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Israel C. (Israel Cook) Russell.

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

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possibly for many consecutive years, in the case of small glaciers, tha
snow may completely cover the true ice, so that one might walk over tha
accumulation and easily mistake it for the snows of a single winter, and
be led to conclude that it was not entitled to be considered as a member
of the great family of glaciers. *

The neVe* is composed of stratified granular snow which is white or
grayish white in color. The snow on high mountains is apt to be exceed-
ingly fine, light, and dry when first formed ; but by partial melting an 1
refreezing it acquires a coarse, granular texture, much like compacted hai ,



INTRODUCTION. 5

and also becomes consolidated and hard. The surface of the neVe* is many
times so softened by the warmth between summer storms, that a thin crust
of ice is formed when the temperature is again lowered. This crust is
buried beneath the next succeeding snowfall and remains in the growing
deposit as a thin stratum of ice. Neves are almost entirely free from
stones or dirt, although even on the highest mountains, the dust borne
from naked cliffs is widely spread over their surfaces and diminishes their
brilliancy. This general dust-covering is frequently not noticeable until
some really clean snow surface is brought in contrast with it. When a lake
on the neVe* is drained and leaves a fresh surface of dazzling whiteness, the
surrounding area frequently shows a gray tint by contrast, thus revealing
the presence of dust which has been sprinkled over it. Sometimes the
covering of dust, especially on the lower portions of the neVes of alpine
glaciers, is sufficiently pronounced to form a definite division plane when
buried by subsequent snowfalls. Illustrations of such an occurrence may
frequently be seen in the walls of fissures. In the great open fissures or
crevasses that break the neves in the region about Mount St. Elias, a
dozen more or less distinct strata separated by bands of blue ice, a fraction
of an inch thick, or by still more conspicuous dust-stained layers, may be
frequently counted. In some instances the layers of granular snow are
fully fifty feet thick, even after having passed from the light, mealy
consistency of freshly fallen snow to the much more compact condition
of the granular neVe* snow, thus indicating the abundance of the snow-
fall in the regions where glaciers have their birth. The surfaces of
neves are renewed many times during the year by fresh, snow. Stones
and dirt falling on them from surrounding cliffs, or swept down by
avalanches from tributary slopes, are buried from sight and enclosed in
the growing deposits.

Below the snow line, the true glacier, composed of compact ice, makes
its appearance at the surface. The horizontal stratification so well marked
in the nev(3 is nearly or quite obliterated, but the ice takes on a character-
istic banded structure, due to alternation of thin sheets of clear, blue ice
with sheets of vesicular, white ice. As has been shown by laboratory
experiments as well as observations on glaciers themselves, this peculiar
banded or ribboned structure is caused, in part at least, by pressure, and is
analogous to the slaty cleavage observable in certain rocks. At the lower
extremities of glaciers in many instances the banded structure is obscure,
or perhaps entirely obliterated, and the ice presents a coarse, granular
appearance not unlike the grain of crystallized marble. As will be



6 GLACIERS OF NORTH AMERICA;

explained, this " glacier grain " is not confined to the extremities of
glaciers, but has been recognized throughout the extent of the glaciers
proper.

The ice below the snow line is frequently dirt-stained and more or less
completely covered with angular stones and large rock masses. This
superficial covering is so general on many glaciers that from a distance no
traces of ice can be seen, and they appear as dark and barren as a newly
plowed field. In a general view of a snow-covered mountain range the
two surface divisions of the glaciers on its sides are usually distinctly
shown by contrast in color. The higher or neve portions are white and
glistening, while the lower portions either reveal the blue tint of compact
ice or are dark with earth and stones.

The debris that falls on a neve from bordering cliffs, and the dust blown
over its surface, sink into the soft snow, principally on account of the
absorption of heat owing to their dark color, and are buried by later snow
storms. As the neve becomes consolidated and acquires motion, this
debris is carried along within its mass. In the region below the snow-
line, however, where the annual melting exceeds the annual snowfall,
the surface of the ice is liquefied, and foreign substances previously buried
become concentrated at the surface. The tendency of the neve is to bury
foreign objects, and of the glacier proper to concentrate them at the
surface. For this reason the lower and consequently the more wasted
portion of a glacier is the more thoroughly dirt-covered.

Moraines. All of the debris carried by glaciers may be designated
in general as morainal material. When arranged in certain more or less
definite ways it is known under specific names. When distributed along
either margin of a glacier it forms lateral moraines. When two glaciers
unite, the right lateral moraine of one of the branches joins the left lateral
moraine of its companion, thus forming a medial moraine in the central
portion of the compound glacier below the junction. When a trunk
glacier is formed by the union of several branches, as is frequently the
case, the number of parallel lines of debris on its surface is correspond-
ingly increased, being always one less than the number of well-defined
branches that unite to form the compound stream. This nomenclature
will be better understood by referring to the following ideal sketch map
of the surface of an alpine glacier, formed by the union of four tribu-
taries. The moraines on the surface of the ice are shown by dots and
the mountain slopes by sketch contours.



INTRODUCTION.



The debris carried to the end of the glacier and deposited about its
extremity, in some cases forms a crescent-shaped ridge, known as a
terminal moraine. Similar moraines about the margin of piedmont and
continental glaciers are usually designated as frontal moraines ; when two




FIG. 1. IDEAL SKETCH MAP OF AN ALPINE GLACIER, SHOWING LATERAL, MEDIAL,
AND TERMINAL MORAINES.

lobes on the outer margin of such glaciers unite, the debris deposited
along the line of junction forms interlobate moraines. Moraines are
subglacial, englacial, or superglacial according to their position.

Other somewhat technical terms iised to designate various modifi-
cations of morainal deposit will be explained in treating of glacial
records.

Crevasses. Moving ice masses, especially when flowing over rough
surfaces or through rugged valleys, are subjected to stresses which cause
them to break and fissures to open. Such open fissures were termed
crevasses by Swiss mountaineers, long before the attention of scientific
men had been called to them. The name has been adopted by glacialists
as a general term for the gaping fissures that so frequently break the



8 GLACIERS OF NORTH AMERICA.

surfaces of moving ice masses. Crevasses occur both in neves and in
true glacial ice, and present varying characteristics which have led to a
somewhat specific classification.

The snow fields at the heads of alpine glaciers are frequently
traversed by fissures several hundred feet long and varying in width
up to 50 feet or more. They are widest in the central portion, and
taper gradually to mere cracks at their extremities, which are frequently
curved in opposite directions. Even the greatest crevasses are at first
simple or compound fractures, too narrow to allow one to insert the
thinnest knife blade, and slowly open in the course of weeks or months.
This widening of crevasses, especially in neves, is due to the stretching
of the material that they traverse. It is frequently stated that ice,
though plastic under pressure, yields to tension only by rupture. The
slow opening of crevasses by the widening of their central portions,
certainly indicates, however, that ice, when subjected to slowly acting
tension, does stretch to some extent without fracture.

As stated above, crevasses begin as narrow cracks and gradually widen.
While camping on the broad neves in the Mount St. Elias region, my
attention was frequently called to the formation of these breaks in the
ice. On one occasion, while sleeping in a tent far out on the neve of the
Agassiz glacier, I was wakened several times during the night by rum-
bling sounds accompanied by sharp crashes, which seemed to proceed
from the ice immediately beneath our tents. With each crash the ice
trembled and vibrated as if an earthquake wave had passed through it..
The sounds came so suddenly and were so startling that some of my
party who were not familiar with the behavior of glaciers, rushed from
the tent in considerable alarm, fearing that a crevasse was about to yawn
beneath them. In the morning we found that a crack in the ice, several
rods in length but without appreciable width, had formed immediately
in front of our tents.

The walls of crevasses in neve regions are of the most exquisite
turquoise blue, the color deepening below the surface until it seems
almost black. The only color in nature that rivals the blue of glacial ice,
is seen when one looks down into the unfathomable sea. The sides of
crevasses are frequently hung with icicles, forming rank on rank of
glittering pendants, and fretted and embossed in the most beautiful
manner with snow wreaths, and partially roofed with curtain-like cornices
of snow. These details are wrought in silvery white, or in innumerable
shades of blue with suggestions of emerald tints. When the sunlight



INTRODUCTION. 9

enters the great chasms, their walls seem encrusted with iridescent
jewels. The still waters with which many of the gulfs are partially filled,
reflect every detail of their crystal walls and make their depth seem
infinite. No dream of fairy caverns ever exceeded the beauty of these
mysterious crypts of the vast cathedral-like amphitheatres of the silent
mountains.

Encircling the upper borders of the neve in most snow-filled amphi-
theatres, there is a great crevasse or a series of nearly parallel and
intersecting fractures that differ in certain ways from the crevasses formed
lower down. The most rapid motion in a neve probably occurs deep
below the surface, where the pressure is greatest and the ice compact. The
light snow forming the surface of the neve is carried bodily forward by
the flow of the ice on which it rests ; this together with the general settling
of the newer and more incoherent snow causes it to break away from the
surrounding cliffs. Great open fissures are thus formed, which border the
upper margin of the neve and separate it from the rocks above in
such a manner, in many instances, as to offer an impassable obstacle.
These breaks frequently mark the boundary between snow work and
rock-climbing, and are known as bergschrunds, or mountain crevasses.
Breaks of this character are among the very first to form when an amphi-
theatre becomes snow-filled, and continue to appear at the same localities
as the glacier advances in age. They occur close to the bordering cliffs
but leave portions of the neve, frequently several rods broad, still clinging
to the rocks. A bergschrund, in the majority of instances, is of the nature
of a fault. The snow left attached to the rocks and forming the upper
margin of the crevasse stands higher than the opposite margin of the
fracture. The snow forming the thrown block, has been affected by a
downward movement, and also by a horizontal movement which opened
the fracture. In observed instances, the vertical displacement is from a
few inches to fifty or sixty feet or more ; and the horizontal movement
shown by the breadth of the crevasse, frequently from fifty to seventy-five
feet. At times compound, or step faults, as a geologist would call them,
are formed and two or more nearly parallel crevasses break the surface.
In winter these breaks are filled and a new layer is added to the surface
of the neve, but during the succeeding spring they form again in about
the same position as the year previous.

In the breaks encircling the head of a neve, the rock beneath the snow
left clinging to the mountain is usually exposed and becomes greatly
shattered by frost and changes of temperature. The blocks thus loosened



10 GLACIERS OF NORTH AMERICA.

are plucked out the succeeding year, when another crevasse forms in the
same locality. It is thought by some students of topography that the
waste from these exposed surfaces leads to the growth of amphitheatres
and cirques and explains many of the peculiarities in the relief of
glaciated mountains.

When a glacier descends a precipice it may become broken and fall in
detached blocks, thus forming veritable ice cascades; but the fragments
unite again at the base of the cliffs and become re consolidated, and the ice
flows on as a continuous stream. At other times the descent is completely
covered with ice so shattered as to be impassable, and presents all degrees
of diversity between ice cascades and ice rapids. The places of steep
descent in the floor of a neve frequently lead to the breaking of the snow
and ice into cubical blocks of all dimensions up to hundreds of feet in
diameter, which bear a striking resemblance to towers and other architec-
tural forms, and add most attractive features to the scenery of glacier-
covered regions. During night marches on the glaciers of Alaska, the
writer could scarcely put aside the idea that these shadowy forms par-
tially illuminated by the northern twilight, were in reality the ruins of
marble temples. In the lower portions of glaciers, where the ice is more
solid and where surface melting is more rapid, the steep descents are
marked by spires and pinnacles having extremely rugged and angular
forms, separated by profound crevasses. These true ice falls are much
more rugged and much more difficult to traverse than similar descents
in the neve, and are seldom accessible even to the most experienced
mountain climbers.

When a glacier passes over a moderate inequality in its bed, it is
fractured so as to form crescent-shaped fissures which are widest just
below the obstruction and gradually close as the slowly moving stream
flows on. In passing over such obstructions the surface of a glacier,
especially in the neve region, sometimes rises so as to have a backward
slope. Instances of this nature- have been observed in the neighborhood
of Mount St. Elias. Marginal crevasses are formed on the sides of well-
defined glaciers, owing to the friction on the sides and the more rapid
flow of the central portion. These breaks trend up stream at angles of
approximately 40, and are broadest at the shore. When the banks of an
ice stream are of snow and ice, counterparts of the marginal crevasses
are formed in them and trend down stream, and are practically continuous
with the breaks in the margin of the glacier itself. The marginal
crevasses in the glacier and the similar breaks in the adjacent bank,



INTRODUCTION. 11

however, are separated by a band of shattered snow sometimes several
rods broad, which sharply defines the margin of the current. These
crevassed banks of snow and ice are common in the St. Elias region, and
have been described by the writer. 1

In the case of glaciers that expand on leaving narrow valleys, stresses
are produced in other directions than in the cases cited above, and longi-
tudinal or more or less regularly radiating breaks are produced. A well-
known instance of this nature is furnished by Rhone glacier.

It may be judged from this brief sketch that the conditions leading to
the fracture of moving ice masses are exceedingly varied and produce
diverse results. The series of more regularly arranged fractures, to which
special attention has been directed, are united by other and less easily
explained breaks, so that the detail of the surface of an ice stream, espe-
cially when modified by melting, becomes at times wonderfully complex.
It is only by selecting isolated and well-defined instances for study that
the laws governing the behavior of ice under the varied stresses produced
in flowing through irregular valleys and over rough surfaces can be at all
understood.

The ice of glaciers is also broken along planes more or less inclined to
their surfaces. Movement takes place along these breaks, and produces
thrusts, analogous to the over-thrusts, or under-thrusts, sometimes seen
in rocks that have been folded and broken. In fact, the counterpart of
many of the structural features observed in rocks, such as faults, folds,
joints, contortions, etc., may be observed in the ice of glaciers.

Surface Features. Owing to the presence of crevasses and to unequal
melting, the surfaces of glaciers are frequently exceedingly rough and
irregular. Foreign matter resting on the ice, when sufficiently thick not
to be warmed through by the sun's heat in a single day, protects the ice
beneath, while adjacent surfaces not so protected are lowered by melting.
Blocks of stone thus shelter the ice beneath and remain on pillars or ped-
estals as the surrounding surface is lowered. A group of such " glacier
tables," as they are called, is shown on page 44. These were observed
by the writer on a small glacier on the High Sierra of California, and
present a fair idea of the character of the mushroom-shaped prominences
common on many glaciers. Glacier tables frequently incline southward
in north temperate latitudes, owing to the greater melting of their sup-

1 "An Expedition to Mount St. Elias, Alaska," National Geographic Magazine
(Washington, D. C.), vol. 3, pp. 127, 128.



12 GLACIERS OF NORTH AMERICA.

porting columns on the south side. Eventually, the upraised block slips
off its pedestal in a southerly direction, leaving a stump of ice to mark
its site. When this happens, the process is renewed and the block again
left in relief by the melting of the surrounding surface. The boulders
and stones carried on the surface of glaciers thus receive many falls,
and become broken and more or less comminuted. This illustrates the
fact that not all of the crushing and commingling of rocks performed by
a glacier takes place deep within or beneath its mass.

Moraines on the surfaces of glaciers are composed in a great measure
of blocks of stone, which protect the ice beneath, as stated above, and
produce still more marked inequalities of the surface. What appear to
be massive embankments of stones and dirt are many times ridges of ice
covered with a veneer of debris only a foot or two thick. The slow melt-
ing of the ice beneath superficial moraines causes the larger and less
angular stones to slide and roll down the sides of the ridges, thus lead-
ing to a rude assortment of the material, with reference to size and
shape. Such an assorting may be seen in the side view of a medial
moraine shown in Figure 8. The friction and impact of the frequently
disturbed rocks cause breakage and the formation of angular gravel, and
even clay. Disintegration and weathering are thus promoted, and the
surface material becomes divided into smaller and smaller masses the far-
ther it is carried or the longer it remains on the ice.

When there are several parallel moraines on a glacier the surface
becomes exceedingly rugged ; and when, in addition, crevasses cross
such a region, it is frequently rendered entirely impassable. I well
remember a long, weary march across the Malaspina glacier, when our
route lay at right angles to fully a score of huge moraines, each one
forming a ridge from 50 to 200 feet broad at the top, and rising 100 or
150 feet above the adjacent troughs. These ridges were completely
sheathed with stones held in sockets of ice, which would frequently slip
from beneath our feet and roll to the bottom of the escarpment. The
sides of these ridges were so steep that we could ascend them only by
choosing zigzag courses. Many were the slips and tumbles experienced
during the ,day. Between the ridges that caused so much delay and
fatigue, there were lanes from a hundred to several hundred yards broad,
floored with comparatively smooth ice, which had been deepened by the
melting of the glacier, where unprotected. When standing on the crests
of the dark, stone-covered ridges one could trace their courses for miles
on either hand, until a change in the slope of the glacier carried them



INTRODUCTION. 13

out of view. At right angles to their trend there was nothing in
sight except the distant mountains and the seemingly endless expanse
of barren and exceedingly desolate debris.

Masses of sand and gravel resting on ice behave in much the same
manner as do rock masses and moraines, except that where they are left
upraised above the wasting surface the grains and pebbles roll and slide
downward, and the pedestal is transformed into a cone of ice sheathed
with a thin covering of loose material. At times, many acres far out
on a glacier, are studded with groups of these peculiarly regular cones
or pyramids, from a few inches to ten or twelve feet in height. Not
infrequently they bear a striking resemblance to Indian tepees ; in fact,
one might easily mistake a group of these structures for an Indian en-
campment.

Still greater inequalities occur when moraines rest on stagnant ice and
basins holding lakelets are formed. The sides of these depressions melt,
and the stones and dirt previously spread out as a general morainal cov-
ering over the surface fall into them. The surface material thus becomes
locally concentrated. As melting progresses, the lakes are drained.
These thick accumulations of debris protect the ice beneath and become
elevated in the same manner as the sand cones described above ; but the
mass of the material being greater, and frequently containing large boul-
ders, the cones formed are of large size, and in many instances have an
elevation of 50 to 150 feet. Although looking like pyramids of rudely
piled boulders, one finds on climbing their sides that they are really
pyramids of ice with a comparatively thin sheathing of stones and dirt.
Large boulders, perched on the summits of these rugged pyramids,
become detached from time to time, and descend in small avalanches to
the depressions below, illustrating, again, the process of breaking and
disintegration which takes place in the debris covering the surfaces of
glaciers.

While large rocks or thick masses of dirt and stones resting on ice pro-
tect it from melting, the reverse is the case with pebbles and other small
objects, particularly those of a dark color, which become warmed through
by the sun's heat during a single day, and lead to the melting of the ice
beneath. Such bodies sink into the ice and are commonly found at the
bottoms of little water-filled wells five or six inches deep. On glaciers,
where there is a scanty covering of pebbles, each individual stone will
be found at the bottom of a water-filled depression. Sometimes the
holes are so abundant that in walking over the surface one really treads



14 GLACIERS OF NORTH AMERICA.

on the summits of thickly set columns of ice, separating the depressions.
Leaves are frequently blown far out on glaciers, and becoming warmed
by the sun sink into the ice in the same manner as the pebbles already
referred to, and even insects, especially butterflies, are conspicuous in
such localities. On one occasion, when traversing an ice stream tribu-
tary to Malaspina glacier, I found a fish, about four inches long, at the
bottom of one of these holes. The nearest water in which it could have
lived was at least twenty miles away. The most probable supposition is
that it had been carried to the place where found by a bird.

Melting* and Drainage. The silence on broad glaciers when the winds
are still and the temperature below freezing is frequently oppressive.
This is especially noticeable on summer nights, for after sunset even in
summer the temperature falls below freezing on the surfaces of large
glaciers ; but when the morning sun warms the air, rills and rivulets are
formed, and the murmuring of running water is heard on every hand.
By midday, brooks and creeks, too deep and rapid to wade and too broad
to vault over, are coursing along in channels of ice. But their exist-
ence is brief. Soon a crevasse is reached, and their floods pour down



Online LibraryIsrael C. (Israel Cook) RussellGlaciers of North America; a reading lesson for students of geography and geology → online text (page 2 of 24)