Landforms, landscapes and the cycle of erosion

Detail

1. Landform and landscape: the basic words

• Landform: a small to medium piece of the earth's surface. Each one has:
• its own shape, size and materials (the rock or sediment it is made of)
• a particular process (erosion or deposition) and agent (running water, groundwater, glaciers, waves or wind) that made it

• Examples: a waterfall, an oxbow lake, a sand dune, a cirque.

• Landscape: several related landforms seen together, e.g. the whole Ganga floodplain with its meanders, levees and oxbow lakes.

• Geomorphic agents of gradation (Class 9 NCERT): the moving forces that wear land down and build it up.
• Gradation means levelling. Degradation is erosion: it lowers high land. Aggradation is deposition: it fills low land.
• The five agents are running water, groundwater, glaciers, waves and wind.

2. Landforms evolve: they have a history

• Evolution of a landform: a landform does not stay the same. It changes step by step over time.
• Cause: the climate changes (wet to dry, warm to cold), or the land is pushed up (uplift).
• Process: the agent at work, or how strongly it works, changes.

• Result: the old form is changed into a new one.

• Two points to stress:

• Each landform passes through stages, like the stages of a human life.

• The agent and process at work decide the form. A glacier makes a U-shaped valley. A river makes a V-shaped valley.

• Running water is the most important agent in humid regions (wet regions with plenty of rain). It works in two ways:

• Overland flow (sheet flow): water moves across the whole slope as a thin sheet.
• Linear flow: water moves along a fixed line, as a stream or river in a channel.

3. From sheet erosion to valleys

• Sheet erosion: rainwater flows over the land as a thin, even sheet and strips off the thin top layer of soil (topsoil). Nobody notices it at first, which is why it is dangerous for farmland.
• How a valley forms, step by step:
• Sheet flow strips topsoil → water starts to gather in small lines.
• Rills: small, narrow channels a few centimetres deep, cut by gathered water.
• Gullies: rills that deepen and widen. They are too deep to plough over.
• Ravines: gullies that grow very deep and branch into a maze. Badland topography (land cut into many deep gullies, of little use for farming) results.

• Valleys: gullies that join into a permanent river channel.

• Where seen in India:

• Chambal ravines: India has about 3.13–3.17 million hectares of ravines. Madhya Pradesh alone has about 2.4 lakh hectares, mostly in Gwalior, Bhind and Morena districts (Planning Commission estimate) [6].
• These Chambal ravines are 1 to 60 m deep. They grow by about 809 hectares a year (about 2.5% a year) [6].

• The ravines also run along the Yamuna, and in the Shiwalik foothills of Punjab (choes: seasonal streams that cut deep gullies, in Hoshiarpur).

• Scale of the problem:

• Land degradation means land losing its quality: soil loss, salt build-up, waterlogging and so on.
• India had 97.84 million hectares of degraded land in 2018-19. This figure is from the Desertification and Land Degradation Atlas of India (ISRO's Space Applications Centre, Ahmedabad, released 2021). The Atlas compares 2003-05 with 2018-19 [7].
• Soil erosion by water and wind causes over 71% of land degradation in India [8].
• An older FAO estimate (1994) said water erosion caused about 80% of degraded land in India. It put the area hit by water or wind erosion at about 144 million hectares [5].

4. Vertical and lateral erosion

Feature	Vertical erosion (downcutting)	Lateral erosion
Meaning	The river cuts down into its bed and deepens it	The river cuts sideways into its banks and widens the valley
Where it dominates	Steep slopes, upper course, soon after uplift	Gentle slopes, middle and lower course
Valley shape	Narrow, deep V-shaped valley, gorge, canyon	Wide valley with a broad, flat floor
Typical features	Waterfalls, rapids, potholes	Meanders, floodplains, oxbow lakes
Stage	Youth	Maturity and old age

• The order over time:
• Early on, downcutting dominates. It slowly wears away waterfalls and cascades (small falls in a series) until they disappear.
• Later, lateral erosion takes over. Valley sides retreat, and the divides (the high ground between two river valleys) are lowered.
• In the end only a low plain with faint relief is left: the peneplain. Relief means the difference in height between high and low points.

5. Base level: the floor of river erosion

• Base level: the lowest level down to which a river can cut its bed.
• Ultimate base level: the sea. No river can erode below sea level at its mouth.

• Temporary (local) base level: a lake, a dam reservoir, or a band of hard rock across the river. It stops downcutting upstream for a while.

• As a river nears base level, its gradient flattens. It cuts down less, and lateral erosion and deposition take over.

6. Davis's cycle of erosion (geographical cycle)

• W.M. Davis (William Morris Davis, American geographer) built his theory of landform change between 1884 and 1934 [2]. He set out the "geographical cycle" in 1899.
• Cycle of erosion: the idea that a landmass, after being raised, is worn down through three stages, youth → maturity → old age. Each stage has its own set of landforms [2].
• How it works:
• Uplift: the cycle begins when an area is pushed up above sea level. Davis assumed the uplift is rapid, and that a long stillstand follows (a long period with no further movement) [3].
• Wearing down: running water and gravity slowly lower the surface [3].

• End: either the region is worn down to base level, or renewed uplift cuts the cycle short [3].

• Davis's trio: "landscape is a function of structure, process and stage."

• Structure: rock type, hardness and how the beds are arranged.
• Process: the agent at work, here running water.

• Stage: how far along the cycle (how much time) the land has gone.

• Peneplain ("almost a plain"): the end product of the cycle. It is a gently rolling, almost featureless plain, worn down close to base level (sea level) by rivers [4].

• Monadnock: an isolated hill or ridge of resistant (hard) rock that still rises above the peneplain because it wore down more slowly [9].
• The name comes from Mount Monadnock in New Hampshire, USA (about 965 m).
• Davis worked out many of his ideas in the Appalachian Mountains (eastern USA). On the old peneplains there, some monadnocks are now the summits of the highest mountains [9].

7. The three stages compared

Stage	Streams	Valleys	Divides	Special features	Main erosion
Youth	Few, poorly linked (poorly integrated: they do not yet form a joined network)	Shallow V-shaped valleys, no real floodplain or a very narrow one	Broad, flat divides with marshes, swamps and lakes	Waterfalls and rapids where hard rock is exposed	Vertical (downcutting)
Maturity	Many, well integrated	Deep V-shaped valleys; wider floodplains in which the river meanders (swings in loops)	Sharp, narrow ridges; flat areas between streams have disappeared	Waterfalls and rapids gone	Vertical and lateral work together
Old age	Few tributaries, with gentle gradients	Vast floodplains	Broad, flat divides with lakes, swamps and marshes	Free meanders, natural levees (raised banks of silt along the river), oxbow lakes (cut-off meander loops); most land is at or slightly above sea level	Lateral erosion and deposition

• Trap: both youth and old age have broad, flat divides with marshes. Only maturity has sharp divides.
• Indian examples of each stage (one river can show all three along its course):
• Youth-type features:
  • Jog Falls on the Sharavathi (Karnataka)
  • Dhuandhar Falls and the marble gorge on the Narmada at Bhedaghat, near Jabalpur (Madhya Pradesh)
  • deep gorges of the Indus, Sutlej and Brahmaputra cut across the Himalaya
• Mature-type features: the middle Ganga plain in Uttar Pradesh, with a wide floodplain and meanders.
• Old-age-type features:
  • the lower Ganga plain in Bihar and West Bengal, with oxbow lakes (mauns or chaurs in north Bihar) and levees
  • the delta lands near the Bay of Bengal
• Peneplain-like surfaces and residual hills:
  • the very old Peninsular plateau, including parts of the Chota Nagpur plateau and the Bundelkhand–Malwa uplands
  • the Aravallis, which are the worn-down stumps of very old fold mountains (see Class 6, "Landforms and Life")

8. Beyond NCERT: other models and rejuvenation

• Walther Penck (1924), Die morphologische Analyse:
• Uplift and erosion happen at the same time. Uplift does not happen first and erosion later.
• So the shape of a slope shows how fast the land is rising:
  • Waxing slope (convex, bulging outward): uplift is speeding up and rivers cut down fast.
  • Uniform slope (straight): uplift and downcutting are in balance.
  • Waning slope (concave, curving inward): uplift is slowing down and slopes lie back.

• Penck did not use Davis's idea of fixed stages tied to time.

• L.C. King (1953), pediplanation:

• King's model is for arid and semi-arid regions (dry lands).
• Slopes retreat parallel to themselves. They move back while keeping the same steepness, instead of being lowered from the top as in Davis's model.
• At the foot of the retreating slopes they leave gently sloping rock surfaces called pediments.

• Pediments join into a pediplain. Isolated hills of hard rock (inselbergs) stand above it. (See the section on landforms made by wind.)

• Rejuvenation: the river gets back the power to cut down. Two causes:

• renewed uplift of the land, or
• a fall in base level, e.g. sea level drops.
• What follows: the river's gradient becomes steeper → downcutting starts again inside the old valley → new landforms form:
  • incised (entrenched) meanders: old meander loops cut deep into rock
  • river terraces: remains of the old valley floor, left standing as steps along the sides
  • knick points: a sudden break in the river's slope, often marked by a waterfall

• Where seen: incised meanders of the Colorado River (Grand Canyon region, USA); the Himalayan rivers in their mountain courses.

• Main criticisms of Davis:

• Uplift is rarely quick and then followed by a long stillstand.
• Climates other than humid ones are left out.
• A complete peneplain is seldom seen, because tectonic activity breaks the cycle again and again.

9. Erosion and human life (Class 9 link)

• Sheet erosion and gullying remove the fertile topsoil. Crop yields fall, reservoirs silt up and farmland is lost.
• Large ravine areas in the Chambal valley are hard to farm or settle [6]. Gully areas like these are a target of India's soil-conservation and land degradation neutrality work.
• Land degradation neutrality: a goal under the UN desertification convention (UNCCD) that new land damage should be balanced by repairing other land, so that the total stays the same or improves.
• India's target is to restore 26 million hectares by 2030.

Prelims Hooks

• Davis's geographical cycle was set out in 1899. Its stages run youth → maturity → old age, and its end product is the peneplain [2][3].
• Monadnock: a hill of resistant rock left standing on a peneplain. It is named after Mount Monadnock, New Hampshire (USA), not a mountain in Scotland or Canada.
• Sharp divides belong to the mature stage. Broad flat divides with marshes belong to youth and old age. This is a common trap.
• Waterfalls and rapids are features of youth. They disappear by maturity.
• Oxbow lakes, natural levees and free meanders are features of old age.
• The order of erosion features from smallest to largest is sheet erosion → rills → gullies → ravines/valleys.
• Penck (1924): uplift and erosion work together. Waxing (convex) and waning (concave) slopes. King (1953): pediplanation and parallel slope retreat in dry lands.
• Ultimate base level is the sea. A lake or a band of hard rock is only a temporary or local base level.
• Rejuvenation leads to incised meanders, river terraces and knick points.
• Chambal ravines lie mainly in Bhind, Morena and Gwalior (Madhya Pradesh) [6]. India's degraded land was 97.84 million hectares in 2018-19 (ISRO SAC Atlas, 2021) [7].

Mains Points

• Stage decides the landscape:
• One Indian river can show all three stages along its course. The Ganga has Himalayan gorges (youth), a meandering middle plain (maturity) and an oxbow-lake-dotted delta (old age).
• The Peninsular plateau shows old-age, almost-peneplained surfaces with residual hills like the Aravallis.

• Use this to answer "structure, process and stage" questions in GS-I.

• Limits of Davis's model:

• Active tectonics in the Himalaya keep restarting the cycle. Rejuvenated rivers, terraces and deep gorges are the result.

• So Penck's model (uplift and erosion together) fits young mountains better. King's pediplanation fits dry peninsular and Rajasthan surfaces better.

• Sheet and gully erosion as a resource problem (GS-III):

• Soil erosion causes over 71% of India's land degradation [8]. About 97.84 million hectares were degraded in 2018-19 [7].
• Chambal ravines grow by about 809 hectares a year [6].
• Effects: loss of farmland, silting of dams, rural poverty, and, in the past, ravines serving as hideouts for dacoits.

• Responses:

  • watershed development
  • contour bunding (low earth banks built along the contour lines of a slope to hold rainwater and soil)
  • ravine afforestation
  • the 26 million hectares land-restoration target for 2030

• Disaster link: in old-stage floodplains, rivers swing freely in loops, banks erode and channels shift. The Kosi in Bihar is an example. These floodplains need floodplain zoning and embankment management (NDMA guidelines).