NCERT Solutions for Class 11 Geography Fundamentals of Physical Geography Chapter 6: Landforms and their Evolution

Chapter at a glance

Landform development passes through youth, maturity and old age stages.

Exercise Questions

Question 1 (i)

In which of the following stages of landform development, downward cutting is dominated?
(a) Youth stage (b) Early mature stage (c) Late mature stage (d) Old stage

Answer

1. Youth stage

Question 1 (ii)

A deep valley characterised by steep step-like side slopes is known as
(a) U-shaped valley (b) Gorge (c) Blind valley (d) Canyon

Answer

A canyon has steep, step-like side slopes (a gorge has almost equally steep sides top to bottom).

4. Canyon

Question 1 (iii)

In which one of the following regions the chemical weathering process is more dominant than the mechanical process?
(a) Humid region (b) Limestone region (c) Arid region (d) Glacier region

Answer

In limestone regions, solution (chemical weathering by carbonated water) is the dominant process.

2. Limestone region

Question 1 (iv)

Which one of the following sentences best defines the term ‘Lapies’?
(a) A small to medium sized shallow depression (b) A landform whose opening is more or less circular at the top and funnel shaped towards bottom (c) A landform formed due to dripping water from surface (d) An irregular surface with sharp pinnacles, grooves and ridges

Answer

4. An irregular surface with sharp pinnacles, grooves and ridges

Question 1 (v)

A deep, long and wide trough or basin with very steep concave high walls at its head as well as in sides is known as:
(a) Cirque (b) Glacial valley (c) Lateral Moraine (d) Esker

Answer

1. Cirque

Question 2 (i)

What do incised meanders in rocks and meanders in plains of alluvium indicate?

Answer

• Meanders in alluvial plains indicate the old (mature) stage of a river: gentle gradient, lateral erosion and deposition on floodplains.
• Incised (entrenched) meanders in rocks indicate that a meandering river was rejuvenated by uplift of the land: the river cut down deeply while keeping its original meandering course, forming deep, steep-sided valleys in hard rock.

Alluvial meanders show an old-stage river on a gentle plain; incised meanders in rock show a meandering river rejuvenated by land uplift that cut down vertically.

Question 2 (ii)

Explain the evolution of valley sinks or uvalas.

Answer

In limestone areas, sinkholes (swallow holes) form by solution of rocks. As solution continues, the rocks along the margins of sinkholes collapse and adjacent sinkholes coalesce (merge). When many sinkholes join, they form a long, narrow to wide trench called a valley sink or uvala.

Uvalas form when adjacent sinkholes enlarge by solution and collapse, merging into long, wide trenches.

Question 2 (iii)

Underground flow of water is more common than surface run-off in limestone areas. Why?

Answer

Limestone is soluble in carbonated water and is highly permeable, with many joints, cracks and bedding planes. Rainwater enters these openings and dissolves the rock, enlarging them into channels, sinkholes and caves. As a result, surface water quickly disappears underground through swallow holes, and most drainage becomes underground.

Because limestone is soluble and highly jointed; water seeps through cracks, enlarges them into sinkholes and caves, and flows underground.

Question 2 (iv)

Glacial valleys show up many linear depositional forms. Give their locations and names.

Answer

1. Lateral moraines — ridges of debris deposited along the sides of glacial valleys.
2. Medial moraines — ridges in the centre of the valley, formed where two glaciers join and their lateral moraines merge.
3. Terminal (end) moraines — long ridges deposited at the end (snout) of a glacier.
4. Ground moraines — debris deposited on the floor of the valley.
5. Eskers — sinuous ridges of sand and gravel deposited by meltwater streams flowing under the glacier.
6. Drumlins — smooth, oval-shaped ridges (like inverted spoons) of glacial till on outwash plains, aligned in the direction of ice flow.

Lateral moraines (valley sides), medial moraines (centre), terminal moraines (snout), ground moraines (floor), eskers (from under-ice streams) and drumlins (in outwash areas).

Question 2 (v)

How does wind perform its task in desert areas? Is it the only agent responsible for the erosional features in the deserts?

Answer

Wind erodes by deflation (lifting and removing loose dust and sand), abrasion (sand-blasting rock surfaces) and impact, creating deflation hollows, mushroom and table rocks. It transports sand and deposits it as dunes and loess.

No, it is not the only agent. Running water — sheet floods and flash floods after occasional heavy rains — is very important in deserts; much of the erosion of pediments, pediplains and playas is done by sheet washing and streams.

Wind erodes by deflation and abrasion and deposits sand as dunes; but it is not the only agent — sheet floods and streams after rain also shape deserts (pediments, playas).

Question 3 (i)

Running water is by far the most dominating geomorphic agent in shaping the earth’s surface in humid as well as in arid climates. Explain.

Answer

In humid regions

• High rainfall produces abundant surface run-off (overland flow and streams).
• Rivers carry out erosion (V-shaped valleys, gorges, waterfalls, potholes, incised meanders, river terraces) and deposition (alluvial fans, floodplains, levees, meanders, oxbow lakes, deltas).
• Over time, rivers reduce the land to a peneplain.

In arid regions

• Rain is scarce but often comes as sudden, heavy downpours. With little vegetation, water runs off as sheet floods and flash floods.
• These carry large amounts of debris, forming pediments, pediplains, bajadas and playas (shallow lakes in basins).
• Wind is important, but much of the desert landscape is shaped by running water.

Why dominant: water is the most widespread agent; it works in all climates, along with gravity it moves enormous loads, and the hydrological cycle keeps renewing it.

In humid areas rivers carve valleys and build plains and deltas; in arid areas sudden sheet floods form pediments, pediplains and playas; water acts everywhere and moves the most material, so it dominates both.

Question 3 (ii)

Limestones behave differently in humid and arid climates. Why? What is the dominant and almost exclusive geomorphic process in limestone areas and what are its results?

Answer

Why different behaviour: limestone dissolves in water containing carbon dioxide (carbonic acid).

• In humid climates, there is plenty of water, so limestone is easily dissolved, forming karst landscapes.
• In arid climates, there is little water, so limestone behaves as a resistant rock, forming ridges and cliffs, and is mainly affected by mechanical weathering.

Dominant process: solution (chemical weathering) by groundwater and surface water — and to some extent deposition.

Results (karst topography)

• Erosional: swallow holes, sinkholes (solution and collapse sinks), uvalas (valley sinks), lapies (pinnacled ridges), limestone pavements, caves and tunnels.
• Depositional: stalactites (hanging from cave roofs), stalagmites (rising from cave floors) and pillars/columns where they join.

Limestone dissolves readily where water is plentiful (humid) but acts as resistant rock where water is scarce (arid). Solution is the dominant process, forming sinkholes, uvalas, lapies and caves, and stalactites, stalagmites and pillars.

Question 3 (iii)

How do glaciers accomplish the work of reducing high mountains into low hills and plains?

Answer

1. Erosion: glaciers erode by plucking (lifting blocks of rock) and abrasion (grinding with debris frozen into ice). This creates cirques at the heads of valleys.
2. As cirques erode headward on different sides of a mountain, the divides between them are sharpened into aretes (sharp ridges) and horns (pyramidal peaks, e.g. Matterhorn).
3. Valleys are deepened and widened into U-shaped valleys; tributary valleys are left as hanging valleys.
4. Continued erosion lowers aretes and horns; the mountains are gradually reduced in height.
5. Glaciers transport and deposit the eroded debris as moraines, outwash plains, eskers and drumlins in low areas.
6. Over a long period, these processes reduce the high mountains to low hills and vast outwash plains.

By plucking and abrasion glaciers carve cirques, aretes, horns and U-shaped valleys, progressively lowering mountains, and deposit the debris as moraines and outwash plains, eventually forming low hills and plains.

Topic checklist

• Stages of landform development
• Running water: erosional and depositional landforms
• Groundwater and karst topography
• Glaciers, waves and currents, wind: landforms
• Comparison of agents in humid and arid areas

Total questions answered: 13 of 13