Landforms and Agents of Erosion
Rocks, weathering and mass movement, and the landforms made by rivers, groundwater, glaciers, wind and waves — erosional and depositional — for UPSC.
SyllabusGS I · Salient features of world physical geography
Prerequisites: Interior of the Earth and Plate Tectonics
The Hook: A Town Built on Landslide Debris
In January 2023, widening cracks in houses and roads forced the evacuation of families in Joshimath, a Himalayan town in Uttarakhand. Geologists pointed out that much of the town stands on loose debris left by old landslides, on a steep slope cut by streams — and a government committee headed by M.C. Mishra had warned about subsidence there as early as 1976.
Joshimath is a lesson in geomorphology: the ground under our feet is the product of weathering, mass movement and erosion, and those processes do not stop because a town has been built on top. Endogenic forces raise the land; exogenic forces — the subject of this page — wear it down.
The Core Concept
The Earth’s surface is shaped by two opposing groups of forces:
| Forces | Source of energy | Processes | Result |
|---|---|---|---|
| Endogenic | Earth’s internal heat | Folding, faulting, volcanism, earthquakes (diastrophism) | Build relief — mountains, plateaus, rift valleys |
| Exogenic | Sun’s energy, gravity | Weathering, mass movement, erosion, transportation, deposition | Wear down relief — gradation |
Gradation has two parts: degradation (wearing down high land) and aggradation (filling up low land with deposits).
Rocks — the raw material
| Rock type | How it forms | Examples | UPSC angle |
|---|---|---|---|
| Igneous | Cooling of magma or lava | Intrusive (plutonic, coarse-grained): granite. Extrusive (volcanic, fine-grained): basalt | “Primary” rocks; the Deccan Traps are basaltic |
| Sedimentary | Compaction and cementation (lithification) of deposited sediments | Sandstone, shale, limestone, coal | Layered (strata); the only rocks that commonly contain fossils; host coal and petroleum |
| Metamorphic | Existing rocks changed by heat and pressure, without melting | Limestone → marble; shale → slate; sandstone → quartzite; granite → gneiss | Questions often pair the original rock with its metamorphic form |
The rock cycle links all three: any rock can be weathered into sediment, metamorphosed, or melted into magma.
Weathering and mass movement
Weathering is the breakdown of rock in place; it does not involve transport.
| Type | Processes | Typical setting |
|---|---|---|
| Physical (mechanical) | Frost wedging (freeze–thaw), exfoliation (peeling of rock sheets from daily heating and cooling), salt crystal growth, unloading | Deserts and cold mountains |
| Chemical | Solution, carbonation (limestone dissolved by carbonic acid), oxidation (rusting of iron minerals), hydration, hydrolysis | Hot, humid climates |
| Biological | Roots prising rocks apart; burrowing animals; acids from lichens | Everywhere life exists |
Mass movement is the downslope movement of rock and soil under gravity, without a transporting agent: creep (slow), solifluction, mudflows, slumps, rockfalls and landslides (fast). Steep slopes, saturated soil, removal of vegetation, slope-cutting for roads and earthquakes all trigger it — the Himalaya and Western Ghats are especially prone.
Erosion and deposition — five agents
graph TD
A[Agents of gradation] --> B[Running water]
A --> C[Groundwater - karst]
A --> D[Glaciers]
A --> E[Wind]
A --> F[Waves and currents]
B --> B1[V-valleys, gorges, meanders, deltas]
C --> C1[Sinkholes, caves, stalactites]
D --> D1[Cirques, U-valleys, moraines]
E --> E1[Mushroom rocks, barchans, loess]
F --> F1[Cliffs, stacks, spits, lagoons]1. Running water (fluvial landforms) — the most widespread agent
| Stage / setting | Erosional landforms | Depositional landforms |
|---|---|---|
| Upper course (youth) | V-shaped valleys, gorges, canyons, waterfalls, rapids, potholes and plunge pools | — |
| Middle course | Meanders, river terraces, incised (entrenched) meanders where land is uplifted | Alluvial fans where a stream leaves the hills; point bars; braided channels |
| Lower course (old age) | — | Floodplains, natural levees, ox-bow lakes (cut-off meanders), deltas at the mouth |
- Gorge vs canyon: a gorge is deep and narrow with nearly vertical sides, about as wide at the top as at the bottom; a canyon has step-like sides, is wider at the top, and typically forms in horizontally bedded sedimentary rocks (the Grand Canyon of the Colorado River).
- Delta vs estuary: a delta forms where a river deposits more sediment than tides and currents can remove (Ganga–Brahmaputra, Godavari). An estuary is a drowned river mouth where tides scour away sediment (the Narmada and the Tapi enter the sea through estuaries).
2. Groundwater (karst topography)
Forms in limestone and dolomite, which dissolve in water containing carbon dioxide. The name comes from the Karst region of the western Balkans.
| Erosional | Depositional (inside caves) |
|---|---|
| Sinkholes (swallow holes), dolines, uvalas (merged sinkholes), lapies (grooved limestone surfaces), caves | Stalactites (hang from the ceiling), stalagmites (rise from the floor), pillars or columns (where the two meet) |
The Borra Caves in the Eastern Ghats of Andhra Pradesh are well-known limestone caves in India.
3. Glaciers
| Erosional | Depositional |
|---|---|
| Cirque (armchair-shaped hollow at the valley head; often holds a tarn lake), arête (sharp ridge between two cirques), horn (pyramidal peak — the Matterhorn), U-shaped valley, hanging valley, fjord (glacial valley drowned by the sea — Norway) | Moraines (terminal, lateral, medial, ground), eskers (winding ridges of sand and gravel from meltwater streams under the ice), drumlins (smooth oval hills; “basket of eggs” topography), outwash plains, erratics |
4. Wind (aeolian landforms in deserts)
| Erosional | Depositional |
|---|---|
| Deflation hollows, mushroom (pedestal) rocks — undercut near the base by sand-blasting, yardangs, inselbergs, pediments and pediplains | Sand dunes, loess (fine wind-blown silt — the Loess Plateau of China) |
Dune types: barchans are crescent-shaped with their horns pointing downwind; parabolic dunes are the reverse, with horns pointing upwind (anchored by vegetation); seif (longitudinal) dunes are long ridges parallel to the wind.
In deserts, running water (sheet floods after rare storms) also shapes landforms — playas (shallow lake basins, often salty) and bajadas (merged alluvial fans).
5. Waves and currents (coastal landforms)
| Erosional | Depositional |
|---|---|
| Sea cliffs, wave-cut platforms, sea caves, arches and stacks (isolated pillars left when arches collapse) | Beaches, bars and barrier bars, spits (sand ridges attached to land at one end), lagoons behind bars, tombolos (bars linking an island to the mainland) |
Chilika Lake (Odisha) and Pulicat Lake (on the Andhra Pradesh–Tamil Nadu border) are lagoons separated from the sea by sand bars and spits.
Memory Anchors
Metamorphic pairs — “Little Sisters Sing Gracefully”: Limestone → marble · Shale → slate · Sandstone → quartzite · Granite → gneiss.
Stalactite vs stalagmite: stalactites hold tight to the ceiling; stalagmites rise from the ground.
Valley shape tells the agent: V = Vigorous river; U = Under ice (glacier).
Barchan horns: “Barchan horns Blow away” — they point downwind. Parabolic is the “Parallel opposite” — horns upwind.
Glacial depositional trio: “Eskers Extend (long ridges), Drumlins Dome (oval hills), Moraines Mark the edges.”
For Prelims
- Weathering is in-place breakdown; erosion involves removal and transport.
- Carbonation dissolves limestone and creates karst landforms.
- Sedimentary rocks are layered and are the main source of fossils, coal and petroleum.
- Ox-bow lakes, levees and deltas belong to the lower course of rivers; potholes, gorges and waterfalls to the upper course.
- Cirque, arête, horn, U-shaped valley, fjord — glacial erosion. Moraine, esker, drumlin — glacial deposition.
- Barchans have horns pointing downwind; loess is wind-deposited silt.
- Stack, arch, sea cave, wave-cut platform — marine erosion. Spit, bar, lagoon, tombolo — marine deposition.
- West-flowing Narmada and Tapi form estuaries; most east-flowing peninsular rivers form deltas.
For Mains
- Dimensions: endogenic vs exogenic balance; climate control on weathering (chemical in humid tropics, physical in deserts and cold regions); human acceleration of mass movement (slope cutting, deforestation, unplanned construction); landforms as resources (alluvial plains, deltas) and as hazards (landslides, coastal erosion).
- Keywords: gradation, base level, peneplain, mass wasting, slope stability, carrying capacity of a slope, sediment budget, coastal squeeze.
- Value-addition: Joshimath (2023) shows how building on old landslide debris raises risk; link Himalayan landslides to the young, fragile geology of the range and to unplanned construction; deltas such as the Sundarbans depend on continued sediment supply, which dams and embankments can reduce.
Common Traps
In the News
- January 2023 — Land subsidence in Joshimath, Uttarakhand, led to evacuations.
- July 2024 — Landslides in the Wayanad district of Kerala caused heavy loss of life. See Disaster Management.
Practice
Prelims
Match the following landforms with the agents that produce them:
| Landform | Agent |
|---|---|
| A. Drumlin | 1. Groundwater |
| B. Barchan | 2. Waves |
| C. Stalagmite | 3. Glacier |
| D. Sea stack | 4. Wind |
Select the correct answer using the code given below.
Solution
- Drumlin → glacier (3): an oval hill of glacial till.
- Barchan → wind (4): a crescent-shaped sand dune.
- Stalagmite → groundwater (1): a deposit rising from a limestone cave floor.
- Sea stack → waves (2): a pillar left after a sea arch collapses.
Answer: C
Consider the following pairs:
| Original rock | Metamorphic rock |
|---|---|
| 1. Limestone | Marble |
| 2. Sandstone | Quartzite |
| 3. Granite | Slate |
Which of the pairs given above is/are correctly matched?
Solution
Pairs 1 and 2 are correct. Pair 3 is wrong: granite metamorphoses into gneiss; slate forms from shale.
Answer: A
How many of the following are depositional landforms?
- Natural levee
- Esker
- Spit
- Alluvial fan
Solution
All four are formed by deposition: levees by rivers in flood, eskers by meltwater streams beneath glaciers, spits by waves and longshore currents, and alluvial fans by streams emerging from hills onto plains.
Answer: D
Mains
Why are the Himalayan towns increasingly vulnerable to land subsidence and landslides? Discuss with reference to geomorphic processes and human factors.
Solution
Introduction: The Himalaya is a young fold mountain system still rising from the India–Eurasia collision. Its slopes are naturally unstable, and recent events such as the Joshimath subsidence (2023) show how human activity amplifies that instability.
Body:
- Geomorphic factors: steep slopes; young, fractured and weathered rocks; towns built on old landslide and glacial debris; high seismicity; intense monsoon rain and cloudbursts that saturate slopes; river undercutting of slope toes.
- Human factors: road widening and slope cutting without support; unregulated multi-storey construction exceeding slope load capacity; poor drainage that lets water seep into debris; deforestation; tunnelling and blasting for infrastructure.
- Measures: carrying-capacity studies for hill towns; landslide hazard zonation; regulated building codes; proper drainage; slope stabilisation; early warning.
Conclusion: Himalayan development must respect geomorphic limits — planning based on slope stability and carrying capacity is cheaper than recurring disaster response.
Quick Revision
| Point | One-liner |
|---|---|
| Endogenic vs exogenic | Build relief vs wear it down |
| Rock types | Igneous (magma), sedimentary (layers, fossils), metamorphic (heat and pressure) |
| Weathering | In-place breakdown; chemical in humid tropics, physical in deserts and cold areas |
| River | V-valley, gorge, pothole → meander, ox-bow → floodplain, levee, delta |
| Karst | Limestone; sinkholes, caves, stalactites, stalagmites |
| Glacier | Cirque, arête, horn, U-valley, fjord; moraine, esker, drumlin |
| Wind | Mushroom rock, deflation hollow; barchan (horns downwind), seif, loess |
| Coast | Cliff, arch, stack; spit, bar, lagoon, tombolo |
Key Takeaways
- Every landform question is really “which agent, and is it erosion or deposition?”
- Climate decides the dominant process: chemical weathering in humid tropics, physical weathering in deserts and cold mountains.
- Human activity on fragile slopes speeds up mass movement — the link between geomorphology and disaster management.
Related Topics
- Interior of the Earth and Plate Tectonics
- Geomorphology — quick revision
- Geomorphology — practice questions
- Drainage System of India
- Soils of India