GLACIER

INTRODUCTION

What is a glacier?

Glacier is a thick mass of ice that originates on land from the compaction and re-crystallization of snow and shows evidence of past or present movement. Since snow is the raw material that eventually produces glacial ice, glaciers must form in areas where more snow falls in winter than melts during the summer. A region that has such a net accumulation is termed a Snowfield and its outer limits are defined by the Snowline. The elevation of the snowline varies greatly. In the frigid polar realm, it may be sea level, while in tropical areas near the equator, the snowline exists only high in the mountains, often at elevations exceeding 4,500 meters. If the accumulation in the snow field is great enough, the pressure of overlying layers transforms the snow below into glacial ice.

Existence

The term ablation is used by glaciologists to include both evaporation and melting of snow and ice. thus, each year a layer of snow is added to what has already accumulated. As snow compacts, by surface melting and refreezing, it turns into a granular ice, then is compressed by overlying layers into hard crystalline ice. When the ice becomes so thick that the lower layers become plastic, outward or downhill flow commences, and an active glacier has come into being.

Movement

Glacier moves under the continuous pressure from the accumulated snow above. The rate of movement is greatest in the middle where there is little obstruction. The sides and the bottom are held back by friction with the valley side spurs and the valley floor. This is demonstrated by planting stakes across the glacier path, as they take a curved shape. (GOH 59)
Movements of different glaciers are:
In the Alps 1 m a day
In Greenland 15 m a day
In Antarctica a few millimeters a day
The glacier has differential movement due to different layers and composition. Veins and fissures are formed due to tension and compression. Melting proceeds due to the action of sunrays. Tiny streams are formed. Yawning gaps called Crevasses are formed due to which ice turns into a series of ridges, pinnacles and Seracs (stepwise like a cascade).
Fine dust causes melting and formation of dust wells. The water freezes at night forming ice needles, the collection of which is called Glacial Star. Sometimes rounded boulders come to rest on the glacier surface causing the formation of the Ice Pyramid.
The ice-sheets reaching the sea often extend into the polar waters and float as Ice Shelves. They terminate into precipitous Cliffs, when they break into individual blocks, these are called Icebergs. At the foot of the mountain ranges, several glaciers may convert to form an extensive ice mass called a Piedmont Glacier. The best known such glacier is the Malaspina Glacier of Alaska which is spread over 4,200 km2.
Load of the Glacier
1. Rock fragments from valley walls, blown by winds or brought by avalanches.
2. Material extracted from the floor.
3. Big boulders and fine particles.
Transportation
1. Near the bottom or dragged along
2. At the top
3. Embedded in layers.
Work of the Glacier
1. Difficulty in studying
2. 3 schools of thought regarding the work done by the glacier.
a. Protectionist School
By Hein (1885); glaciers are the protectors of the crust.
b. Erosionist School
By Hess (1904); glaciers are powerful agents of erosion and excavation.
c. Modern Thought
Stable ice does the work of protection while moving ice does erosion.

THE ICE AGE & TYPES OF GLACIER

Today, only two ice caps are present, in Greenland and in Antarctica. The former covers an area of 1,872,000 km2, while the later ins more than 13 million sq. km. They are made up of compact sheets of ice, hardened and crystallized to a depth of over a kilometer. A part form Antarctica and Greenland, glaciation is still evident on the highlands of many parts of the world, which lie above the snow line.
The glacier or ice masses of the world can be said to belong to two distinct classes:

1. Valley or Mountain Glaciers

These are also known as Alpine Glaciers and flow like tongues of ice through the mountain valleys. These glaciers are formed by the descending Neve or Firn (the water in an effort to percolate down through the glacier gets frozen again leading to the formation of granular ice which is known as Neve or Firn. It is the main source of nourishment for the glacier), and later on are also fed by direct snowfall, avalanches and drift of snow blown by the wind from higher levels. Its shape and form undergoes transformation according to the configuration of the valley. The mountain glacier are ordinarily not very extensive, their length ranging from 2 to 3 miles. But certain mountain glaciers are so vast that they go on flowing for a distance of 50 to 60 miles and appear like huge rivers of ice. The thickness of ice also varies from 10 ft. to tens of thousands feet.

2. Continental Glaciers or Ice Sheets

In the Greenland and Antarctica, all the precipitation is in the form of snow. The snow that falls from year to year goes on being accumulated because very little of it is wasted either by melting or evaporation (ablation). The result is that these regions are covered by an extensive ice-mass, a gigantic ice-dome that hides beneath it all the surface irregularities irrespective of its height or depth. This is known as ice-sheet or continental glacier. It is estimated that the thickness of the ice-sheets of Greenland varies from 2,000 to 7,000 feet while the average thickness of an ice-sheet of Antarctica is 4,000 feet. The ends of the ice sheets break into blocks due to action of sea waves and keep floating as Icebergs. The coastal regions are free from the covering of ice and mountain peaks stand out as Nunataks. In these glaciers streams flow beneath the ice mass. The upper ice layers move faster than the lower ones, this phenomenon is called Englacial Drift.
This type of glacial does not contain any debris so it does no erosional work.

Difference b/w Valley & Continental Glaciers
These two distinct classes of glaciers differ not only in their location but also in form, manner of modifying the relief, mode of nourishment, and nature of topography that results from their actions.

1. Form
Mountain or valley glaciers are contained within a rock basin or valley; while the continental glaciers are like a dome or sheet or shield, which covers beneath it all the irregularities of relief and configuration.

2. Manner of modifying the Relief
The mountain glaciers produce hollows and caves increasing the ascent of the relief. But the continental glaciers level down or iron out the irregularities of the surface over which they flow.

3. Mode of Nourishment
The mountain glaciers are nourished by Nivation, which consists of frost action resulting in snow niches (corners) that gradually dig themselves in and become the starting point of mountain glacier. The continental glacier, receive their nourishment from low level clouds, descending currents and the ice-needles descending from the clouds.

4. Resulting Topography
The regions visited by mountain glaciers show a concave relief or topography in which the curve is sharp towards upper end. The marks left by continental glaciers are, on the other hand, convex in shape and their curvature is slight. The mountain glaciers leave behind a topography predominantly marked with erosional features while the continental glaciers leave behind a predominance of depositional features.

GLACIAL EROSION

Introduction

Glaciers are capable of carrying-on great amounts of erosional work. The ice has scraped, scoured (polished), and torn rock debris from the floors and walls of the valleys, and carried them downslope.
Ways in which erosional work takes place
Glaciers primarily erode land in two different ways:

1. Plucking

As a glacier flows over fractured bedrock surface, it loosens and lifts blocks of rocks, incorporates them into flow and carries them off. This process, known as plucking, occurs when melted water penetrates cracks and joints along the rock floor of the glacier and refreezes. As the water expands, it exerts a tremendous leverage that pries (pushes) the rock loose. In this manner, sediments become part of the glacier’s load.

2. Abrasion

It is the second major erosion process. As the ice, with its load of rock fragments moves along, it acts as a giant rasp (wear) file and grinds the surface below as well as rocks within the ice. The pulverized rock produced by the glacial grist mill is appropriately called Rock Flour. When the embedded material consists of large fragments, long scars and grooves called Glacial Striations (texture) may gouge out. These linear scratches on the bedrock surface provide clues to the direction of glacial movement. On the other hand, all abrasive action produces striations. When the sediments consist primarily of fine silt particles, the rock surfaces over which glacier moves may become highly polished.

Difference b/w Continental & Alpine Glacial Erosion

The erosional effect of Alpine and Continental glaciers are quite different from each other. A visitor to an Alpine-Glaciated region is likely to see sharp and very angular topography. The reason is that as Alpine glaciers move down valley, they tend to accentuate the irregularities of mountain landscape by creating steeper canyon walls and making bold peaks even more jagged. By contrast, continental ice-sheets generally over-ride the terrain and hence tend to subdue rather than accentuate the irregularities they encounter. Although erosional accomplishment of continental glaciers can be tremendous, landforms carved by these ice-masses usually do not inspire the same degree of wonderment or awe as do the erosional features created by Alpine glaciers. In regions where the erosional effects of continental ice-sheets are significant, glacial scoured surfaces and subdued terrain are the rule. By contrast, in mountainous areas, erosion by Alpine glaciers yield many truly spectacular features. Much of the rugged mountain scenery so celebrated for its majestic beauty is the product of glacial erosion.

Erosional Landforms (Landforms of Highland Glaciation or Alpine)

1. Corrie, Cirque or Cwm

Cirques are bowl-shaped, steep sided depressions in the bedrock with gently slopping floors. It is formed by the downslope movement of the glacier from the snow covered valley head and the intensive shattering of the upland slopes. The depression so produced accumulates the Firn or Neve.
The process of plucking also steeps the back-wall and the movement of ice deepens the depression into a steep, Horse-Shoe-Shaped-Basin, called a Cirque (in French), Corrie (in Scotland) and cwm (in Wales).
There is rocky ridge at the exit of the corrie and when the ice eventually melts, water collects behind the barrier to from a Corrie Lake or Tarn. (GOH 61, Strahler 524)

2. Aretes and Pyramidal Peaks

When two corries cut back on opposite sides of a mountain, knife edged ridges are formed called aretes ( a French word). When the aretes are horizontally arranged they give rose to the Comb-Ridges.
When three or more cirques cut back together, their ultimate recession will form an Angular Horn or Pyramidal Peak. (GOH 61)

3. Bergschrund

At the heads of the glacier, where it begins to leave the snowfield of a corrie, a deep vertical crack opens up called a bergschrund (in German) or rimaye (in French).
This happens in summer when, although the ice continues to move out of the corrie, there is no new snow to replace it. In some cases, not one but several cracks occur. The bergschrund represents a major obstacle to the climbers. Further down where the glacier negotiates a bend or sudden slope, more Crevasses or cracks are formed. (GOH 61)
The above characteristic forms belong to upper slopes above the level of the glacier’s surface. Erosion here is accomplished by Sapping and Undermining.
In valleys and the lower slopes erosion is carried out by Plucking and Abrasion.

4. U-shaped Glacial Trough

The glacier as it moves down the narrow v-shaped valley, tends to iron out the irregularities of the side walls straighten them. It is fed by many corries-like tributaries that join a river. It wears away the sides and floor of the valley. It scratches and grinds the bedrock removing any rock debris and surface soil. The interlocking spurs are thus blunted to form Truncated Spurs and the floor of the valley is deepened.
The glaciated valley takes the characteristic U-shape with a wide, flat floor and very steep sides. After the disappearance of the ice, the deep sections of the glacial troughs may be filled with water forming ribbon lakes. (GOH 62)

5. Hanging Valleys

The main valley is eroded much more rapidly and to a greater extent than the smaller tributary valleys, as it contains a much larger glacier.
After the ice has melted, a tributary valley, therefore hangs above the main valley so that its stream plunges down as a water fall. Such hanging valleys may form a natural head of water for generating Hydroelectricity.
6. Rock Basins and Rock Steps (Bastions)
A glacier erodes and excavates the bedrock in an irregular manner. The unequal excavation give rise to many rock basins later filled by lakes in the valley trough.
Ribbon-like falls result from the hanging valleys. The steep walls over which these walls are formed are not quite smooth. The tributary glaciers have cut them into Steps due to the additional weight of ice at the point of convergence of the two valleys. A series of such steps may be formed due to different degrees of resistance to glacial erosion of the bedrock. This series is called Rock Bastion (battlement).

7. Moraines

The moraines are made up of the pieces of rock, boulders that are shattered by frost action, imbedded in the glaciers and brought down the valley.
The moraines are important from the point of view of the depositional work done by the glaciers. The glacial pavement bears the streaks and scratches and is smoothened into a rounded surface.

8. Fjords (body of water)

when the floor of a trough open to the sea lies below sea level, the sea water will enter as the ice front recedes, thus producing a narrow estuary known as a Fjord. Among the most spectacular landforms associated with glacial erosion are fjords. Fjords may originate either by submergence of the coast or by glacial erosion to a depth below sea level. These are deep, steep sided inlets of the sea that exist in many high latitude areas of the world where mountains are adjacent to the ocean. Norway, British Columbia, Greenland, New Zealand, Chile and Alaska all have coastlines characterized by fjords. They represent glacial troughs that were partially submerged as the ice left the valley and sea level rose following the Ice Age. The depth of fjords are often dramatic, in some instances exceeding 1,000 to 15,000 meters. The great depths of these flooded troughs is only partly explained by the post Ice Age rise in sea level. Fjords are observed to be opening up today along the Alaskan coast, where some glaciers are melting back rapidly and the fjord waters are extended along the troughs. (Strahler 529)

9. Horns

These are sharp, pyramid-like peaks. These are found by the enlargement of cirques produced by plucking and frost action. A group of cirques along the single high mountain create the spires of rock called horns. As the cirques enlarge and converge, an isolated horn is produced. The most famous example is the Matterhorn in the Swiss Alps.

10. Col

When on the opposite sides of a ridge, two cirques go on widening the boundary wall, the cutback action is able to reduce the intervening ridge. If the two cirques have been at the same level or height, they would come to become tangent to each other at some point. When this happens, the intervening rock wall gives way and a saddle-like formation comes into existence. This is called a col and is often used as a pass through the mountain ranges. The Canadian Pacific Railway passes through such a col.

11. Tarn

Small lake occupying a rock basin in a cirque or a glacial trough is called “Tarn”.

12. Trough Lakes

The major troughs usually contain large elongated trough lakes sometimes also called Finger Lakes. Finger Lakes are like tarns but bigger in scale.

(Landforms of Glaciated Lowlands)

1. Roche Moutonnees
Unlike the running water, glacier is not diverted from its path by slide barriers or obstacles. On the contrary, in motion tries to level down whatever comes in its way. The irregularities of the surface are ironed out so that they become rounded on the onward side (stoss side) while their leeward sides remain broken and irregular. Such rounded hummocks appear from a distance like sheep back in a flock and they are called Roche Moutonnees. These are asymmetrical ridges. They are supposed to be the residuals of the relief before the glaciers passed over them. Roche Moutonnees are most prominently developed where the ice action has been most powerful and continuous. Typical profiles of this type are seen in the passes of the Alps, in Southern Sweden and Finland, in the valleys of the Adirondacks and also on the borders of the Ice-sheet in Greenland. (Strahler 534)

2. Crag and Tail
The crag is a mass of a hard rock with a precipitous slope on the upstream side which protects the soft-ward leeward side slope from completely worn down by the oncoming ice. It therefore has a gentle tail, strewn with the eroded rock debris. For example; Castle rock of Edinburgh, Scotland.

Glacial Deposition

Glaciers are capable of acquiring and transporting a huge load of debris as they slowly yet steadily advance across the land. Ultimately these materials must be deposited when the ice melts. In regions where glacial sediments are deposited, the sediments can play a truly significant role in forming the physical landscape. For example in many areas once covered by continuous ice-sheets of the recent ice age, the bedrock rarely exposed because glacier deposits of tens or even hundreds of meters thick completely mantle (blanket) the terrain. The generation of these deposits is to reduce the local relief and thus level the topography. Indeed much of the country’s scenery results directly from glacial deposits.

Types of Glacial Deposits

The term glacial drift has long been applied to include all varieties of rock debris deposited in close association with glaciers. Drift is of two major types:

1. Till
This is an unsorted glacial deposit or a heterogeneous mixture comprising of a variety of eroded materials; boulders, angular stones, sticky clay and fine rock flour. It is spread out in sheets, not mounds and forms gently undulating till or drift plains. It is deposited directly form the ice without water transport. The landform is rather monotonous and featureless. The degree of fertility of such glacial plains depends very much on the composition of the depositional materials. Such plains are found in East Anglia, and the north mid-west of USA (arable lands).
Moraines of Valley Glaciers are composed largely of till, whereas the Valley Train (deposit of alluvium extending down valley from a melting glacier is the valley train) is composed of stratified drift or out wash.

2. Out wash or Stratified Drift
Materials deposited by glacial melt-water are called out wash. Out wash is sorted according to the size and weight of the fragments. Since ice is not capable of sorting activity, these sediments are not deposited directly by the glacier as the till is, but rather they reflect the sorting action of the glacial melt-water that was responsible for dropping them. Accumulations of out wash often consist largely of sand and gravel, i.e., bad load material, because the finer rock flour remains suspended and is commonly carried far from the glacier by the melt-water streams. An indication that out wash consists primarily of sand and gravel can be seen in many areas where these deposits are mined as aggregate for road works and other construction projects.

Depositional Landforms of Alpine Glaciers

1. Moraines

Perhaps the most wide spread feature created by glacial deposition are moraines, which are simply layers or ridges of till. Several types of moraines are identifiable; some are common only to mountain valleys, and others are associated with areas affected by either continental or Alpine glaciers. Lateral and Medial moraines fall in the first category, while End moraines and Ground moraines in the second.

a. Lateral Moraines
The sides of an Alpine glacier accumulate large quantities of debris from the valley walls. When the glacier wastes away, these, these materials are left as ridges, called Lateral Moraines, along the sides of a valley. (Strahler 525)

b. Medial Moraines
These are formed when two alpine glaciers or ice streams coalesce to form ice streams. This medial moraine rides upon the ice in mid stream. The till that was once all along the edges of each glacier joins to single dark stripe of debris within the enlarged glacier. The creation of the stripes within the ice stream is one obvious proof that glacial ice moves, because Medial moraine could not form if they did not flow down the valley. (Strahler 525)

c. End Moraine or Terminal Moraine
As the name implies, End Moraines form at the terminus of a glacier. At the terminus of a glacier debris accumulates in a heap. This heap is usually in the form of curved embankment lying across the valley floor and bending up-valley along each wall of the trough to merge with the lateral moraines, creating a ridge of till hundreds of meters high. As the end of the glacier wastes back, scattered debris is left behind. Successive halts in ice-retreat produce successive moraines, termed recessional moraines. (Strahler 525, 527)

d. Ground Moraine
As the glacier recedes layer of till is laid down, forming a gentle undulating surface of Ground Moraine. This cover is often inconspicuous because it forms no prominent or recognizable topographic feature. Nevertheless, the ground moraine may be thick and may obscure or entirely bury the hills and valleys that existed before glaciation. Where thick and smoothly spread, the ground moraine forms an extensive, level till plain, but this condition is likely only in regions already fairly flat to start with.

e. Interlobate Moraine
Moraine formed between two adjacent lobes of an ice sheet.
Depositional Landforms of Continental Glaciers

1. Out wash Plains
At the same time the end moraine is forming, water from the glacier cascades over the till, sweeping it out infront of the growing ridge as debris. Melt water generally emerges ice in rapidly moving streams that are chalked with suspended material and is substantial bed local as well. As the water moves the glacier, it moves onto the relatively flat surface beyond and rapidly loses velocity. As a consequence, much of its bed load is dropped and the melt-water begins weaving a complex pattern of braided channels. In this way a broad, ramp-like surface called an out wash plain is built adjacent to the downstream edge of most end moraines. The deposits are in reality great alluvial fans upon which are spread layer upon layer of sands and gravel. The term glaciofluvial is often applied to stream laid stratified drift.

2. Kettles
Often out wash plains are pockmarked with basins or depressions known as kettles. Kettles also occur in deposits of till. Kettles form when a block of stagnant ice becomes wholly or partially buried in drift and ultimately melts, leaving a pit in glacial sediment. Likewise typical depth of most kettles is kettles is less than 10 m, although the vertical dimensions of some approach 50 m. In many cases, water eventually fills the depression and forms a pond or lake.

3. Drumlins
These are swarms (crowd) of oval, elongated whale back hammocks composed wholly of boulder clay, with their elongation in the direction of the ice flow, i.e., on the downstream side. They are low hills varying from a few meters to 120 m in heights and may be a kilometer or two long. They appear a little steeper at the onset side and taper off at the leeward end. They are arranged diagonally and so are commonly described as having a basket of eggs topography. Large numbers of them are found in County Down in Northern Ireland and the glaciated plain around the Great Lakes in North America. It consists of glacial till. Drumlins invariably lie in a zone behind the terminal or recessional moraines. Drumlins are formed under moving ice, by a kind of plastering action in which layer upon layer of bouldery clay is spread upon the drumlin. This would have been possible only if the ice were so heavily choked with debris that the excess had to be left behind.(GOH 64).

4. Esker
These are long, narrow, sinuous ridges composed of sand and gravel which mark the former sides of sub-glacial melt-water streams (ice tunnel). They vary from a few meters to 60 m in height and may be several kilometers long.
As the sand and gravel is highly porous, water is rapidly drained off from their crests, so they do not support trees except in Finland.
Examples are found in Maine, USA (length = 160 km) and in Scandinavia e.g., Punkaharju Esker of Finland. The Eskers may take various shapes or get arranged in groups according to the pattern of glacier withdrawal. (Strahler 535,537)

5. Kame
These are a number of hillocks and ridges of sand and gravel. They are not terminal moraines. They may be in the form of alternating ridges and depressions (lakes).
They are deposited by sub-glacial streams and are arranged asymmetrically in the direction of the ice-front. Due to the presence of kettle lakes, the topography is called “Knob and Kettle“ Topography.