Books: The Elements of Geology
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William Harmon Norton >> The Elements of Geology
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GLACIER MOTION. The motion of the glaciers of the Alps seldom
exceeds one or two feet a day. Large glaciers, because of the
enormous pressure of their weight and because of less marginal
resistance, move faster than small ones. The Muir advances at the
rate of seven feet a day, and some of the larger tide glaciers of
Greenland are reported to move at the exceptional rate of fifty
feet and more in the same time. Glaciers move faster by day than
by night, and in summer than in winter. Other laws of glacier
motion may be discovered by a study of Figures 96 and 97. It is
important to remember that glaciers do not slide bodily over their
beds, but urged by gravity move slowly down valley in somewhat the
same way as would a stream of thick mud. Although small pieces of
ice are brittle, the large mass of granular ice which composes a
glacier acts as a viscous substance.
CREVASSES. Slight changes of slope in the glacier bed, and the
different rates of motion in different parts, produce tensions
under which the ice cracks and opens in great fissures called
crevasses. At an abrupt descent in the bed the ice is shattered
into great fragments, which unite again below the icefall.
Crevasses are opened on lines at right angles to the direction of
the tension. TRANSVERSE CREVASSES are due to a convexity in the
bed which stretches the ice lengthwise (Fig. 99). MARGINAL
CREVASSES are directed upstream and inwards; RADIAL CREVASSES are
found where the ice stream deploys from some narrow valley and
spreads upon some more open space. What is the direction of the
tension which causes each and to what is it due?
LATERAL AND MEDIAL MORAINES. The surface of a glacier is striped
lengthwise by long dark bands of rock debris. Those in the center
are called the medial moraines. The one on either margin is a
lateral moraine, and is clearly formed of waste which has fallen
on the edge of the ice from the valley slopes. A medial moraine
cannot be formed in this way, since no rock fragments can fall so
far out from the sides. But following it up the glacial stream,
one finds that a medial moraine takes its beginning at the
junction of the glacier and some tributary and is formed by the
union of their two adjacent lateral moraines. Each branch thus
adds a medial moraine, and by counting the number of medial
moraines of a trunk stream one may learn of how many branches it
is composed.
Surface moraines appear in the lower course of the glacier as
ridges, which may reach the exceptional height of one hundred
feet. The bulk of such a ridge is ice. It has been protected from
the sun by the veneer of moraine stuff; while the glacier surface
on either side has melted down at least the distance of the height
of the ridge. In summer the lowering of the glacial surface by
melting goes on rapidly. In Swiss glaciers it has been estimated
that the average lowering of the surface by melting and
evaporation amounts to ten feet a year. As a moraine ridge grows
higher and more steep by the lowering of the surface of the
surrounding ice, the stones of its cover tend to slip down its
sides. Thus moraines broaden, until near the terminus of a glacier
they may coalesce in a wide field of stony waste.
ENGLACIAL DRIFT. This name is applied to whatever debris is
carried within the glacier. It consists of rock waste fallen on
the neve and there buried by accumulations of snow, and of that
engulfed in the glacier where crevasses have opened beneath a
surface moraine. As the surface of the glacier is lowered by
melting, more or less englacial drift is brought again to open
air, and near the terminus it may help to bury the ice from view
beneath a sheet of debris.
THE GROUND MORAINE. The drift dragged along at the glacier's base
and lodged beneath it is known as the ground moraine. Part of the
material of it has fallen down deep crevasses and part has been
torn and worn from the glacier's bed and banks. While the stones
of the surface moraines remain as angular as when they lodged on
the ice, many of those of the ground moraine have been blunted on
the edges and faceted and scratched by being ground against one
another and the rocky bed.
In glaciers such as those of Greenland, whose basal layers are
well loaded with drift and whose surface layers are nearly clean,
different layers have different rates of motion, according to the
amount of drift with which they are clogged. One layer glides over
another, and the stones inset in each are ground and smoothed and
scratched. Usually the sides of glaciated pebbles are more worn
than the ends, and the scratches upon them run with the longer
axis of the stone. Why?
THE TERMINAL MORAINE. As a glacier is in constant motion, it
brings to its end all of its load except such parts of the ground
moraine as may find permanent lodgment beneath the ice. Where the
glacier front remains for some time at one place, there is formed
an accumulation of drift known as the terminal moraine. In valley
glaciers it is shaped by the ice front to a crescent whose convex
side is downstream. Some of the pebbles of the terminal moraine
are angular, and some are faceted and scored, the latter having
come by the hard road of the ground moraine. The material of the
dump is for the most part unsorted, though the water of the
melting ice may find opportunity to leave patches of stratified
sands and gravels in the midst of the unstratified mass of drift,
and the finer material is in places washed away.
GLACIER DRAINAGE. The terminal moraine is commonly breached by a
considerable stream, which issues from beneath the ice by a tunnel
whose portal has been enlarged to a beautiful archway by melting
in the sun and the warm air (Fig. 107). The stream is gray with
silt and loaded with sand and gravel washed from the ground
moraine. "Glacier milk" the Swiss call this muddy water, the gray
color of whose silt proves it rock flour freshly ground by the ice
from the unoxidized sound rock of its bed, the mud of streams
being yellowish when it is washed from the oxidized mantle of
waste. Since glacial streams are well loaded with waste due to
vigorous ice erosion, the valley in front of the glacier is
commonly aggraded to a broad, flat floor. These outwash deposits
are known as VALLEY DRIFT.
The sand brought out by streams from beneath a glacier differs
from river sand in that it consists of freshly broken angular
grains. Why?
The stream derives its water chiefly from the surface melting of
the glacier. As the ice is touched by the rays of the morning sun
in summer, water gathers in pools, and rills trickle and unite in
brooklets which melt and cut shallow channels in the blue ice. The
course of these streams is short. Soon they plunge into deep wells
cut by their whirling waters where some crevasse has begun to open
across their path. These wells lead into chambers and tunnels by
which sooner or later their waters find way to the rock floor of
the valley and there unite in a subglacial stream.
THE LOWER LIMIT OF GLACIERS. The glaciers of a region do not by
any means end at a uniform height above sea level. Each terminates
where its supply is balanced by melting. Those therefore which are
fed by the largest and deepest neves and those also which are best
protected from the sun by a northward exposure or by the depth of
their inclosing valleys flow to lower levels than those whose
supply is less and whose exposure to the sun is greater.
A series of cold, moist years, with an abundant snowfall, causes
glaciers to thicken and advance; a series of warm, dry years
causes them to wither and melt back. The variation in glaciers is
now carefully observed in many parts of the world. The Muir
glacier has retreated two miles in twenty years. The glaciers of
the Swiss Alps are now for the most part melting back, although a
well-known glacier of the eastern Alps, the Vernagt, advanced five
hundred feet in the year 1900, and was then plowing up its
terminal moraine.
How soon would you expect a glacier to advance after its neve
fields have been swollen with unusually heavy snows, as compared
with the time needed for the flood of a large river to reach its
mouth after heavy rains upon its headwaters?
On the surface of glaciers in summer time one may often see large
stones supported by pillars of ice several feet in height (Fig.
108). These "glacier tables" commonly slope more or less strongly
to the south, and thus may be used to indicate roughly the points
of the compass. Can you explain their formation and the direction
of their slope? On the other hand, a small and thin stone, or a
patch of dust, lying on the ice, tends to sink a few inches into
it. Why?
In what respects is a valley glacier like a mountain stream which
flows out upon desert plains?
Two confluent glaciers do not mingle their currents as do two
confluent rivers. What characteristics of surface moraines prove
this fact?
What effect would you expect the laws of glacier motion to have on
the slant of the sides of transverse crevasses?
A trunk glacier has four medial moraines. Of how many tributaries
is it composed? Illustrate by diagram.
State all the evidences which you have found that glaciers move.
If a glacier melts back with occasional pauses up a valley, what
records are left of its retreat?
PIEDMONT GLACIERS
THE MALASPINA GLACIER. Piedmont (foot of the mountain) glaciers
are, as the name implies, ice fields formed at the foot of
mountains by the confluence of valley glaciers. The Malaspina
glacier of Alaska, the typical glacier of this kind, is seventy
miles wide and stretches for thirty miles from the foot of the
Mount Saint Elias range to the shore of the Pacific Ocean. The
valley glaciers which unite and spread to form this lake of ice
lie above the snow line and their moraines are concealed beneath
neve. The central area of the Malaspina is also free from debris;
but on the outer edge large quantities of englacial drift are
exposed by surface melting and form a belt of morainic waste a few
feet thick and several miles wide, covered in part with a
luxuriant forest, beneath which the ice is in places one thousand
feet in depth. The glacier here is practically stagnant, and lakes
a few hundred yards across, which could not exist were the ice in
motion and broken with crevasses, gather on their beds sorted
waste from the moraine. The streams which drain the glacier have
cut their courses in englacial and subglacial tunnels; none flow
for any distance on the surface. The largest, the Yahtse River,
issues from a high archway in the ice,--a muddy torrent one
hundred feet wide and twenty feet deep, loaded with sand and
stones which it deposits in a broad outwash plain (Fig. 110).
Where the ice has retreated from the sea there is left a hummocky
drift sheet with hollows filled with lakelets. These deposits help
to explain similar hummocky regions of drift and similar plains of
coarse, water-laid material often found in the drift-covered area
of the northeastern United States.
THE GEOLOGICAL WORK OF GLACIER ICE
The sluggish glacier must do its work in a different way from the
agile river. The mountain stream is swift and small, and its
channel occupies but a small portion of the valley. The glacier is
slow and big; its rate of motion may be less than a millionth of
that of running water over the same declivity, and its bulk is
proportionately large and fills the valley to great depth.
Moreover, glacier ice is a solid body plastic under slowly applied
stresses, while the water of rivers is a nimble fluid.
TRANSPORTATION. Valley glaciers differ from rivers as carriers in
that they float the major part of their load upon their surface,
transporting the heaviest bowlder as easily as a grain of sand;
while streams push and roll much of their load along their beds,
and their power of transporting waste depends solely upon their
velocity. The amount of the surface load of glaciers is limited
only by the amount of waste received from the mountain slopes
above them. The moving floor of ice stretched high across a valley
sweeps along as lateral moraines much of the waste which a
mountain stream would let accumulate in talus and alluvial cones.
While a valley glacier carries much of its load on top, an ice
sheet, such as that of Greenland, is free from surface debris,
except where moraines trail away from some nunatak. If at its edge
it breaks into separate glaciers which drain down mountain
valleys, these tongues of ice will carry the selvages of waste
common to valley glaciers. Both ice sheets and valley glaciers
drag on large quantities of rock waste in their ground moraines.
Stones transported by glaciers are sometimes called erratics. Such
are the bowlders of the drift of our northern states. Erratics may
be set down in an insecure position on the melting of the ice.
DEPOSIT. Little need be added here to what has already been said
of ground and terminal moraines. All strictly glacial deposits are
unstratified. The load laid down at the end of a glacier in the
terminal moraine is loose in texture, while the drift lodged
beneath the glacier as ground moraine is often an extremely dense,
stony clay, having been compacted under the pressure of the
overriding ice.
EROSION. A glacier erodes its bed and banks in two ways,--by
abrasion and by plucking.
The rock bed over which a glacier has moved is seen in places to
have been abraded, or ground away, to smooth surfaces which are
marked by long, straight, parallel scorings aligned with the line
of movement of the ice and varying in size from hair lines and
coarse scratches to exceptional furrows several feet deep. Clearly
this work has been accomplished by means of the sharp sand, the
pebbles, and the larger stones with which the base of the glacier
is inset, and which it holds in a firm grasp as running water
cannot. Hard and fine-grained rocks, such as granite and
quartzite, are often not only ground down to a smooth surface but
are also highly polished by means of fine rock flour worn from the
glacier bed.
In other places the bed of the glacier is rough and torn. The
rocks have been disrupted and their fragments have been carried
away,--a process known as PLUCKING. Moving under immense pressure
the ice shatters the rock, breaks off projections, presses into
crevices and wedges the rocks apart, dislodges the blocks into
which the rock is divided by joints and bedding planes, and
freezing fast to the fragments drags them on. In this work the
freezing and thawing of subglacial waters in any cracks and
crevices of the rock no doubt play an important part. Plucking
occurs especially where the bed rock is weak because of close
jointing. The product of plucking is bowlders, while the product
of abrasion is fine rock flour and sand.
Is the ground moraine of Figure 87 due chiefly to abrasion or to
plucking?
ROCHES MOUTONNEES AND ROUNDED HILLS. The prominences left between
the hollows due to plucking are commonly ground down and rounded
on the stoss side,--the side from which the ice advances,--and
sometimes on the opposite, the lee side, as well. In this way the
bed rock often comes to have a billowy surface known as roches
moutonnees (sheep rocks). Hills overridden by an ice sheet often
have similarly rounded contours on the stoss side, while on the
lee side they may be craggy, either because of plucking or because
here they have been less worn from their initial profile.
THE DIRECTION OF GLACIER MOVEMENT. The direction of the flow of
vanished glaciers and ice sheets is recorded both in the
differences just mentioned in the profiles of overridden hills and
also in the minute details of the glacier trail.
Flint nodules or other small prominences in the bed rock are found
more worn on the stoss than on the lee side, where indeed they may
have a low cone of rock protected by them from abrasion. Cavities,
on the other hand, have their edges worn on the lee side and left
sharp upon the stoss.
Surfaces worn and torn in the ways which we have mentioned are
said to be glaciated. But it must not be supposed that a glacier
everywhere glaciates its bed. Although in places it acts as a rasp
or as a pick, in others, and especially where its pressure is
least, as near the terminus, it moves over its bed in the manner
of a sled. Instances are known where glaciers have advanced over
deposits of sand and gravel without disturbing them to any notable
degree. Like a river, a glacier does not everywhere erode. In
places it leaves its bed undisturbed and in places aggrades it by
deposits of the ground moraine.
CIRQUES. Valley glaciers commonly head as we have seen, in broad
amphitheaters deeply filled with snow and ice. On mountains now
destitute of glaciers, but whose glaciation shows that they have
supported glaciers in the past, there are found similar crescentic
hollows with high, precipitous walls and glaciated floors. Their
floors are often basined and hold lakelets whose deep and quiet
waters reflect the sheltering ramparts of rugged rock which tower
far above them. Such mountain hollows are termed CIRQUES. As a
powerful spring wears back a recess in the valley side where it
discharges, so the fountain head of a glacier gradually wears back
a cirque. In its slow movement the neve field broadly scours its
bed to a flat or basined floor. Meanwhile the sides of the valley
head are steepened and driven back to precipitous walls. For in
winter the crevasse of the bergschrund which surrounds the neve
field is filled with snow and the neve is frozen fast to the rocky
sides of the valley. In early summer the neve tears itself free,
dislodging and removing any loosened blocks, and the open fissure
of the bergschrund allows frost and other agencies of weathering
to attack the unprotected rock. As cirques are thus formed and
enlarged the peaks beneath which they lie are sharpened, and the
mountain crests are scalloped and cut back from either side to
knife-edged ridges.
In the western mountains of the United States many cirques, now
empty of neve and glacier ice, and known locally as "basins,"
testify to the fact that in recent times the snow line stood
beneath the levels of their floors, and thus far below its present
altitude.
GLACIER TROUGHS. The channel worn to accommodate the big and
clumsy glacier differs markedly from the river valley cut as with
a saw by the narrow and flexible stream and widened by the weather
and the wash of rains. The valley glacier may easily be from one
thousand to three thousand feet deep and from one to three miles
wide. Such a ponderous bulk of slowly moving ice does not readily
adapt itself to sharp turns and a narrow bed. By scouring and
plucking all resisting edges it develops a fitting channel with a
wide, flat floor, and steep, smooth sides, above which are seen
the weathered slopes of stream-worn mountain valleys. Since the
trunk glacier requires a deeper channel than do its branches, the
bed of a branch glacier enters the main trough at some distance
above the floor of the latter, although the surface of the two ice
streams may be accordant. Glacier troughs can be studied best
where large glaciers have recently melted completely away, as is
the case in many valleys of the mountains of the western United
States and of central and northern Europe (Fig. 114). The typical
glacier trough, as shown in such examples, is U-shaped, with a
broad, flat floor, and high, steep walls. Its walls are little
broken by projecting spurs and lateral ravines. It is as if a V-
valley cut by a river had afterwards been gouged deeper with a
gigantic chisel, widening the floor to the width of the chisel
blade, cutting back the spurs, and smoothing and steepening the
sides. A river valley could only be as wide-floored as this after
it had long been worn down to grade.
The floor of a glacier trough may not be graded; it is often
interrupted by irregular steps perhaps hundreds and even a
thousand feet in height, over which the stream that now drains the
valley tumbles in waterfalls. Reaches between the steps are often
basined. Lakelets may occupy hollows excavated in solid rock, and
other lakes may be held behind terminal moraines left as dams
across the valley at pauses in the retreat of the glacier.
FJORDS are glacier troughs now occupied in part or wholly by the
sea, either because they were excavated by a tide glacier to their
present depth below sea level, or because of a submergence of the
land. Their characteristic form is that of a long, deep, narrow
bay with steep rock walls and basined floor. Fjords are found only
in regions which have suffered glaciation, such as Norway and
Alaska.
HANGING VALLEYS. These are lateral valleys which open on their
main valley some distance above its floor. They are conspicuous
features of glacier troughs from which the ice has vanished; for
the trunk glacier in widening and deepening its channel cut its
bed below the bottoms of the lateral valleys.
Since the mouths of hanging valleys are suspended on the walls of
the glacier trough, their streams are compelled to plunge down its
steep, high sides in waterfalls. Some of the loftiest and most
beautiful waterfalls of the world leap from hanging valleys,--
among them the celebrated Staubbach of the Lauterbrunnen valley of
Switzerland, and those of the fjords of Norway and Alaska.
Hanging valleys are found also in river gorges where the smaller
tributaries have not been able to keep pace with a strong master
stream in cutting down their beds. In this case, however, they are
a mark of extreme youth; for, as the trunk stream approaches grade
and its velocity and power to erode its bed decrease, the side
streams soon cut back their falls and wear their beds at their
mouths to a common level with that of the main river. The Grand
Canyon of the Colorado must be reckoned a young valley. At its
base it narrows to scarcely more than the width of the river, and
yet its tributaries, except the very smallest, enter it at a
common level.
Why could not a wide-floored valley, such as a glacier trough,
with hanging valleys opening upon it, be produced in the normal
development of a river valley?
THE TROUGHS OF YOUNG AND OF MATURE GLACIERS. The features of a
glacier trough depend much on the length of time the preexisting
valley was occupied with ice. During the infancy of a glacier, we
may believe, the spurs of the valley which it fills are but little
blunted and its bed is but little broken by steps. In youth the
glacier develops icefalls, as a river in youth develops
waterfalls, and its bed becomes terraced with great stairs. The
mature glacier, like the mature river, has effaced its falls and
smoothed its bed to grade. It has also worn back the projecting
spurs of its valley, making itself a wide channel with smooth
sides. The bed of a mature glacier may form a long basin, since it
abrades most in its upper and middle course, where its weight and
motion are the greatest. Near the terminus, where weight and
motion are the least, it erodes least, and may instead deposit a
sheet of ground moraine, much as a river builds a flood plain in
the same part of its course as it approaches maturity. The bed of
a mature glacier thus tends to take the form of a long, relatively
narrow basin, across whose lower end may be stretched the dam of
the terminal moraine. On the disappearance of the ice the basin is
rilled with a long, narrow lake, such as Lake Chelan in Washington
and many of the lakes in the Highlands of Scotland.
Piedmont glaciers apparently erode but little. Beneath their lake-
like expanse of sluggish or stagnant ice a broad sheet of ground
moraine is probably being deposited.
Cirques and glaciated valleys rapidly lose their characteristic
forms after the ice has withdrawn. The weather destroys all
smoothed, polished, and scored surfaces which are not protected
beneath glacial deposits. The oversteepened sides of the trough
are graded by landslips, by talus slopes, and by alluvial cones.
Morainic heaps of drift are dissected and carried away. Hanging
valleys and the irregular bed of the trough are both worn down to
grade by the streams which now occupy them. The length of time
since the retreat of the ice from a mountain valley may thus be
estimated by the degree to which the destruction of the
characteristic features of the glacier trough has been carried.
In Figure 104 what characteristics of a glacier trough do you
notice? What inference do you draw as to the former thickness of
the glacier?
Name all the evidences you would expect to find to prove the fact
that in the recent geological past the valleys of the Alps
contained far larger glaciers than at present, and that on the
north of the Alps the ice streams united in a piedmont glacier
which extended across the plains of Switzerland to the sides of
the Jura Mountains.
THE RELATIVE IMPORTANCE OF GLACIERS AND OF RIVERS. Powerful as
glaciers are, and marked as are the land forms which they produce,
it is easy to exaggerate their geological importance as compared
with rivers. Under present climatic conditions they are confined
to lofty mountains or polar lands. Polar ice sheets are permanent
only so long as the lands remain on which they rest. Mountain
glaciers can stay only the brief time during which the ranges
continue young and high. As lofty mountains, such as the Selkirks
and the Alps, are lowered by frost and glacier ice, the snowfall
will decrease, the line of permanent snow will rise, and as the
mountain hollows in which snow may gather are worn beneath the
snow line, the glaciers must disappear. Under present climatic
conditions the work of glaciers is therefore both local and of
short duration.
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