacier gradually
thins out in consequence of the packing of its mass, however large and
numerous may be its accessions. The analogy fails also in one important
point, that of the acceleration of speed with the steepness of the
slope. The motion of the glacier bears no such direct relation to the
inclination of its bed. And though in a glacier, as in a river, the axis
of swiftest motion is thrown alternately on one or the other side of the
valley, according to its shape and slope, the very nature of ice makes
it impossible that eddies should be formed in the glacier, and the
impressive feature of whirlpools is altogether wanting in them. What
have been called glacier-cascades bear only a remote resemblance to
river-cascades, as in the former the surface only is thrown into
confusion by breaking, without affecting the primitive structure;[I] and
I reiterate my formerly expressed opinion that even the stratification
of the upper regions is still recognizable at the lower end of the
glacier of the Rhone.
The internal structure of the glacier has already led me beyond the
limits I had proposed to myself in the present article. But I trust my
readers will not be discouraged by this dry discussion of various
theories concerning it, and will meet me again on the glacier, when we
will examine together some of its more picturesque features, its
crevasses, its rivulets and cascades, its moraines, its boulders, etc.,
and endeavor also to track its ancient course and boundaries in earlier
geological times.
* * * * *