Israel C. (Israel Cook) Russell.

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

. (page 22 of 24)
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in size and their neVes broaden, the snow line descends lower and lower,
even though the general climatic conditions remain sensibly the same.
This is on account of the reaction on the atmosphere of the ice bodies
previously formed. The snow and ice-covered mountains chill the winds
blowing over them more than formerly, and cause heavier precipitations.
Each storm that sweeps over the white peaks, even in summer, is now
accompanied by a fall of snow. Glaciers thus tend to change their
mete6rological environment in such a way as to favor their own growth.

The trunk glacier in the valley below continues to advance century
after century and increases in thickness, particularly towards its terminus.
In its middle and upper course, and especially in the neve* region, the
depth of the frozen flood is but little greater than during the earlier
centuries of its maturity.

An increase in the thickness of a glacier which flows from a lofty
mountain while due mainly to a decrease in the gradient of the valley it
occupies, is aided also by an increase in the load of debris it carries. Each
of these changes tends to increase the friction of flow, but as the advance of
ice from above is continuous, the lower portion of the glacier, where the
current is more sluggish, must increase in depth or in breadth, or more
precisely in area of cross section, in order to enable the flood to pass.

Another agency which modifies a glacier's life also increases in power
as the glacier advances, namely, the heat of the atmosphere. The lower
a glacier descends the more rapidly is it melted. The gradient of its sur-
face is thus controlled by two groups of antagonistic agencies. Decrease
in flow causes the ice to increase in thickness, while melting lowers its
surface. Still more complexity is apparent when one remembers that an
increase of temperature is accompanied by an increased rate of flow.
The records left by former alpine glaciers in Alaska show that for
many miles after leaving their neves they increase in thickness, but as
the effect of the increasing temperature of the lower region they invaded
was felt, they gradually decreased in thickness and receded.


Returning to our imaginary glacier, we observe that in its onward
march it carries the mouths of lateral valleys. If these are without glaciers,
or if the ice bodies in their upper courses fail to reach the main line of
drainage, an ice dam is formed in the main valley, the drainage in the
lateral valleys is held in check, and lakes appear. The accumulated water
may escape either over the surface of the ice dam, usually at its junction
with the rocky side of the valley ; but more frequently the water disap-
pears beneath the ice and finds its way downward through sub- or englacial
tunnels. These lakes on the margin of the glacier are subject to great
fluctuations on account of changes in the icy channels through which they
discharge. Not infrequently they are drained suddenly, and floods occur
in the valley below the end of the glacier that confined them. The
empty basins refill when the outlet becomes again closed by movements
in the dam of ice. Not infrequently their surfaces are whitened with
floating bergs, from which boulders are scattered over the bottom. In
spite of the vicissitudes that beset their lives, however, we learn by watch-
ing their basins as they are successively filled and emptied, that deposits of
sand and clay are formed in them, and under favorable conditions may
even fill the depressions up to the level of the wall of ice which in part
forms their boundaries. Important deposits thus originate, and may
remain as a part of the geological record when the glacier melts and the
lakes along its margins are drained.

In our fancied study of the life of a glacier we see it approach a por-
tion of the valley where the descent is rugged and broken. The yellow
waters issuing from it are churned into foam as they leap from ledge to
ledge. On reaching the place of steep descent the glacier becomes greatly
broken, and for a time falls in detached masses, forming a veritable ice
cascade. The precipice of rock is heightened by a wall of ice, which has
a serrate crest and is broken into towers and pinnacles, separated by blue
crevasses. From time to time a great tower falls with a crash, and sends
a cloud of ice fragments rolling down the valley. The steep descent soon
becomes blocked with fallen fragments. The face of the rocky precipice
is concealed from view, except at its extremities, where it joins the sides
of the valley. The broken and pinnacled ice descending the precipice is
cemented together again at the bottom, and the glacier flows on, with but few
scars caused by its rough passage remaining. At the ice fall the descend-
ing mass impinges with great force on the rocks at the bottom of the slope,
and must there have its erosive power augmented. Rock basins worn
out at such localities will be seen when the glacier retreats. The debris


carried on the glacier above where it makes its steep descent is, to a con-
siderable extent, engulfed in crevasses, but a portion escapes these pitfalls
and still darkens the surface below the cascade. A part of the stones
that were engulfed soon returns to the surface, by reason of the melting of
the ice ; but some of them remain for a long time in an englacial or sub-
glacial position. At each sharp descent in the bed of the glacier an ice
cascade appears. When the grade is steep but not precipitous, the ice
is greatly broken and presents the appearance of a rapid rather than a

The surface of the glacier throughout its lower course, and especially
at its extremity, is dark with debris. The medial moraines so prominent
in its middle course, and from a distance appearing like winding highways
leading from the land of flowers to the land of snow, become less well
defined and less definitely separated one from another. The reason for the
flattening and spreading that the moraines suffer is not far to seek. Owing
to the more rapid melting of the clear ice the portions protected by debris
are left in relief. The medial moraines where best defined are really roof-
like ridges of ice veneered with dirt and stones. From time to time a
rock breaks away from its insecure position and rolls and slides down the
steep slope to the clear ice below, thus tending to widen the moraine-
covered tract. On gaining a new position the fallen block again protects
the ice beneath and is again left in relief by the melting of the adjacent
surface, and the process is repeated. The falls that the rock masses experi-
ence and the larger ones receive the roughest treatment result in
many fractures. The blocks of stone are thus reduced in size and prepared
for future usefulness in the formation of soil. Both medial and lateral
moraines are broadened in this way, and in time lose their stream-like
character, and the general surface of the glacier becomes covered with
debris. This process is assisted also by the appearance at the surface of
stones previously enclosed in the ice, but not associated with well-defined

The accidents that happen to the blocks forming surface moraines, on
account of the unequal melting of the glaciers, are repeated on a smaller
scale by individual blocks of stone large enough to shelter the ice on which
they rest. When the conditions are favorable, such blocks of stone are
raised on pedestals and form glacier tables. These growths on the
glacier, like the flowers that bloom beside it, look toward the sun that gave
them birth, and as they increase in height incline toward his mid-day
position. Finally, they slide from their pedestals and the process is


repeated. Other features due to differential melting are also present, but
belong to the minor details of the surface where waste exceeds supply, and
are not conspicuous from a distance.

The glacier, now in its full strength, advances from the extremity of
the valley that sheltered its youth and guided its early life, and invades
the piedmont plain. The low lands are densely forested. Majestic spruce
trees and aged moss-covered hemlocks stand in thick, serrate ranks across
the glacier's path, but are mowed down as easily as the grass before a
scythe. Crushed, broken, and splintered, the trunks are piled in huge
confused heaps and overridden and buried by the slow but resistless march
of the ice. Where the waters flowing from the glacier are abundantly
loaded with sand and gravel, they build alluvial deposits about its margin.
The streams in their passage over these alluvial cones subdivide and send
off distributaries into the forest to the right and left, and the trees are
surrounded and buried by sand and gravel while yet standing. A fringe
of dead trees, in part denuded of their branches, marks the areas where
the stream-borne deposits have made recent conquests. Under these con-
ditions the glacier advances over the buried forests, and all vestiges of its
existence are blotted out. Centuries later the still erect trunks may be

The rOle that a glacier plays in its full strength when it invades a plain
or enters a broad valley may be said to depend on how well it improved
the opportunities of its youth. If the debris it gathered from the amphi-
theatre in which it was cradled and from the cliffs that sheltered its early
course is sufficiently abundant, it retains its stream-like character and
builds protecting embankments along its sides as it advances. If the
lateral moraines are not massive in comparison with the volume of ice, the
glacier expands and forms a semicircular terminus in which a constantly
increasing area of surface is exposed to the sun and rain. Radial cre-
vasses appear in the expanding ice foot and still farther assist in its

A critical point of the life of a glacier has now been reached. Will it
have strength to resist the increased warmth of the region it has invaded
and continue to advance, or must it halt at an intangible boundary, where
the annual melting balances the flow of ice from higher regions ?

In our fancied watch we will assume that the glacier is not only unable
to advance against the invisible enemies that beset its progress, but that
a climatic cycle favorable to glacial growth has passed its maximum and
begun to swing in the opposite direction. Some distant cause, possibly


the upheaval of a barrier in the ocean, or the slow growth of a coral reef,
has deflected an ocean current that formerly bathed the adjacent coast, and
the winds fail to bring as much moisture to the mountains as formerly.
Perhaps the relation of the earth to the sun is undergoing a secular
change, and the mean annual temperature, and the relations of the seasons,
are so modified that for thousands of years conditions unfavorable to the
existence of perennial snow will gradually supplant and crowd out the
favorable conditions. Probably a combination of these and other changes
equally gradual in their effects are at work in modifying the size and
shape of the spheroid of 32, and the snow line is rising at the rate of a
few feet in a century.

The terminus of the glacier seemingly remains stationary for a time,
but in reality is seldom at the same line for many successive years. Minor
climatic changes experienced scores of years, or perhaps even centuries
previously, at the fountains from which it flows, are transmitted like
pulsations through its length, and its extremity advances or retreats a few
feet in a year or in a score of years. Again, climatic changes may promote
or retard melting in the glacier proper, and lead to less sluggish responses
in the position of its terminus. Halts, advances, and retreats may also be
caused by the concentration of debris in its wasting extremity.

The glacier has now passed its period of strength, and many changes
in its appearance and in its behavior become apparent. The flow of even
the most rapid threads of the current in the central part of the stream is
no longer sufficient to renew the surface each year at its extremity. The
terminus instead of advancing is now slowly retreating. Melting from
the surface of the glacier proper is in excess of the supply of ice from
higher regions, and a general shrinking and lowering of the surface is in
progress. The stones and dirt previously held in the ice become more
rapidly concentrated at the surface. Clear ice, especially in the lower
portions of the glacier proper, is no longer visible except in crevasses.
Vegetation begins to take root on the previously desolate fields of debris.
A fringe of flowers and ferns soon margins the stagnant portions, and
transforms them during the summer season into luxuriant gardens. The
forest, previously swept away as a thing too insignificant to be worthy the
attention of the legions of the ice king, re-advances year by year, and in
time flourishes on the moraines resting on the ice under which relics of
their ancestors lie buried. Animals wander through the forest where
moss and thick undergrowths conceal pitfalls in the ice beneath, and are
entrapped. Man, unattentive to the wonders about him, may tread the


silent aisles of the forest without knowing that a dead glacier lies buried
beneath the carpet of vegetation on which he treads. As the ice slowly
melts from beneath the forest^covered moraine, openings are formed, and
lakelets surrounded with rank vegetation give variety to the scene. As
these pools enlarge, the soil and boulders on their banks are undermined,
and uprooted shrubs and trees fall into them. These relics of the forest,
and the peaty soil formed where the depressions are partially drained,
become buried in morainal debris. Many a puzzling record is thus
preserved which will lead students astray in future centuries.

The forest-covered border of the fringing moraine is separated from
the clear ice by broad areas of desolation, on which scarcely a lichen has
taken root. On these monotonous wastes striking changes are likewise
in progress. Scores and perhaps hundreds of lakes are formed, similar to
those in the forest-covered moraine, but producing more apparent results,
since the debris about them is not covered with a mask of vegetation.
As these walls of ice about the lakelets melt, the stones and dirt on the
adjacent surface are precipitated into them, and accumulate deeply over
their bottom. When the general surface is lowered by melting, these
deeply filled holes are transformed into prominences thickly covered with
earth and stones. The irregular piles assume a pyramidal form, because
of the displacement and sliding down of the rock masses on their borders.
In time the sites of the basins of muddy water are marked by huge pyra-
mids of ice, a hundred feet or more in height but concealed from sight by
a veneer of stones. From our observing station, scores and hundreds
of those monuments are in view. Between them are many lakelets not
yet filled, and others that have been drained of their water but are yet
unsightly depressions in which the stones are covered with slimy mud.

While the changes noted near the extremity of the glacier have been
in progress, other signs of old age have appeared far up its course. The
ice no longer fills the valley as deeply as before, but an apparent settling
of its surface has occurred. The mountain slopes rising above its borders
are either bare or sheathed with debris left stranded as the current sub-
sided. The height of the ice during its maximum is marked on each
side of the valley by an abandoned lateral moraine. This ridge is
frequently a well-defined terrace or shelf on the steep slope. The
border overlooking the valley is higher than the margin adjacent to the
mountain, and forms a ridge composed of boulders and stones of many
sizes and shapes. Between the ridge and the upward slope of the moun-
tain, there is frequently a level-floored space, formed of sand and gravel


swept by torrents from the region above. Streams descending lateral
gullies are deflected by the outer ridge, and, should the general grade of
the valley be sufficiently gentle, may form lakelets and swamps.

The slope of the abandoned lateral moraines, or their gradient, is
plainly less than the gradient of the surface of the ice still remaining in
the bottom of the trough. Far down the valley, near where it opens out
on the piedmont plain, their elevation a thousand feet above the
surface to the stagnant ice, but as one traces them up stream this interval
becomes less and less, until in proximity to the lower border of the mantle
of perennial snow, the two coincide. This is the reverse of the process
noted during the advance of the glacier, when the strong current caused
the ice to thicken most rapidly at a distance below the source of supply,
and shows that Summer is invading the realm of Winter.

The highest lateral moraine left stranded as the ice melts, marks the
limit between the rugged and angular crags that have been long exposed
to the action of frost and rain, and the surfaces which have been worn
and rounded by flowing ice. Above, the lines in the sculpturing are
more or less vertical, in conformity with the direction of flow of the
streams and rills that made them ; below, the most pronounced elements
in the relief trend in the direction of the major axis of the valley, and
appear to be approximately horizontal.

Most interesting of all the records on the sides of the valley left
exposed by the shrinking of the ice, but scarcely discernible from a distance,
are lines and grooves on the ice-worn surfaces. When the rocks are
hard and fine-grained, their polished surfaces glitter in the sunlight like
burnished granite fresh from the builder's hand. A near view will
generally show, however, that the tools that did the polishing were not
carefully selected. The glaciated surfaces are covered by scorings of
varying character, from light, delicate lines, such as might be traced
with a diamond's point, up to deep grooves and heavy irregular gouges,
seemingly made by huge boulders of hard rock forced along under heavy
pressure. The more even lines, especially, are in parallel series and trend
in the direction of ice movement. Similar markings are also to be found
at the extremity of the glacier wherever its retreat has exposed the hard
rocks over which it passed. A close inspection will reveal many localities
where the glacial ice charged with sand and stones rests directly in con-
tact with polished and striated surfaces. The manner in which the
graving tools that made the inscriptions are held in the icy matrix is thus


As we watch the slowly receding extremity of the glacier, we note
that its rate of retreat is not uniform. At times it remains practically
stationary for many years, and the debris carried forward on its surface
is concentrated in its extremity. When the glacier again retreats, an
irregular accumulation of boulder, stone, sand, and earth, indiscriminately
commingled, is left as an embankment across the valley. The crest of
the ridge forms a curve concave up stream, and is apt to have its more
gentle slope on the side facing the shrunken glacier. Several such
crescent-shaped piles may be left as the glacier slowly retires. Many of
these abandoned terminal moraines form dams above which the drainage
from the glacier and from adjacent slopes is retarded, and lakes are
formed. These lakes, like those originating along the sides of the glacier
during its advance, are short-lived. They rise rapidly as the ice with-
draws, and in fact are flooded and begin to overflow long before the
retreat of the ice has allowed them to fully expand. While yet small,
they are turbid with glacial silt and have a peculiar yellowish green color.
When they reach a large size, their waters are more or less perfectly
clarified, and may appear as a plain of blue in which the majestic moun-
tains are mirrored. Where glacial streams enter, there is always a fringe
of yellow shading off through innumerable tints to the clear blue beyond
and showing that abundant sediments are there being deposited. Unfor-
tunately, as it would seem, various agencies conspire to deface and to
remove these pleasing results of glacial agencies. The lake basins are
rapidly filled with sediment, and the overflowing water cuts deep channels
through the unconsolidated material that holds them in check. For these
reasons, they pass rapidly through their predestined changes, and finally
are completely drained and their bottoms transformed into smiling
meadows. Dark evergreen forests border the even meadows of waving
grass, and flowers beautify their surfaces. The now sluggish streams
meander in many graceful curves between banks that are outlined by
the pink of budding willows in spring and with the gold of aspen leaves
in autumn. Deer and other inhabitants of the region find there a
sheltered retreat. The once frozen solitude is transformed into a park,
more beautiful and richer in delicate charms than man has yet designed.
The plowshare of ice that descended from the mountains broke the flinty
rocks and prepared the land for the harvest.

While watching the transformation from stern winter to smiling
summer in the lower valley, the extremity of the glacier has receded
above that portion of its bed where great crevasses and ice cascades broke


its surface during its descent. At the base of these steep slopes, now
worn and rounded, other lakes are born. Their cradles are hollows in the
solid rock. No moraine piles hold the water in check, but the rims of
their basins are polished and striated ledges. Could we see their bottom,
we would find that they, also, are smoothed and rounded. As the ice
withdraws from the higher portions of its bed the number of rock-enclosed
tarns increases, and fresh charms are added to the diversity and beauty of
the region. In neighboring valleys and adjacent slopes where the rocks
are essentially the same but have not been subjected to glacial action, rock
basins are absent. Evidently their abundance in the abandoned beds of
the retreating glacier is the result of the abrasion that the rocks suffered
from the ice that moved over them. They are plentiful where the descent
is precipitous, and absent at lower levels, where the grade of the ice stream
was gentle. The region where they occur is notably free from moraines
and other glacial deposits ; lower down the valley where they are absent
there are heavy accumulations of debris. That their rarity in the lower
part of the valley is not due to the masking of the hard-rock surfaces by
glacial deposits is shown by their absence from areas where such deposits
are lacking. The regions where the rock basins are most abundant are
regions where abrading and transporting agencies were active ; the lower
portion of the valley where they do not occur are where the glacier did
least work, and where deposition and not erosion was the rule. Where
the glacier was heavily charged with debris it became sluggish and did but
little abrading. In many instances rock-basin lakes are situated at the
bottom of steep inclines, down which the ice descended and pressed with
great force on the rocks where there is an abrupt change to a more gentle
grade. The rock-enclosed lakes are, in general, longer-lived than those
held by moraines, but in time they too become filled with sediment and
are transformed into brilliant gardens of alpine flowers.

Had our glacier advanced boldly into the piedmont plain, possibly
another variety of rock basin .would have been excavated, but in the fan-
cied instances before us it is not practicable to include a complete analysis
of glacial action.

The last stages in the decline and death of a glacier are slow and
frequently much prolonged. The blue ice visible below the margin of the
white neve contracts more and more, until during certain seasons, or for a
series of years, it is completely concealed by a covering of snow. Fluctua-
tion still occurs, however, and during years, or a succession of years, when
the winter snows are light, or summer melting accelerated, the glacier


proper may be seen for a short time in late summer or early autumn. In
its old age the glacier reaches a second childhood in which the character-
istics of its early years again appear. So complete is this similarity in

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Online LibraryIsrael C. (Israel Cook) RussellGlaciers of North America; a reading lesson for students of geography and geology → online text (page 22 of 24)