Landforms and Their Evolution

Written and reviewed by Subhra Gupta, Humanities faculty at UniDrill (M.A. Sociology, University of Delhi · 7 years teaching Humanities). Based on NCERT Geography Class XI (Fundamentals of Physical Geography), Chapter 6.

Every landform has a birth date

Look at a river delta, a Himalayan peak and a desert dune. Each one began at some moment, has been changing ever since, and will one day disappear. A landform is a small to medium parcel of the earth's surface. Several related landforms together make a landscape, which is a large tract. Every landform has its own shape, size and material, and each is the product of particular geomorphic processes and agents.

Evolution here means the stages of change: either one landform turning into another, or a landform changing after it has formed. A change in climate or a vertical or horizontal movement of the land can change how strong a process is, or replace it with a different one. The textbook compares the stages of a landmass to the stages of life: youth, maturity and old age.

After weathering has loosened the surface, five agents do the erosion and then the deposition. Most of what you need is in this overview table.

Running water: the most important agent in humid regions

Running water works in two ways: overland flow that spreads across the land as a sheet, and linear flow in streams and rivers. Most erosional landforms come from vigorous, youthful rivers on steep gradients. As the river cuts down, its slope gentles and its speed drops. Slow water deposits more, so the gentler the channel, the greater the deposition. Once downward cutting weakens, lateral erosion of the banks increases and the hills and valleys are reduced to plains.

From rill to peneplain. Sheet flow first concentrates into narrow paths and scours small rills. Rills grow into gullies, gullies deepen, widen and join into a valley network. Early on, down-cutting dominates and removes waterfalls and cascades. In the middle stage streams cut more slowly and lateral erosion of valley sides becomes severe. The divides between basins are lowered until a lowland of faint relief remains, with a few low, resistant hills called monadnocks standing here and there. This almost flat plain made by stream erosion is a peneplain.

Pause and predict: a stream has a very steep gradient and is carving a deep, narrow channel through hard rock. Is it cutting down or sideways, and which stage is it in?

Check your answer: it is cutting down, because on steep gradients erosion concentrates on the channel bottom and lateral erosion is small. That is typical of youth.

Erosional landforms of running water. The table below sets out the five you are most likely to meet.

River terraces are "paired" when they occur at the same height on both sides of the river.

Depositional landforms of running water. Deposition takes over where the stream slows down.

The best way to separate an alluvial fan from a delta is the location. A fan builds at the foot of mountains. A delta builds where the river enters the sea. Fan deposits are not as neatly layered as delta deposits, which are clearly sorted and stratified.

Meanders are a pattern, not a landform. Rivers on very gentle gradients in large flood and delta plains rarely flow straight. Loops called meanders develop for three reasons: water on a very gentle gradient works sideways on its banks, the alluvial banks are loose and uneven so water can push against them, and the Coriolis force deflects flowing water just as it deflects wind. On a bend, water erodes and undercuts the outer bank, which shows up as a steep scarp called the cut-off bank, and deposits material on the inner bank, which has a long, gentle profile. As loops deepen they get cut off at their narrowest part and are left behind as oxbow lakes. The Burhi Gandak river near Muzaffarpur in Bihar shows many such cut-offs.

Beyond the textbook. For the exam: youth, mature and old describe the stages of a river landscape, ending in a peneplain. In reality: that three stage "life" scheme is a model. Many later geomorphologists point out that landscapes can reach a balance between uplift and erosion and stay in it for long periods, so landforms do not always age in a fixed order. Climate change and uplift can restart the cycle at any time. Know the stages for CUET, but remember they describe a tidy sequence that real landscapes often interrupt.

Groundwater: karst topography

Groundwater matters for landforms only in rocks that dissolve. Surface water sinks through rocks that are permeable, thinly bedded, jointed and cracked, then flows sideways along bedding planes and joints. Moving water does very little mechanical removal, so the result is visible only in limestone and dolomite rich in calcium carbonate, where solution and precipitation do the work. Any limestone or dolomite region that shows landforms formed this way is called karst topography, named after the Karst region in the Balkans next to the Adriatic Sea.

Why stalagmites sit under stalactites. Water charged with carbon dioxide dissolves calcium carbonate. When that water evaporates, or loses its carbon dioxide as it trickles over rough rock, it leaves calcium carbonate behind. Water that drips from the roof, or through the thin pipe of a stalactite, lands directly below it, so the stalagmite grows exactly where the stalactite is dripping. Surface streams in limestone areas often vanish down swallow holes, flow underground and re-emerge through a cave opening further downstream.

Pause and predict: a student says that groundwater carves landforms everywhere, so granite regions must have caves like limestone regions. What is wrong with that?

Check your answer: granite does not dissolve in water in any useful way, and the mechanical work of groundwater is insignificant. Landforms of groundwater appear only in soluble rocks such as limestone and dolomite.

Beyond the textbook. For the exam: a sinkhole is a solution or collapse feature formed naturally by solution in limestone. In reality: sinkholes can also be triggered by human activity such as heavy pumping of groundwater or leaking underground water pipes, which weaken the support under the soil mantle. That is why a sinkhole covered by soil can look like solid ground and then suddenly drop away.

Glaciers: rivers of ice

Glaciers are masses of ice that move under gravity. They move as sheets over land (continental glaciers, or piedmont glaciers when a vast sheet spreads at the foot of mountains) or as linear flows in broad valleys (mountain and valley glaciers). Their movement is slow, from a few centimetres to a few metres a day, or even less. In India, glaciers lie in the higher Himalayas in Uttarakhand, Himachal Pradesh and Jammu and Kashmir. The Bhagirathi river is fed by meltwater from the snout (Gaumukh) of the Gangotri glacier, and the Alaknanda is fed by the Alkapuri glacier. The two rivers meet at Devprayag to form the Ganga.

Glacial erosion is tremendous because of the friction of the ice's weight. Large angular blocks plucked from the land are dragged along the floors and sides of valleys, causing great damage through abrasion and plucking. Glaciers can damage even unweathered rock and reduce high mountains to low hills and plains. Once the slope is gentle enough, the glacier stops and leaves low hills, outwash plains and depositional forms.

The one comparison NTA loves: till versus outwash. Till is dumped directly by ice, so it is unsorted and angular. Outwash is moved by meltwater, so it is layered, sorted and slightly rounded. The same idea applies to alluvium versus till: water sorts, ice does not.

Waves and currents: the coast

Coasts are the most dynamic and destructive zone. Waves cause most coastal change. Storm waves and tsunami waves can change the coast more in a short time than normal breaking waves. Besides waves, coastal landforms depend on the shape of the land and the sea floor and on whether the coast is advancing seaward or retreating landward. Two types explain the main ideas.

Erosion on a rocky coast. Waves hammer the base of hill sides until they form cliffs. The cliff retreats and leaves a wave-cut platform (terrace) in front. Hollows at the base of the cliff widen into sea caves, and when the cave roofs collapse the cliff recedes further. Resistant remnants left standing just off the shore are sea stacks, which are also temporary. Eventually the hills and cliffs disappear, leaving a narrow coastal plain that may be covered by alluvium, shingle or sand.

Deposition and the lagoon sequence. Material removed from cliffs is broken into smaller pieces and rolled round, then deposited offshore, where longshore currents and waves build beaches along the shore and bars (long ridges of sand or shingle parallel to the coast) in the nearshore zone. The sequence for CUET runs like this:

The coast in one line: offshore bar (submerged) → shows above water as a barrier bar → attached to a headland or one end of a bay as a spit → closes the bay mouth, creating a lagoon → sediment fills the lagoon → coastal plain.

Beaches are temporary: a sandy beach in one season can shrink to a narrow strip of coarse pebbles in another. Most beaches are sand, while shingle beaches have pebbles and cobbles. Behind the beach, wind-lifted sand piles up as sand dunes, forming long ridges parallel to the coast on low sedimentary coasts. Large rivers that bring plenty of sediment build deltas on such coasts.

Beyond the textbook. For the exam: offshore bars, barriers, beaches, dunes and mangroves act as the first defence against storm and tsunami waves. In reality: this is why digging out beach sand, damming rivers that feed a coast with sediment, or clearing mangroves is risky. It disturbs the sediment budget, so the coastal forms erode and homes behind them take the first blow of the next storm.

Wind: the desert sculptor

Wind and sheet floods are the two dominant agents in hot deserts. Dry, barren floors heat the air above them quickly, so the hot light air rises with turbulence, eddies and whirlwinds. Wind works through deflation (lifting and removing dust and small particles), abrasion (sand and silt acting as tools that grind the land) and impact (the sand-blasting force of grains striking rock).

A common misconception. People think deserts are shaped by wind alone. Rain is scarce, but it falls torrentially in a short time. Rocks without vegetation break down fast under sharp day and night temperature swings, and sheet floods then wash the debris away. Wind moves the fine material, while most mass erosion is done by sheet floods or sheet wash. Desert stream channels are broad, smooth and indefinite and carry water only briefly after rains.

Pediments grow by parallel retreat of slopes (backwasting): the steep wash slope and the cliff above it retreat while the pediment extends at the expense of the mountain.

Dune types. Dunes need a sand supply and an obstacle to get started.

When sand is plentiful, regular dunes merge and lose their shapes. Most desert dunes shift, and a few stabilise, especially near human habitation.

Two views. For the exam: a pediplain, not a peneplain, is the end product in deserts, and wind is one of two dominant agents with sheet floods. Another reading: many modern studies of arid landscapes stress that most desert surfaces are shaped over very long times by past wetter climates as well as present dryness, so what you see today may not be the work of present conditions alone.

Exam traps

60-second revision

Where this chapter connects

• Previous: Geomorphic Processes explains weathering, mass movements and erosion, which set up the work done in this chapter.
• Related: Drainage System uses the same river ideas for the Ganga, Brahmaputra and peninsular rivers, and Movements of Ocean Water explains the waves and currents that shape coasts.
• Next: Composition and Structure of Atmosphere begins the study of the atmosphere, the source of the climate that controls most of these agents.