Chapter 3 — 1.3 Works of a Glacier

A glacier is a huge mass of ice, formed from the accumulation and compaction of snow over many years, which moves slowly downhill or outward under the pull of gravity and the pressure of its own weight. The word comes from the French glace, meaning ice. Glaciers are the largest store of fresh water on earth; about ten per cent of the land surface is covered by ice today, and during the last Ice Age, which ended about ten thousand years ago, ice covered nearly a third of it, reaching as far south as London, Berlin and New York.

Glaciers form only where more snow falls in winter than melts in summer, so that the surplus accumulates. The line above which snow lies permanently on the ground throughout the year is the snowline. Its height depends chiefly on latitude and on the amount of snowfall. At the poles the snowline is at sea level. In the Alps it lies at about 2,700 m. In the Himalaya it is higher, at about 4,500 m on the wetter eastern side in Sikkim and Darjeeling, where the heavy monsoon snowfall lowers it, and at about 5,000 to 6,000 m on the drier western side in Kashmir and Ladakh. Kanchenjunga, at 8,586 m, thus carries some four thousand metres of permanent snow and ice. The snowline is also lower on the shaded northern slopes than on the sunny southern slopes of a range in the northern hemisphere.

Above the snowline the fresh snow, which is light and full of air, is gradually compressed by the weight of the layers above it. The flakes lose their points, the air is squeezed out and the snow turns into granular ice called firn or névé. With further pressure over years the firn becomes solid, bluish glacier ice, and when this ice is thick enough, usually more than about 50 m, it begins to move. The area above the snowline where ice accumulates is the zone of accumulation; the area below it, where the ice melts, evaporates and breaks off faster than it is supplied, is the zone of ablation. The lower end of a glacier, where melting finally balances supply, is its snout, from which a milky meltwater stream usually issues.

If accumulation exceeds ablation the glacier grows and advances; if ablation exceeds accumulation the glacier shrinks and its snout retreats up the valley. Most Himalayan glaciers are retreating at present because of global warming; the Gangotri glacier has been retreating at about 20 m a year. This matters because the glaciers feed the Ganga, Indus and Brahmaputra during the dry months when there is no rain.

Glaciers are classified by their size and the kind of ground they occupy into three main types.

Continental glaciers or ice sheets are enormous, thick masses of ice that cover whole continents or large islands, burying all but the highest peaks and spreading outward in all directions from a central dome. The Antarctic ice sheet covers about 140 lakh square kilometres and is up to 4,000 m thick; it holds about 90 per cent of the world's ice. The Greenland ice sheet covers about 17 lakh square kilometres and is up to 3,000 m thick. During the Pleistocene Ice Age similar sheets covered Canada, the northern United States, Scandinavia and northern Russia, and the landforms they left are still visible there. Where an ice sheet reaches the sea it spreads out as a floating ice shelf, from which huge flat-topped icebergs break off. A smaller version of an ice sheet, covering a mountain plateau or a small island, is an ice cap, as in Iceland.

Valley glaciers or alpine glaciers or mountain glaciers are rivers of ice that occupy pre-existing river valleys in high mountains and flow down them from a snowfield at the head. They are much smaller than ice sheets, a few kilometres to a few tens of kilometres long, and their movement is guided by the valley walls. They are named after the Alps, where they were first studied, but the largest outside the polar regions are in the Himalaya and Karakoram. The Siachen glacier in the Karakoram, about 76 km long, is the longest in India and the second longest outside the polar regions; the Baltoro (about 60 km) and Biafo (about 63 km) are also in the Karakoram; the Gangotri (about 30 km) in Uttarakhand, and the Zemu (about 26 km) in Sikkim are other well-known examples. A valley glacier fed by several tributary glaciers from side valleys is called a compound or trunk glacier.

Piedmont glaciers form where several valley glaciers emerge from their mountain valleys on to a plain or lowland at the foot of the mountains and spread out and merge into a single broad, fan-shaped sheet of ice. The word piedmont means foot of the mountain. The Malaspina glacier in Alaska, about 65 km wide, is the classic example. Piedmont glaciers are intermediate in size between valley glaciers and ice sheets.

Other named forms include cirque glaciers, small masses of ice occupying armchair-shaped hollows high on a mountain, hanging glaciers clinging to steep faces, and tidewater glaciers that end in the sea. In the Himalaya the glaciers are all of the valley type, often covered with a thick layer of rock debris that insulates the ice and gives them a dirty grey appearance.

Although ice is a solid, a thick mass of it behaves like a very stiff fluid and flows slowly under its own weight. The movement of a glacier was first proved in the Alps in the nineteenth century by driving a line of stakes across a glacier and observing, over months, that the line bowed downstream, with the stakes in the middle moving fastest.

A glacier moves in two ways. Basal sliding is the sliding of the whole ice mass over its bed on a thin film of meltwater, produced by the pressure of the ice and by friction; this is the chief mode of movement in temperate glaciers such as those of the Himalaya and Alps, whose base is at melting point. Internal deformation or plastic flow is the movement of ice crystals past one another within the glacier, so that the ice deforms and creeps downhill like a slow-flowing paste; this is the only mode in polar glaciers whose base is frozen to the bed. The upper 50 m or so of a glacier is brittle and cannot flow; it is carried along on the plastic ice beneath and cracks into deep fissures called crevasses wherever the glacier passes over a steep step in its bed or round a bend. A crevasse may be tens of metres deep and, hidden under snow, is the chief danger to mountaineers. Where the glacier drops over a very steep slope it breaks into a chaotic mass of ice pinnacles called an ice fall; the Khumbu ice fall below Mount Everest is famous.

The rate of movement of a glacier is very slow compared with a river, usually between a few centimetres and a few metres a day. Valley glaciers in the Himalaya move about 10 to 30 cm a day; some Alaskan glaciers move several metres a day; the fastest, in Greenland, exceed 20 m a day. The speed depends on the thickness of the ice (thicker ice exerts more pressure and flows faster), the steepness of the slope, the temperature (warmer ice flows faster) and the amount of meltwater at the base. Movement is fastest in the centre and at the surface, where friction with the valley walls and the bed is least, and slowest along the sides and bottom, exactly as in a river. It is faster in summer than in winter.

The movement of a glacier is the source of its power to erode. A glacier 300 m thick presses on its bed with a weight of about 27 tonnes per square metre, and the rock fragments frozen into its base act as the teeth of a file. Moreover, unlike a river, a glacier fills its entire valley from wall to wall and floor to a great height, so its erosion acts on the whole cross-section at once. This is why glaciers deepen and widen valleys far more dramatically than rivers.

A glacier erodes the rock over which it moves by two main processes, which together make it the most powerful of all the agents of erosion in the mountains.

Plucking or quarrying is the tearing away of blocks of rock from the bed and walls of the valley. Meltwater at the base of the glacier seeps into the joints and cracks of the bedrock and freezes there, so that the rock becomes frozen into the base of the ice. As the glacier moves on, it pulls the loosened blocks out of the bed and carries them away. Plucking is most effective on well-jointed rocks and on the downstream side of rock knobs, where the pressure of the ice is lower and freezing is easier; it produces the rough, broken, step-like surfaces seen on the lee side of a roche moutonnée and the steep back wall of a cirque. The blocks plucked out become the tools for the second process.

Abrasion is the wearing down of the bed and walls by the rock fragments frozen into the base and sides of the glacier. Pressed against the bedrock by the enormous weight of the ice and dragged along, the fragments scratch, scrape and polish the rock. Large angular fragments cut long parallel grooves and scratches called striations, which show the direction of ice movement and remain as evidence long after the ice has gone. Finer fragments smooth and polish the surface. In the process the rock is ground into a fine powder called rock flour, which gives glacial meltwater streams their milky, grey-green colour and is the reason why the Teesta at Sevoke and the Bhagirathi at Gangotri look cloudy. Abrasion is greatest where the ice is thickest and moving fastest, and it is the process that deepens and smooths the floor of a glacial valley.

Frost action, though strictly a form of weathering, works closely with glacial erosion. On the rock walls above the glacier, water freezes and thaws in cracks and shatters the rock, and the fragments fall on to the ice as lateral moraine; at the head of the glacier this frost shattering cuts back the walls of the cirque. A gap between the ice and the head wall, called the bergschrund, allows meltwater to reach the rock and freeze there.

The results of glacial erosion differ from those of river erosion in three ways. First, a glacier erodes the whole width and a great height of its valley at once, whereas a river erodes only a narrow channel. Second, a glacier can erode below the level of its outlet, scooping out rock basins that later hold lakes, whereas a river cannot cut below its base level. Third, a glacier straightens its valley by cutting off spurs, whereas a river winds round them. These differences explain the characteristic glacial landforms described in the next topics.

The erosional landforms of a glacier begin at the very top of the mountain, in the hollows where snow first collects.

A cirque is a deep, armchair-shaped or amphitheatre-shaped hollow with a steep, semicircular back wall and a flat or basin-shaped floor, cut into the side of a mountain at the head of a glacial valley. It is called a corrie in Scotland and a cwm in Wales. A cirque begins as a small hollow on a shaded slope in which snow collects and lasts through the summer. Freeze-thaw around the edges of the snow patch shatters the rock and enlarges the hollow, a process called nivation. As the snow deepens into a small glacier, the ice rotates in the hollow, plucking rock from the back wall and abrading the floor, so that the hollow becomes deeper and its wall steeper. The cirque floor is often scooped out below the level of its lip, and when the ice melts a small circular lake called a tarn or cirque lake occupies the basin. Cirques are typically a few hundred metres to a kilometre or two across, with back walls hundreds of metres high. They are common on the northern and eastern flanks of the Himalayan peaks, and the lakes of the Sikkim Himalaya such as Gurudongmar and the tarns of the Alps occupy old cirques.

When two cirques develop on opposite sides of a ridge and cut back towards each other, the ridge between them is narrowed by plucking and frost shattering into a sharp, knife-edged, jagged crest called an arête, from the French word for a fish bone. Arêtes are the sharp, serrated ridges that make the skyline of high mountains look like the teeth of a saw; they are dangerous to climb. When the two cirques finally break through the ridge, a gap called a col is formed, which often serves as a pass.

When three or more cirques cut back into a mountain from different sides, the summit left between them is reduced to a steep-sided, sharp-pointed pyramid called a pyramidal peak or horn. The Matterhorn (4,478 m) on the Swiss–Italian border is the classic example, with four cirques and four arêtes meeting at its point; in India the summits of Nanda Devi, Kamet and Kanchenjunga, and Mount Everest itself, are pyramidal peaks carved by cirques on their several faces. The sharp, angular scenery of the high Himalaya, so different from the rounded hills of the Deccan, is thus entirely the work of ice.

Cirque, arête and horn are the landforms of the zone of accumulation, above the snowline; they should be drawn together, since they form together.

When a glacier occupies a river valley in the mountains it completely remodels it, and the changes are so distinctive that a glaciated valley can be recognised at a glance long after the ice has gone.

A river cuts a V-shaped valley with a narrow floor and sloping sides that wind round the interlocking spurs. A glacier, filling the valley from wall to wall to a depth of hundreds of metres, erodes the floor and both walls at the same time by plucking and abrasion. It deepens the floor into a flat or gently rounded trough, and it steepens the sides into nearly vertical cliffs. The result is a U-shaped valley or glacial trough, whose cross-section has a broad, flat floor and steep walls, quite unlike the V of the river. The trough is also straightened, because the ice cannot flow round the bends as easily as water and cuts through the spurs. Where the glacier scoured deeper at places of thicker ice or softer rock, rock basins were formed which now hold long, narrow ribbon lakes, such as the Lake District lakes of England and the lakes of the Alps. The Yosemite valley of California and the Lachen and Lachung valleys of north Sikkim are examples of glacial troughs; the upper valleys of the Alaknanda and Bhagirathi in Uttarakhand also show the U shape above their present glaciers.

The spurs that once projected into the river valley from both sides are cut off by the passing ice, leaving steep triangular faces of rock on the valley walls called truncated spurs. Their presence on both sides of a straight valley is a sure sign of past glaciation.

A main valley glacier is thick and heavy and erodes its floor deeply. The small tributary glaciers that join it from side valleys are thin and erode their floors much less. When the ice melts, the floor of each tributary valley is left high above the floor of the main valley, and the tributary valley appears to hang on the wall of the trough. This is a hanging valley. The stream that now occupies the hanging valley plunges into the main valley as a waterfall; the Bridalveil Fall in Yosemite and many falls of the Alps and the Himalaya are of this kind. Hanging valleys are thus a product of the difference between the erosive power of the trunk glacier and its tributaries.

Where the valley floor is stepped, with a series of flat treads and steep risers, it is called a glacial stairway; each step marks a place where the ice was thicker or the rock more resistant. After the ice melts the river that reoccupies the trough is far too small for its huge valley and is called a misfit stream. The Himalayan valleys of Lahaul, Spiti and north Sikkim show these features on a grand scale.

Three more erosional features are regularly asked in the examination.

A roche moutonnée is a knob or hummock of hard bedrock on the floor of a glaciated valley, shaped by the ice into a form with two distinct sides. The side facing the direction from which the ice came, the stoss or upstream side, is smooth, gently sloping, polished and striated, because the ice pressed hard against it and abraded it. The side facing downstream, the lee side, is steep, rough and jagged, because there the pressure of the ice was released, meltwater froze in the joints and the ice plucked blocks away. The name, given by the Swiss geologist de Saussure in the eighteenth century, means sheep-like rock; a group of them seen from a distance looks like a flock of sheep lying down, or like the wavy wigs of the period that were smoothed with mutton fat. Roches moutonnées range from a few metres to hundreds of metres in length and are found throughout the glaciated valleys of the Alps, Scandinavia and the Himalaya. They are valuable because their shape tells the direction in which the vanished ice moved.

A crag and tail forms where a mass of very hard rock, such as a volcanic plug, stood in the path of the ice. The ice was forced to pass round the crag, which protected the softer rock on its lee side from erosion. The result is a steep-faced crag on the upstream side with a long, gently sloping tail of protected rock and deposited material stretching away downstream. The Castle Rock of Edinburgh, with the Royal Mile running down its tail, is the classic example.

A fjord is a long, narrow, deep inlet of the sea with steep, often vertical, walls, formed when a glacial trough that reached the coast was drowned by the sea after the ice melted and the sea level rose. Because a glacier can erode below sea level, the floor of a fjord is often very deep, more than 1,000 m in Norway's Sognefjord, which is about 200 km long, yet near the mouth there is a shallow sill or threshold where the ice was thinner and erosion less; this sill separates the deep water of the fjord from the open sea. Fjords occur on the coasts of Norway, Greenland, Alaska, British Columbia, southern Chile, New Zealand and Scotland, that is, on mountainous coasts in high latitudes that were glaciated. They make magnificent natural harbours and are among the most beautiful scenery in the world. India has no fjords, because its glaciated mountains are far from the sea.

Together with the cirque, arête, horn, U-shaped valley and hanging valley, these complete the list of erosional landforms that the examination expects the student to define, illustrate and, where possible, draw.

A glacier transports an enormous load of rock material, and it does so in a way quite different from a river. All the rock debris carried by a glacier, whatever its size and wherever it lies in the ice, is called moraine; the same word is used for the deposits made of it after the ice melts. The load comes from two sources: the rock plucked and abraded from the bed and walls of the valley by the ice itself, and the rock shattered by frost from the cliffs above the glacier and dropped on to its surface.

A river carries its load in different ways according to size, and sorts it as it goes. A glacier, being solid, carries everything together, from fine rock flour to boulders as large as a house, without sorting and without rounding, because the fragments do not roll or strike one another. Hence glacial deposits are unsorted and angular, which is how they are told apart from river deposits. Moreover a river deposits whenever its velocity falls, but a glacier deposits only when it melts, so the load may be carried for hundreds of kilometres from its source and set down far away.

Moraine in transit is named by its position in the glacier:

• Lateral moraine is the debris carried along the two edges of a valley glacier, derived mainly from frost-shattered rock falling from the valley walls. It forms two dark stripes along the sides of the ice.
• Medial moraine forms where two glaciers join; the inner lateral moraines of the two unite into a single dark stripe running down the middle of the combined glacier. A glacier with several tributaries may show several medial moraines side by side.
• Englacial moraine is debris carried inside the ice, having fallen into crevasses or been buried by snow.
• Subglacial or ground moraine is the debris carried at the base of the glacier, produced by plucking and abrasion, and dragged along between the ice and the bed.
• Terminal or end moraine is the debris pushed along at the snout of the glacier, like earth before a bulldozer.

Himalayan glaciers are so heavily loaded with surface debris that in their lower parts the ice is completely hidden under a blanket of rock several metres thick; the Gangotri, Zemu and Siachen glaciers look like rivers of rubble rather than of ice. This debris cover insulates the ice from the sun and slows its melting.

When the glacier melts, all this moraine is dropped where it lies, and the transported material becomes the depositional landforms of the next topic. The unsorted mixture of clay, sand, pebbles and boulders left directly by the ice is called till or boulder clay; material carried away and re-deposited by meltwater streams, which sort it, is called fluvio-glacial or outwash material.

When a glacier melts, the moraine it carried is left behind as ridges and sheets of unsorted till. These are the depositional landforms of the glacier itself, as distinct from those made by its meltwater.

A terminal moraine or end moraine is a crescent-shaped ridge of till stretching across the valley at the farthest point reached by the snout of the glacier. It is built partly by the ice pushing debris ahead of it and partly by material carried to the snout and dumped there as the ice melts, over the years during which the snout stayed in one place. It may be tens of metres high and, in the case of ice sheets, hundreds of kilometres long; the terminal moraines of the Pleistocene ice sheet form long hills across northern Germany, Poland and the northern United States, and Long Island near New York is a terminal moraine. In the Himalaya, terminal moraines mark the former extent of glaciers well below their present snouts; at Gangotri the moraines show that the glacier once reached several kilometres farther down the valley. A terminal moraine may dam the valley, and the meltwater then collects behind it as a moraine-dammed lake. Such lakes in the Himalaya are dangerous, because the moraine dam can burst and release a sudden flood, a glacial lake outburst flood, as happened at Kedarnath in 2013 and in Sikkim in 2023.

Recessional moraines are a series of smaller ridges behind the terminal moraine, each marking a place where the snout paused for some years during its retreat. They record the history of the glacier's shrinking.

Lateral moraines, after the ice has gone, remain as long, low ridges of debris along both sides of the valley, marking the former height of the ice surface; in the Himalaya they form embankments tens of metres high above the present glaciers, showing how much the ice has thinned.

Medial moraines are seldom preserved as ridges, because they are spread over the valley floor as the ice melts.

Ground moraine is the sheet of till, often many metres thick, spread over the whole floor of the valley or over a vast lowland by a melting ice sheet. It forms a gently rolling, poorly drained surface with many lakes and hollows, characteristic of the till plains of Canada, the northern United States, Finland and the north German plain. The clay-rich till is hard to plough but often fertile.

Moraines differ from river deposits in being unsorted, unstratified and made of angular fragments of every size from clay to boulders; a spoonful of till may contain a boulder and rock flour side by side. This is the surest way to distinguish a glacial deposit from an alluvial one in the field and in the examination.

Two landforms are made directly by moving ice from the till it carries beneath it.

A drumlin is a smooth, elongated, oval or egg-shaped hill of till, shaped like the back of a whale or an inverted spoon, formed under a moving ice sheet. Its long axis lies parallel to the direction of ice movement; its upstream or stoss end is steeper, higher and blunter, and its downstream or lee end is longer, lower and gently tapering, so that the whole form points the way the ice went. Drumlins are typically a few hundred metres to a kilometre or two long, 100 to 500 m wide and 15 to 50 m high. They are believed to form when the ice, overloaded with debris at its base, plasters the till around some obstacle or simply moulds the till beneath it into streamlined shapes as it flows over it. They rarely occur singly; hundreds of them together make a drumlin field or, from its appearance, a basket-of-eggs topography. The classic fields are in Ireland (County Down), New York State, Wisconsin and Finland. The word comes from the Irish droimnín, a little ridge. Drumlins are found on lowlands once covered by ice sheets rather than in valley-glacier country, so India has none of note; they should be contrasted with the roche moutonnée, which is bedrock with its steep face downstream, whereas a drumlin is till with its steep face upstream.

An erratic or erratic boulder is a large block of rock, sometimes weighing hundreds or thousands of tonnes, carried by a glacier far from its place of origin and dropped where the ice melted, so that it now rests on bedrock of an entirely different kind. The word means wanderer. Erratics are unmistakable because their rock type does not match the local rock; blocks of Norwegian granite lie on the chalk of eastern England and on the plains of Germany, blocks of Canadian Shield granite lie in the American Midwest, and blocks of Alpine rock lie on the Jura mountains. Geologists trace erratics back to their source to map the direction and extent of vanished ice sheets. When an erratic is left balanced on a smaller rock or a narrow base it is called a perched block. In the Himalaya, huge erratics of gneiss and granite lie in the valleys of Lahaul, Ladakh and north Sikkim well below the present glaciers, proving that the ice once reached far lower. The Big Rock near Okotoks in Alberta, Canada, about 16,500 tonnes, is one of the largest known erratics.

Both drumlins and erratics are landforms of deposition by ice itself, and both serve as compasses pointing to the former direction of ice flow: the drumlin by its shape and the erratic by its source.

A melting glacier gives off enormous quantities of water, which flows in streams over, through and beneath the ice and out from the snout. These meltwater streams pick up the till, wash away the clay, sort the rest by size and deposit it in layers. The resulting landforms are called fluvio-glacial or glacio-fluvial, because they are the joint work of ice and running water. Unlike till, their material is sorted and stratified, and unlike moraine it is rounded by the water.

An outwash plain or sandur (an Icelandic word) is a broad, flat, gently sloping plain of sand and gravel spread out beyond the terminal moraine by the braided meltwater streams issuing from the snout. The coarse gravel is dropped nearest the ice and the finer sand farther out, so the plain is sorted from coarse to fine away from the glacier. Outwash plains in front of the Pleistocene ice sheets cover large areas of the north German plain, Poland, Iceland and the northern United States, and their sandy soils support heathland and pine forest rather than good farmland. Hollows in the outwash left by the melting of buried blocks of ice form small round lakes called kettle holes or kettle lakes, which dot the plains of Minnesota and Wisconsin.

An esker is a long, narrow, winding ridge of sorted sand and gravel, from a few metres to tens of metres high and up to hundreds of kilometres long, that snakes across the country like a raised railway embankment. It is the bed of a stream that flowed in a tunnel beneath or within the ice; when the ice melted, the deposits of the tunnel were left standing as a ridge. Eskers run roughly parallel to the direction of ice movement and are common in Finland, Sweden, Ireland and Canada, where they are quarried for gravel and used as natural roads across marshy country.

A kame is a small, irregular, steep-sided mound or hummock of stratified sand and gravel, deposited by meltwater in a hollow or crevasse on the surface of a stagnant glacier and let down on to the ground as the ice melted. A kame terrace is a flat-topped bench of the same material deposited by a stream flowing along the edge of a glacier between the ice and the valley wall, and left as a shelf on the valley side when the ice melted. Kames often occur together with kettle holes in a jumbled landscape called kame-and-kettle topography.

Varves are thin paired layers of sediment, a coarser light layer from summer and a finer dark layer from winter, laid down in lakes fed by glacial meltwater; by counting varves geologists have dated the retreat of the ice year by year.

In the Himalaya, outwash gravel fills the valley floors below the snouts, and the Bhabar belt at the foot of the mountains is partly built of gravel carried down by meltwater-fed rivers.

Because both are agents of exogenetic forces that erode, transport and deposit, and because examination questions often ask for a comparison, the work of a glacier and a river should be set side by side.

The most important single difference is that ice is solid. Because it is solid, it moves slowly, it cannot flow round obstacles easily and so straightens its valley, it carries everything without sorting, it erodes the whole valley at once rather than a channel, and it deposits only when it melts. Because water is liquid, it flows fast and around every spur, sorts its load by size and drops it wherever it slows down. From these physical facts every other difference follows.

The two agents also work together. Most Himalayan rivers begin as meltwater from a glacier snout: the Ganga at Gaumukh, the Teesta at the Zemu glacier, the Indus and Brahmaputra from the glaciers of Tibet. The river then reoccupies the glacial trough below the snout as a misfit stream, carries the outwash gravel down to the plains, and builds it into the fans of the Bhabar and the Duars. Rock flour from glacial abrasion gives these rivers their colour and a part of the fertility of the plains. During the Pleistocene, when the glaciers extended much farther down, the Himalayan rivers were vastly larger than today and laid down much of the alluvium of the Ganga plain. The story of the rivers of Bengal therefore begins with the story of the glaciers.

The Himalaya and Karakoram hold the largest area of glacier ice outside the polar regions, sometimes called the Third Pole; India alone has close to ten thousand glaciers, covering some 37,000 km² in the Indus, Ganga and Brahmaputra basins.

The important glaciers of India are: the Siachen in the Karakoram of Ladakh, about 76 km long, the longest in India and the site of the highest battlefield in the world; the Baltoro and Biafo in the Karakoram; the Bara Shigri in Lahaul, about 28 km, the longest in Himachal Pradesh; the Gangotri in Uttarakhand, about 30 km, whose snout Gaumukh is the source of the Bhagirathi; the Satopanth and Bhagirath Kharak that feed the Alaknanda; the Milam and Pindari in Kumaon; and the Zemu in Sikkim, about 26 km, on the east face of Kanchenjunga, the source of the Teesta, which flows through West Bengal. Many of the lakes of north Sikkim, such as Gurudongmar and Cholamu, lie in glacially scoured basins. These glaciers are all of the valley type, heavily covered with debris in their lower parts, and they show every erosional landform of this chapter: cirques on the peaks, arêtes on the ridges, pyramidal peaks such as Kanchenjunga, U-shaped troughs, hanging valleys, roches moutonnées, moraine ridges and outwash gravel.

The glaciers are the water towers of north India. Rain falls only in the monsoon, but the glaciers melt through the hot dry months of April, May and June, when the rivers would otherwise run nearly dry, and they keep the Ganga, Indus and Brahmaputra flowing all the year. Hydroelectric plants, canals and the drinking water of hundreds of millions of people depend on this melt.

Nearly all Himalayan glaciers are now retreating because of global warming. The Gangotri glacier has retreated by about three kilometres in the last two centuries and its snout moves back about 20 m a year; the Pindari, Milam and Zemu are all shrinking, and the glaciers are also thinning, as the old lateral moraines standing high above the ice show. The consequences are serious. In the short term, faster melting increases the river flow and swells the moraine-dammed lakes at the snouts, which can burst and cause glacial lake outburst floods; the South Lhonak lake outburst in Sikkim in October 2023 destroyed the Teesta III dam and caused heavy damage down the valley into Jalpaiguri. In the long term, as the glaciers shrink, the dry-season flow of the rivers will fall, threatening irrigation and water supply in the plains. Black carbon from burning fuel settling on the ice darkens it and speeds the melting.

The study of the work of glaciers is thus not only a matter of scenery but of the future water security of West Bengal and India, which is why the Madhyamik syllabus asks students to understand it.

The work of glaciers is examined in the same forms as the work of rivers, with a strong emphasis on diagrams, because nearly every glacial landform has a distinctive shape.

One-mark questions ask for terms and facts: the longest glacier of India (Siachen), the source of the Teesta (Zemu glacier), the armchair-shaped hollow at a valley head (cirque), the sharp ridge between two cirques (arête), the peak carved by several cirques (pyramidal peak or horn), the valley formed by a glacier (U-shaped), the drowned glacial valley on the coast (fjord), the egg-shaped hill of till (drumlin), the winding ridge of sand and gravel (esker), the boulder carried far from its source (erratic), the classic piedmont glacier (Malaspina), the lake in a cirque (tarn). Match-the-column items pair landforms with agents, so the student must be sure which forms belong to ice and which to water or wind.

Two-mark questions ask for definitions: What is a snowline? What is meant by moraine? What is a hanging valley? What is a roche moutonnée? Give the definition and one example.

Three-mark questions ask for the formation of a landform with a diagram, or a difference: How is a cirque formed? How does a hanging valley form? Distinguish between a drumlin and a roche moutonnée. Distinguish between till and outwash.

Five-mark questions ask for a group of landforms: Describe with diagrams three erosional landforms of a glacier; Describe the depositional landforms of a glacier; Compare the work of a river and a glacier; Describe the different types of moraine. The answer should devote one paragraph and one neat diagram to each landform, name the process (plucking, abrasion, melting, meltwater) and give one example.

Guidance for diagrams: the cirque should be drawn as a half bowl with a steep back wall, a floor hollowed below the lip and a tarn; the arête and horn are best drawn together with the cirque on a single mountain; the U-shaped valley should be a cross-section with a flat floor, steep walls and a hanging valley with a waterfall on one side; the roche moutonnée must show the direction of ice with the smooth side upstream; the drumlin must show the steep side upstream; the moraine diagram should be a plan of a glacier with lateral and medial moraines. Always draw the arrow of ice movement.

Common errors to avoid: confusing the steep sides of the drumlin (upstream) and the roche moutonnée (downstream); calling a fjord a river landform; describing till as sorted; writing that a glacier cannot erode below sea level; and forgetting that moraine is both the load in transit and the deposit left behind. Use exact figures, the length of Siachen (76 km) and Gangotri (30 km), the retreat rate of Gangotri (about 20 m a year), the height of the Himalayan snowline (about 4,500–6,000 m), because they show command of the chapter.