Elements of Geology — Ch. V: The Work of Glaciers, pt 1
CHAPTER V THE WORK OF GLACIERS
THE DRIFT.
The surface of northeastern North America, as far south as the Ohio and Missouri rivers, is generally covered by the drift,--a formation which is quite unlike any which we have so far studied.
A section of it, such as that illustrated in Figure 87, shows that for the most part it is unstratified, consisting of clay, sand, pebbles, and even large bowlders, all mingled pell- mell together.
The agent which laid the drift is one which can carry a load of material of all sizes, from the largest bowlder to the finest clay, and deposit it without sorting.
The stones of the drift are of many kinds.
The region from which it was gathered may well have been large in order to supply these many different varieties of rocks.
Pebbles and bowlders have been left far from their original homes, as may be seen in southern Iowa, where the drift contains nuggets of copper brought from the region about Lake Superior.
The agent which laid the drift is one able to gather its load over a large area and carry it a long way.
The pebbles of the drift are unlike those rounded by running water or by waves.
They are marked with scratches.
Some are angular, many have had their edges blunted, while others have been ground flat and smooth on one or more sides, like gems which have been faceted by being held firmly against the lapidary's wheel.
In many places the upper surface of the country rock beneath the drift has been swept clean of residual clays and other waste.
All rock rotten has been planed away, and the ledges of sound rock to which the surface has been cut down have been rubbed smooth and scratched with long, straight, parallel lines.
The agent which laid the drift can hold sand and pebbles firmly in its grasp and can grind them against the rock beneath, thus planing it down and scoring it, while faceting the pebbles also.
Neither water nor wind can do these things.
Indeed, nothing like the drift is being formed by any process now at work anywhere in the eastern United States.
To find the agent which has laid this extensive formation we must go to a region of different climatic conditions.
THE INLAND ICE OF GREENLAND.
Greenland is about fifteen hundred miles long and nearly seven hundred miles in greatest width.
With the exception of a narrow fringe of mountainous coast land, it is completely buried beneath a sheet of ice, in shape like a vast white shield, whose convex surface rises to a height of nine thousand feet above the sea.
The few explorers who have crossed the ice cap found it a trackless desert destitute of all life save such lowly forms as the microscopic plant which produces the so- called "red snow."
On the smooth plain of the interior no rock waste relieves the snow's dazzling whiteness; no streams of running water are seen; the silence is broken only by howling storm winds and the rustle of the surface snow which they drive before them.
Sounding with long poles, explorers find that below the powdery snow of the latest snowfall lie successive layers of earlier snows, which grow more and more compact downward, and at last have altered to impenetrable ice.
The ice cap formed by the accumulated snows of uncounted centuries may well be more than a mile in depth.
Ice thus formed by the compacting of snow is distinguished when in motion as GLACIER ICE.
The inland ice of Greenland moves.
It flows with imperceptible slowness under its own weight, like, a mass of some viscous or plastic substance, such as pitch or molasses candy, in all directions outward toward the sea.
Near the edge it has so thinned that mountain peaks are laid bare, these islands in the sea of ice being known as NUNATAKS.
Down the valleys of the coastal belt it drains in separate streams of ice, or GLACIERS.
The largest of these reach the sea at the head of inlets, and are therefore called TIDE GLACIERS.
Their fronts stand so deep in sea water that there is visible seldom more than three hundred feet of the wall of ice, which in many glaciers must be two thousand and more feet high.
From the sea walls of tide glaciers great fragments break off and float away as icebergs.
Thus snows which fell in the interior of this northern land, perhaps many thousands of years ago, are carried in the form of icebergs to melt at last in the North Atlantic.
Greenland, then, is being modeled over the vast extent of its interior not by streams of running water, as are regions in warm and humid climates, nor by currents of air, as are deserts to a large extent, but by a sheet of flowing ice.
What the ice sheet is doing in the interior we may infer from a study of the separate glaciers into which it breaks at its edge.
THE SMALLER GREENLAND GLACIERS.
Many of the smaller glaciers of Greenland do not reach the sea, but deploy on plains of sand and gravel.
The edges of these ice tongues are often as abrupt as if sliced away with a knife (Fig. 92), and their structure is thus readily seen.
They are stratified, their layers representing in part the successive snowfalls of the interior of the country.
The upper layers are commonly white and free from stones; but the lower layers, to the height of a hundred feet or more, are dark with debris which is being slowly carried on.
So thickly studded with stones is the base of the ice that it is sometimes difficult to distinguish it from the rock waste which has been slowly dragged beneath the glacier or left about its edges.
The waste beneath and about the glacier is unsorted.
The stones are of many kinds, and numbers of them have been ground to flat faces.
Where the front of the ice has retreated the rock surface is seen to be planed and scored in places by the stones frozen fast in the sole of the glacier.
We have now found in glacier ice an agent able to produce the drift of North America.
The ice sheet of Greenland is now doing what we have seen was done in the recent past in our own land.
It is carrying for long distances rocks of many kinds gathered, we may infer, over a large extent of country.
It is laying down its load without assortment in unstratified deposits.
It grinds down and scores the rock over which it moves, and in the process many of the pebbles of its load are themselves also ground smooth and scratched.
Since this work can be done by no other agent, we must conclude that the northeastern part of our own continent was covered in the recent past by glacier ice, as Greenland is to-day.
VALLEY GLACIERS
The work of glacier ice can be most conveniently studied in the separate ice streams which creep down mountain valleys in many regions such as Alaska, the western mountains of the United States and Canada, the Himalayas, and the Alps.
As the glaciers of the Alps have been studied longer and more thoroughly than any others, we shall describe them in some detail as examples of valley glaciers in all parts of the world.
CONDITIONS OF GLACIER FORMATION.
The condition of the great accumulation of snow to which glaciers are due--that more or less of each winter's snow should be left over unmelted and unevaporated to the next--is fully met in the Alps.
There is abundant moisture brought by the winds from neighboring seas.
The currents of moist air driven up the mountain slopes are cooled by their own expansion as they rise, and the moisture which they contain is condensed at a temperature at or below 32 degrees F. , and therefore is precipitated in the form of snow.
The summers are cool and their heat does not suffice to completely melt the heavy snow of the preceding winter.
On the Alps the SNOW LINE--the lower limit of permanent snow--is drawn at about eight thousand five hundred feet above sea level.
Above the snow line on the slopes and crests, where these are not too steep, the snow lies the year round and gathers in valley heads to a depth of hundreds of feet.
This is but a small fraction of the thickness to which snow would be piled on the Alps were it not constantly being drained away.
Below the snow fields which mantle the heights the mountain valleys are occupied by glaciers which extend as much as a vertical mile below the snow line.
The presence in the midst of forests and meadows and cultivated fields of these tongues of ice, ever melting and yet from year to year losing none of their bulk, proves that their loss is made good in the only possible way.
They are fed by snow fields above, whose surplus of snow they drain away in the form of ice.
The presence of glaciers below the snow line is a clear proof that, rigid and motionless as they appear, glaciers really are in constant motion down valley.
THE NEVE FIELD.
The head of an Alpine valley occupied by a glacier is commonly a broad amphitheater deeply filled with snow.
Great peaks tower above it, and snowy slopes rise on either side on the flanks of mountain spurs.
From these heights fierce winds drift the snows into the amphitheater, and avalanches pour in their torrents of snow and waste.
The snow of the amphitheater is like that of drifts in late winter after many successive thaws and freezings.
It is made of hard grains and pellets and is called NEVE.
Beneath the surface of the neve field and at its outlet the granular neve has been compacted to a mass of porous crystalline ice.
Snow has been changed to neve, and neve to glacial ice, both by pressure, which drives the air from the interspaces of the snowflakes, and also by successive meltings and freezings, much as a snowball is packed in the warm hand and becomes frozen to a ball of ice.
THE BERGSCHRUND.
The neve is in slow motion.
It breaks itself loose from the thinner snows about it, too shallow to share its motion, and from the rock rim which surrounds it, forming a deep fissure called the bergschrund, sometimes a score and more feet wide.
SIZE OF GLACIERS.
The ice streams of the Alps vary in size according to the amount of precipitation and the area of the neve fields which they drain.
The largest of Alpine glaciers, the Aletsch, is nearly ten miles long and has an average width of about a mile.
The thickness of some of the glaciers of the Alps is as much as a thousand feet.
Giant glaciers more than twice the length of the longest in the Alps occur on the south slope of the Himalaya Mountains, which receive frequent precipitations of snow from moist winds from the Indian Ocean.
The best known of the many immense glaciers of Alaska, the Muir, has an area of about eight hundred square miles (Fig. 95).
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.
