Chapter 1 — 1.1 Exogenetic Forces

The surface of the earth that we see today, with its mountains, plateaus, plains, valleys and deltas, is the result of two great groups of forces working against each other over millions of years. Geographers call them endogenetic forces and exogenetic forces. The word endo means inside and exo means outside, while genetic refers to origin. So endogenetic forces are those that originate inside the earth, and exogenetic forces are those that originate on or above the earth's surface.

Endogenetic forces come from the heat and pressure inside the earth. They act suddenly, as in earthquakes and volcanic eruptions, or very slowly, as in the folding and faulting that raised the Himalayas and the Western Ghats. Their general effect is to create unevenness on the surface: they lift up mountains, sink down basins and tilt plateaus. For this reason they are also called constructional forces. They mainly work vertically, pushing the crust up or pulling it down.

Exogenetic forces, in contrast, are powered by the sun and by gravity. Solar energy drives the water cycle, the winds and the changes of temperature; gravity pulls water, ice and loose rock downhill. The agents through which these forces act are running water, glaciers, wind, sea waves and groundwater. Their general effect is the opposite of the endogenetic forces: they wear down the high places and fill in the low places, and so they are called destructional forces or levelling forces. They work mainly horizontally, moving material from one place to another across the surface.

The two groups never stop working. As soon as an uplift raises a block of land, rain, rivers and wind begin to attack it. The Himalayas are still rising by a few millimetres each year, yet the Ganga, the Brahmaputra and hundreds of other streams carry millions of tonnes of their rock down to the plains and the Bay of Bengal every year. The landscape at any moment is a balance between the building of the endogenetic forces and the levelling of the exogenetic forces.

A simple way to remember the distinction is this: endogenetic forces produce the first order and second order relief features, that is, the continents, ocean basins, mountains and plateaus, while exogenetic forces carve the third order relief features such as valleys, gorges, waterfalls, dunes and deltas on top of them. This chapter and the three sections that follow are devoted to the third group of features and the forces that make them.

Exogenetic forces are the natural forces that operate on the surface of the earth, drawing their energy chiefly from the sun and from gravity, and that continuously alter the surface by wearing it down in one place and building it up in another. They are sometimes called external forces or exogenic processes.

The ultimate source of energy for almost every exogenetic force is solar radiation. The sun heats the land and the sea unevenly. This uneven heating produces differences in air pressure, and air moves from high to low pressure as wind. The sun evaporates water from the oceans, lakes and soil; the vapour rises, cools, condenses and falls as rain or snow; the rain feeds rivers and the snow, when it collects year after year, forms glaciers. Daily and seasonal changes in temperature caused by the sun make rocks expand and contract, and this cracks them. Even sea waves are mostly raised by winds, which in turn are born of solar heating. So the sun, at a distance of 150 million kilometres, is the engine that drives the sculpture of the earth's surface.

The second source is gravity. Gravity gives running water, moving ice and loose rock their tendency to move downhill. A river flows from its source in the mountains to the sea because gravity pulls it. A glacier creeps down its valley because gravity pulls it. A landslide on a Darjeeling hillside after heavy rain is gravity acting directly on rock and soil. Without gravity, the sun's energy could make the rocks crack but could not move the fragments away.

Exogenetic forces work through certain agents. An agent is the medium that actually does the cutting and carrying. The principal agents are running water (rivers), moving ice (glaciers), wind, sea waves and groundwater. Each agent is important in a particular kind of climate: rivers in humid regions, glaciers in cold high-latitude and high-altitude regions, wind in hot deserts, waves along coasts and groundwater in limestone regions.

The work of exogenetic forces has three characteristic features. First, it is slow and continuous; a river may take a million years to cut a gorge, but it never stops. Second, it is selective; soft rocks are removed faster than hard rocks, so the hard rocks stand out as ridges and the soft rocks form valleys. Third, it is directed towards a level; every agent tends to reduce the surface towards the level of the sea, which is why the total effect is called levelling or gradation. These three ideas, slowness, selectivity and the tendency to level, will reappear in every landform studied in this unit.

The combined effect of all the exogenetic processes that lay bare and wear away the surface of the land is called denudation. The word comes from the Latin denudare, meaning to strip naked. Just as a sculptor strips away stone to reveal a statue, denudation strips away the rock cover of the land, lowering hills and exposing fresh rock.

Denudation is not a single process but a chain of processes that follow one another. The usual sequence is:

• Weathering – the breaking down of rocks in place, where they lie, by the action of temperature, water, air and living things. No movement of material is involved.
• Mass wasting or mass movement – the downhill movement of the weathered material under the direct pull of gravity, without the help of any moving agent such as a river or glacier. Landslides, soil creep and rockfalls are examples.
• Erosion – the wearing away of rocks and the removal of the loosened material by a moving agent such as a river, glacier, wind, wave or groundwater.
• Transportation – the carrying of the eroded material by the agent from one place to another.
• Deposition – the laying down of the transported material when the agent loses energy.

Strictly, denudation refers to the first three, weathering, mass wasting and erosion, which together lower the land. Transportation and deposition are the follow-up processes by which the removed material is carried away and laid down elsewhere. Together, all five make up the larger idea of gradation, discussed in a later topic.

It is important to see how these processes depend on one another. Weathering prepares the material; without weathering, hard solid rock would resist erosion much more strongly. Erosion removes the weathered layer and exposes fresh rock to further weathering, so the two processes feed each other. If the weathered material were never removed, it would form a thick blanket that protects the rock underneath, and weathering would slow down. Mass wasting acts as the link, bringing weathered material from the hillsides down to the valley floor, where the river can pick it up.

The rate of denudation varies enormously from place to place. It is highest where relief is steep, rainfall is heavy and vegetation is thin, as in the Himalayan foothills and the Chotanagpur plateau edges. It is lowest in flat, dry, well-vegetated regions. Human activities such as deforestation, mining and road-cutting can multiply the rate many times, which is why landslides have become so common in the Darjeeling hills.

Weathering is the disintegration and decomposition of rocks in situ, that is, in the place where they lie, by the action of the elements of weather, of water, of the atmosphere and of living organisms. The essential point is that the rock is broken down but is not carried away; weathering is a static process. Because it prepares loose material for the agents of erosion, it is called the first step of denudation.

Weathering is classified according to the manner in which the rock is broken down. When the rock merely breaks into smaller pieces without any change in its chemical composition, it is physical or mechanical weathering. When the minerals of the rock are altered chemically into new substances, it is chemical weathering. When plants, animals or human beings bring about the breakdown, it is biological weathering. This topic deals with the first kind.

The most common form of physical weathering in India is thermal weathering, caused by changes of temperature. In deserts and on bare rock surfaces of the Deccan and Chotanagpur, the day temperature may reach 45 °C or more and the night temperature may fall to 15 °C. Rock is a poor conductor of heat, so the outer layer expands by day and contracts by night while the interior stays the same. After thousands of such cycles the outer shell peels off in curved sheets, like the skin of an onion. This is called exfoliation, and it produces rounded domes such as the granite hills near Bangalore and the hills of Purulia. Rocks made of several minerals with different rates of expansion, such as granite, crumble grain by grain, a process called granular disintegration. Sudden cooling by rain on a hot rock can also split it, which is called block disintegration when the rock breaks along its joints into rectangular blocks.

In cold regions and high mountains the chief process is frost action or freeze-thaw. Water enters cracks by day, freezes at night and expands by about nine per cent of its volume, wedging the crack wider. Repeated freezing and thawing shatters the rock into sharp, angular fragments that collect at the foot of cliffs as scree or talus. This is seen on the higher slopes of the Himalaya above Sandakphu and in Ladakh.

Other forms include pressure release, in which deep rocks expand and crack when the overlying load is removed, salt weathering in coastal and desert areas, where growing salt crystals prise the grains apart, and wetting and drying of clay-rich rocks such as shale, which swell and shrink until they flake. In every case the rock becomes smaller but its chemistry does not change.

Chemical weathering is the decomposition of rocks through chemical reactions between the minerals of the rock and water, oxygen, carbon dioxide and the weak acids found in nature. Unlike physical weathering, it changes the composition of the rock, turning hard minerals into softer, often soluble, substances. It is most active in hot and humid climates, because chemical reactions speed up with heat and need water as a medium. The heavy rainfall and high temperature of West Bengal, Kerala and the north-eastern states make chemical weathering the dominant type there.

The important processes are:

• Oxidation – oxygen dissolved in water combines with iron in minerals, forming iron oxides. The rock turns reddish or yellowish, becomes crumbly and loses strength. The red soils of Birbhum, Bankura and Purulia show its effect.
• Carbonation – rainwater absorbs carbon dioxide from the air and becomes weak carbonic acid. This acid reacts with limestone (calcium carbonate) to form calcium bicarbonate, which is soluble and is washed away. Limestone regions such as the Meghalaya plateau and parts of Jaintia hills develop caves and sinkholes in this way.
• Hydrolysis – water itself reacts with silicate minerals such as feldspar in granite, breaking them down into clay minerals. This is how granite decays into kaolin, the china clay used in ceramics.
• Hydration – some minerals absorb water into their structure and swell, weakening the rock; anhydrite changing to gypsum is an example.
• Solution – rock salt and gypsum dissolve directly in water.

Chemical weathering in the tropics proceeds deep below the surface and produces a thick layer of decomposed rock called regolith. In monsoon regions with alternating wet and dry seasons, the washing out of silica and the concentration of iron and aluminium oxides produce laterite, the hard, red, brick-like material that covers much of the Rarh region of West Bengal and gives Purulia and Bankura their characteristic red landscape.

Biological weathering is the breakdown of rock by living organisms, and it usually works through both physical and chemical means. The roots of trees grow into cracks and widen them as they thicken; the banyan trees growing out of old walls in Kolkata show this vividly. Burrowing animals such as rats, earthworms and termites loosen the soil and expose rock to air and water. Lichens and mosses that grow on bare rock secrete acids that dissolve minerals. Bacteria in the soil produce humic acids that speed up chemical decay. Human beings, through mining, quarrying, ploughing and road construction, break rock at a rate that no natural agent can match, and so they are often included as a biological agent of weathering.

Once rock has been loosened by weathering, it does not stay on a slope for ever. Under the pull of gravity the loose material moves downhill, sometimes a few millimetres a year, sometimes hundreds of metres in a few seconds. This downslope movement of weathered rock, soil and debris under gravity, without the direct help of a transporting agent like a river, glacier or wind, is called mass wasting or mass movement. Water is often present and makes the movement easier by adding weight and reducing friction, but it does not carry the material the way a river does.

Several conditions favour mass wasting. A steep slope gives gravity a greater downhill component. Heavy rainfall or snowmelt saturates the soil, adds weight and lubricates the particles. Removal of vegetation takes away the roots that bind the soil. Earthquakes shake loose material free. Undercutting of the slope by a river, by waves or by road cutting removes the support at the base. Weak, jointed or clay-rich rocks fail more readily than massive hard rocks.

Mass movements are classified by their speed and by the amount of water involved:

• Slow movements: Soil creep is the almost invisible downhill movement of the top layer of soil, revealed by tilted fence posts, bent tree trunks and cracked walls. Solifluction is the slow flow of water-saturated soil over frozen ground in cold regions.
• Rapid flows: Earthflow and mudflow occur when rain-soaked soil or volcanic ash flows like thick liquid down a slope; mudflows in the Himalayan foothills can bury roads and villages.
• Slides: A landslide is the sudden sliding of a mass of rock and soil along a definite surface. A rockslide involves bedrock; a slump is a rotational slide in which the mass tilts backward as it moves down a curved surface.
• Falls: A rockfall is the free fall of rock fragments from a steep cliff, producing scree at the base.

Mass wasting is a major concern in the Darjeeling and Kalimpong hills of West Bengal. The combination of very heavy monsoon rainfall (over 3,000 mm), steep slopes, weak and highly weathered rock, deforestation for tea gardens and settlements, and road cutting has made landslides an almost yearly disaster along the Hill Cart Road and the Teesta valley. Preventive measures include afforestation, terracing, building retaining walls, providing proper drainage and avoiding construction on unstable slopes.

In the chain of denudation, mass wasting plays the role of a feeder. It brings weathered material from the slopes to the valley floor, where the river can pick it up and carry it away. Without it, rivers would be starved of the load they need for erosion and deposition.

Erosion is the wearing away of the earth's surface and the removal of rock material by a moving natural agent such as running water, glacier, wind, sea waves or groundwater. The word comes from the Latin erodere, to gnaw away. Erosion differs from weathering in one essential respect: weathering is breakdown in place, whereas erosion involves movement. The agent picks up the loosened material and takes it away, and in doing so it also uses that material as a tool to cut the rock over which it passes. Erosion is therefore both a cutting and a removing process, and it is dynamic.

Every agent of erosion works through a few common mechanisms, though the names differ from agent to agent:

• Hydraulic action – the sheer force of moving water or wind loosens and lifts particles.
• Abrasion or corrasion – the load carried by the agent (pebbles in a river, boulders in a glacier, sand in wind) scrapes and grinds the surface like sandpaper.
• Attrition – the pieces of load strike one another and are worn smaller and rounder.
• Solution or corrosion – the agent dissolves soluble rock chemically; important for rivers and groundwater in limestone areas.

The agents of erosion and the regions where each is most powerful are as follows. Running water is the most widespread and important agent, working wherever rainfall is sufficient to feed rivers, and it is the master agent in humid regions such as the Ganga plain and the Western Ghats. Glaciers operate where snow accumulates faster than it melts, that is, in polar regions and above the snowline in high mountains such as the Himalaya. Wind is the chief agent in hot deserts and along sandy coasts, where vegetation is thin and loose sand is plentiful, as in the Thar. Sea waves attack coastlines everywhere, cutting cliffs and building beaches. Groundwater works underground in limestone regions, dissolving caves and passages, as in the Meghalaya plateau.

The rate of erosion depends on the energy of the agent, the nature of the rock, the slope of the land and the vegetation cover. Fast water on a steep slope over soft, jointed rock with no vegetation erodes fastest. Human beings have greatly increased erosion by clearing forests, overgrazing and unwise cultivation; soil erosion by rain and rivulets on the bare laterite of the Rarh region has cut the land into a network of gullies known locally as khoai, seen around Santiniketan.

Erosion is the central process of denudation. It is the stage at which the land is actually lowered and the landforms that we study, valleys, gorges, cirques, yardangs and cliffs, are carved.

Material that has been eroded must be carried somewhere, and eventually it must be put down. These two processes, transportation and deposition, complete the work begun by weathering and erosion.

Transportation is the carrying of eroded material, called the load, by the agent from the place of erosion to the place of deposition. The load carried by a river or a wind is moved in four ways. Fine clay and silt are held up in the water or air and carried along as suspension; this gives the Ganga its muddy colour. Dissolved minerals are carried in solution, invisible to the eye. Sand grains that are too heavy to remain suspended hop and bounce along the bed in a series of jumps, which is called saltation. Pebbles and boulders roll or slide along the bottom as traction or bed load. The ability of an agent to carry a load has two measures: its capacity, which is the total quantity it can carry, and its competence, which is the largest size of particle it can move. Both increase sharply with the velocity of the agent; a river in flood can roll boulders that it could not move in the dry season.

Deposition occurs when the agent loses energy and can no longer carry all its load. The heaviest particles are dropped first and the finest last, so deposits are usually sorted by size. A river deposits when its velocity falls, which happens when the slope decreases at the foot of mountains, when the channel widens, when the river enters a lake or the sea, or when its volume falls in the dry season. A glacier deposits when it melts. Wind deposits when it meets an obstacle or when its speed drops. Because glacial ice does not sort its load, glacial deposits are a jumble of all sizes, unlike the neatly sorted deposits of water and wind.

Deposition builds the depositional landforms that fill in the low places of the earth: alluvial fans, floodplains, deltas, moraines, sand dunes and loess plains. The whole of the Bengal delta and the plains of north Bengal are built of material transported from the Himalaya and deposited by the Ganga, the Brahmaputra and their tributaries. Every year the Ganga alone carries about 1,600 million tonnes of sediment towards the Bay of Bengal, of which a large part is deposited in the delta and on the sea floor.

Transportation and deposition are thus the constructive half of the work of exogenetic forces. Erosion lowers the high land; deposition raises the low land; and both are directed towards the same end, the reduction of the relief of the earth's surface to one level.

The general tendency of all exogenetic processes is to bring the uneven surface of the earth to a single level by cutting down the high parts and filling up the low parts. This levelling process is called gradation. The term was introduced by the American geologist G. K. Gilbert. A surface that has been reduced to a uniform gentle slope is said to be graded.

Gradation has two complementary halves. Degradation is the wearing down of the high land by weathering, mass wasting and erosion; it lowers the surface. Its result is the group of erosional landforms such as V-shaped valleys, gorges, waterfalls, cirques, U-shaped valleys, mushroom rocks and yardangs. Aggradation is the building up of the low land by the deposition of the material that has been removed from the high land; it raises the surface. Its result is the group of depositional landforms such as alluvial fans, floodplains, deltas, moraines and sand dunes. Degradation and aggradation always go together; the sediment that leaves the Himalaya by degradation arrives in the Bengal basin by aggradation.

Degradation is sometimes described as the sum of three processes, weathering, mass wasting and erosion, which together are also called denudation. Aggradation involves transportation followed by deposition. So gradation as a whole can be written as the sum of all five exogenetic processes.

The two halves can be compared in a table:

Gradation is a very slow process. The Aravalli range, once as high as the Himalaya, has been degraded over hundreds of millions of years to a chain of low hills. The Ganga plain, on the other hand, has been aggraded by a layer of alluvium that in places is several thousand metres thick. In practice the earth's surface is never completely graded, because endogenetic forces keep raising new mountains and because the sea level itself changes. Nevertheless, the direction of the exogenetic forces is always the same: towards a level surface, and that level is the base level of erosion, discussed next.

If rivers went on cutting down for ever, they would eventually cut the continents below the level of the sea. This does not happen, because a river cannot erode its bed below the level of the water body into which it flows. The lowest level to which a river, or any other agent of erosion, can wear down the land is called the base level of erosion. The concept was given by the American geographer J. W. Powell in 1875.

The ultimate or permanent base level is the mean sea level, because all the rivers of the world finally reach the sea and cannot cut below its surface. If every mountain were degraded down to this level, the whole land surface would become a nearly flat plain just above the sea, a surface that Davis called a peneplain.

Besides the sea, there are temporary or local base levels. A lake into which a river flows acts as a base level for the stretch of river above it, because the river cannot cut below the lake surface. A band of very hard rock across the river bed holds up erosion above it and acts as a base level until the river finally wears it through. The point at which a tributary joins the main river is the base level for that tributary. A waterfall marks a temporary base level for the river upstream of it. These local base levels are temporary because in time the lake fills up, the hard rock is cut through and the waterfall retreats, after which the river continues to cut down towards the sea.

The base level controls the long profile of a river. From its source the river flows steeply, then more and more gently, until near its mouth it flows almost level, and the whole curve flattens out at sea level. A river that has just the gradient needed to carry its load, neither eroding nor depositing, is said to be at grade, and its smooth concave profile is called the profile of equilibrium.

Base level is not fixed for all time. If the sea level falls, or if the land is uplifted, the river gets a new, lower base level and begins to cut down again with fresh energy; this is called rejuvenation, and it produces features such as river terraces and incised meanders. If the sea level rises, the lower part of the valley is drowned and the river begins to deposit. The idea of base level is therefore the key that links the work of exogenetic forces with the concept of gradation: gradation is the reduction of the land towards the base level.

The exogenetic forces act through certain agents, and the importance of each agent depends on the climate of the region. The ideal way to understand this is to look at the world's climatic regions and ask which agent is most active in each.

In humid regions, where rainfall is abundant throughout the year or in a long wet season, the dominant agent is running water. Rivers, rills and rain-wash carry out most of the erosion, transportation and deposition. The Ganga basin, the Western Ghats, the Brahmaputra valley and the whole of monsoon Asia are river-dominated landscapes. Chemical weathering is strong because of the warmth and moisture, and the landforms are those of river action: V-shaped valleys, waterfalls, meanders, floodplains and deltas.

In cold regions, where the temperature stays below freezing for most of the year, precipitation falls as snow and accumulates into ice. Here the dominant agent is the glacier. Antarctica, Greenland, the Arctic islands and the high Himalaya above about 5,000 m are glaciated. Frost action is the chief form of weathering, and the landforms are those of ice: cirques, U-shaped valleys, moraines and drumlins.

In arid regions, where rainfall is below 250 mm a year and vegetation is scanty, the dominant agent is wind. The Sahara, the Arabian desert, the Thar and the deserts of central Asia and Australia are wind-dominated. Physical weathering by extreme temperature change is strong, and the landforms are those of wind: mushroom rocks, yardangs, sand dunes and loess plains. Even in deserts, however, the occasional violent rainstorm produces flash floods that carve wadis and build alluvial fans, so running water still plays a part.

Along coasts, whatever the climate, sea waves are the dominant agent, cutting cliffs, caves and arches on rocky shores and building beaches, spits and bars on gentle shores. In limestone regions such as Meghalaya, the Vindhyas and the Karst region of Europe, groundwater is the special agent, dissolving the rock to form sinkholes, caves, stalactites and stalagmites.

Two further points deserve notice. First, the agents often work together: in the Himalaya, glaciers erode the highest peaks, their meltwater feeds rivers that erode the middle slopes, and mass wasting operates everywhere. Second, the agent that dominates a region may change with time; the landscape of northern Europe was shaped by ice during the last Ice Age and is now being reshaped by rivers. Landforms left behind by an agent that no longer operates are called relict landforms.

The American geographer William Morris Davis proposed in 1899 that the landscape of a region evolves through a regular sequence of stages, just as a living being passes through youth, maturity and old age. He called this sequence the geographical cycle or the cycle of erosion. In his own words, landscape is a function of structure, process and stage: the structure of the rocks, the process (the agent) acting on them, and the stage that the cycle has reached.

Davis assumed that a block of land is rapidly uplifted from the sea and then left undisturbed while rivers erode it. Under these conditions the landscape passes through three stages.

Youth: The land is high above the base level, so the rivers have great energy and cut downward vigorously. Valleys are narrow, deep and V-shaped; gorges, waterfalls, rapids and potholes are common. The interfluves, the flat uplands between the valleys, are broad and largely untouched. Relief, the difference between the highest and lowest points, is increasing.

Maturity: The rivers have cut down nearly to base level, so vertical erosion slows and lateral erosion becomes important. Valleys widen, the interfluves are narrowed into sharp ridges, and the whole surface is dissected into a network of hills and valleys. Relief is at its maximum in early maturity and then begins to decline. Waterfalls disappear, and meanders and small floodplains appear in the lower courses.

Old age: The rivers flow sluggishly across a nearly level plain in wide meanders with broad floodplains, oxbow lakes and natural levees. The hills have been worn down to low, rounded rises. Relief is very small. The final product is a gently rolling surface just above base level, which Davis named the peneplain (almost a plain). Occasional hard-rock hills that resist erosion stand out on the peneplain as monadnocks, named after Mount Monadnock in New Hampshire.

If the land is uplifted again before the cycle is complete, the rivers gain new energy and begin the cycle afresh; this is called rejuvenation, and it leaves marks such as river terraces, incised meanders and knick points in the river profile.

Davis's model has been criticised because uplift and erosion in reality go on together rather than one after the other, and because climate changes during the long life of a cycle. Nevertheless, the terms youth, maturity and old age are still used to describe the character of a river and its valley, and the idea of a peneplain remains useful. The Chotanagpur plateau and the Deccan are sometimes cited as uplifted old peneplains.

Because the examination frequently asks the student to distinguish between the two groups of forces, it is worth setting the comparison out clearly. Both groups are natural, both are continuous and both shape the surface, but they differ in origin, direction, effect, speed and result.

The two groups are in constant competition, and the landscape at any time is the result of the balance between them. When the endogenetic forces are stronger, as in a young fold-mountain region, the land rises faster than it is worn down and relief increases. When the exogenetic forces are stronger, as in an old shield region, relief decreases towards a peneplain. Over the whole history of the earth the two have kept a rough balance, which is why the continents have neither been worn flat nor grown into unlimited heights.

Several other paired terms from this chapter should be distinguished in the same way:

• Weathering is breakdown in place with no transport; erosion is wearing away with removal by a moving agent.
• Erosion requires a moving agent such as a river; mass wasting is movement by gravity alone.
• Degradation lowers the land and gives erosional landforms; aggradation raises the land and gives depositional landforms.
• Physical weathering changes size only; chemical weathering changes composition.
• Permanent base level is sea level; a temporary base level is a lake, waterfall or hard-rock band.

Being able to state these differences in two or three clear points, with one example each, is the surest way to score in the two-mark and three-mark questions of the Madhyamik examination.

West Bengal is an unusually good laboratory for this chapter, because within one state almost every exogenetic agent can be seen at work. In the Darjeeling Himalaya, the Kanchenjunga slopes above the snowline carry glaciers, and frost shattering, rockfall and landslides are the everyday processes of the hills. The Teesta, Jaldhaka, Torsa and Mahananda rise there and rush down as youthful rivers, cutting gorges and dropping gravel fans where they meet the plains of the Terai and Duars. The Rarh region of Purulia, Bankura, Birbhum and western Bardhaman is an old, laterite-covered plateau edge where chemical weathering has produced red soil and rain-wash has cut the khoai badlands; its rivers, the Damodar, Ajay, Mayurakshi and Kangsabati, are mature streams that flood in the monsoon. The Ganga delta of the south, from Murshidabad to the Sundarbans, is the greatest depositional landform of the country, built by aggradation and still growing seaward. The coast at Digha and Bakkhali shows wave action, beaches and dunes. Only the true desert agent, wind, is missing, and even that can be seen in a small way on the coastal dunes.

In the Madhyamik examination this section is tested through several types of questions. The one-mark multiple-choice and fill-in-the-blank items ask for terms: the agent that dominates a desert, the name of the process of breaking rock in situ, the ultimate base level, the scientist who proposed the cycle of erosion. Very short answers of two marks ask for definitions, such as of exogenetic forces, denudation, gradation or mass wasting, or for a single difference, such as between weathering and erosion. Three-mark questions ask the student to explain a process with an example, for instance exfoliation or carbonation, or to distinguish two terms in three points. The five-mark descriptive questions ask for a full account, such as the types of weathering with examples, or the stages of the cycle of erosion with diagrams, or a comparison of endogenetic and exogenetic forces.

A few habits help in answering. Always begin with a one-line definition. Support each point with an example, preferably an Indian or West Bengal one. Draw a labelled diagram wherever a landform or a process can be drawn, because a neat diagram earns marks and saves words. Use the exact terms of the chapter; write in situ for weathering, base level for the limit of erosion and peneplain for the end of the cycle. Keep the sequence of processes in order: weathering, mass wasting, erosion, transportation, deposition.

Finally, remember that the three sections that follow, on the work of rivers, glaciers and wind, are applications of this section. The mechanisms of erosion, the modes of transport, the causes of deposition and the tendency towards base level explained here will be used again in every one of them, so this chapter should be understood thoroughly rather than memorised.