Landforms and their Evolution — NCERT Solutions

Given: Limestone areas show more underground drainage than surface run-off.

Reason:

1. Limestone is a soluble rock. Rainwater, being slightly acidic (carbonic acid), dissolves limestone along its numerous joints, bedding planes, and fractures through the process of carbonation.
2. This dissolution widens the joints and cracks into large openings, caves, tunnels, and conduits underground.
3. As a result, water quickly percolates downward through these openings rather than flowing over the surface.
4. The surface in limestone areas is often dry and porous, with sinkholes and swallow holes that swallow surface streams, directing water underground.
5. Underground, the water flows through a well-developed network of caves and passages.

Therefore, the highly permeable and soluble nature of limestone makes underground flow far more common than surface run-off in karst regions.

Given: Linear depositional landforms in glacial valleys.

Glacial valleys contain several linear depositional features formed by the deposition of glacial debris (till and outwash). These are:

1. Lateral Moraines: Long, linear ridges of debris deposited along the sides (margins) of a glacier within the valley.
2. Medial Moraines: Linear ridges of debris found in the middle of a glacial valley, formed when two glaciers merge and their lateral moraines join together.
3. Terminal (End) Moraines: Linear or crescent-shaped ridges deposited at the snout (front end) of the glacier, marking the furthest advance of the ice.
4. Ground Moraine: A sheet-like deposit of till spread over the valley floor beneath and behind the glacier.
5. Eskers: Long, narrow, winding ridges of stratified sand and gravel deposited by meltwater streams flowing within or beneath the glacier, found on the valley floor.
6. Drumlins: Elongated, streamlined hills of glacial till, found in groups on the valley floor, with their blunt end facing the direction from which the glacier came.

Thus, glacial valleys display a rich variety of linear depositional landforms at their sides, middle, floor, and terminus.

Given: Role of wind in desert geomorphology.

How wind performs its task:
Wind acts as a geomorphic agent in deserts through three main processes:

1. Deflation: Wind picks up and removes loose, fine particles (dust and sand) from the surface, lowering the land surface and creating deflation hollows or basins.
2. Abrasion: Wind-carried sand particles act like sandpaper and scrape, scratch, and polish rock surfaces, undercutting rocks to form features like mushroom rocks (pedestal rocks) and yardangs.
3. Attrition: Sand particles carried by wind collide with each other and become smaller and more rounded over time.

Through these processes, wind creates erosional landforms such as deflation hollows, mushroom rocks, inselbergs, yardangs, and ventifacts, and depositional landforms such as sand dunes (barchans, seifs, longitudinal dunes, transverse dunes) and loess deposits.

Is wind the only agent?
No, wind is not the only agent responsible for erosional features in deserts. Running water also plays a significant role:

• Occasional but intense rainfall causes flash floods that carve out valleys, gorges, and alluvial fans.
• Dry river channels called wadis (ephemeral streams) are formed by running water.
• Pediments and bajadas are also partly shaped by water action.

Thus, while wind is the dominant agent in deserts, running water also contributes significantly to shaping desert landforms.

Given: The role of running water as the most dominant geomorphic agent.

Introduction:
Running water (rivers and streams) is the most powerful and widespread geomorphic agent on Earth. It shapes the landscape through erosion, transportation, and deposition in both humid and arid regions.

In Humid Regions:
In humid climates, rivers receive abundant water from rainfall and are perennial (flow throughout the year). They perform extensive work:

• Erosion: Rivers erode their beds and banks through hydraulic action, abrasion, corrosion (solution), and attrition. They cut deep valleys, gorges, canyons, and waterfalls in the youth stage.
• Transportation: Rivers carry enormous quantities of sediment — dissolved load, suspended load, and bed load — downstream.
• Deposition: In the mature and old stages, rivers deposit sediment to form floodplains, meanders, oxbow lakes, deltas, and alluvial fans.
• The entire landscape in humid regions — from mountains to plains to coasts — is largely shaped by river action over geological time.

In Arid Regions:
Even in arid (desert) climates, running water is a significant agent despite infrequent rainfall:

• Occasional intense rainstorms produce flash floods that carry large amounts of debris.
• These ephemeral streams (wadis) carve valleys, gorges, and pediments.
• Alluvial fans and bajadas are formed at the base of mountains by water deposition.
• Many desert landforms previously attributed solely to wind are now known to have been shaped significantly by water action.

Why Running Water Dominates over Other Agents:

• Wind erosion is limited to surface layers and is less effective on hard rocks.
• Glaciers are confined to high altitudes and polar regions.
• Groundwater acts slowly and only in specific rock types.
• Running water, however, operates over vast areas, has high kinetic energy, and can carry large loads over long distances.

Conclusion:
Thus, whether in the lush humid tropics or the dry arid deserts, running water remains the most dominant and universal geomorphic agent, continuously sculpting the Earth's surface through its erosional and depositional activities.

Given: Behaviour of limestone in different climates and the dominant geomorphic process in limestone areas.

Why Limestone Behaves Differently in Humid and Arid Climates:

Limestone ($CaCO_3$) is a soluble rock. Its behaviour depends on the availability of water and carbon dioxide:

• In Humid Climates: There is abundant rainfall and moisture. Rainwater absorbs $CO_2$ from the atmosphere and soil to form carbonic acid:

$$H_2O + CO_2 \rightarrow H_2CO_3$$
This weak acid readily dissolves limestone:
$$CaCO_3 + H_2CO_3 \rightarrow Ca(HCO_3)_2$$
The calcium bicarbonate formed is soluble and is carried away in solution. Thus, in humid climates, limestone is rapidly dissolved and a distinctive karst landscape develops.

• In Arid Climates: There is very little rainfall and moisture. Without sufficient water, the chemical process of carbonation-solution cannot operate effectively. Limestone in arid regions therefore behaves more like any other hard rock — it resists weathering and may form cliffs and ridges. Mechanical weathering (temperature changes causing expansion and contraction) is more significant here.

Dominant Geomorphic Process in Limestone Areas:
The dominant and almost exclusive geomorphic process in limestone areas is chemical weathering through carbonation-solution (also called karstification or karst processes).

Results (Karst Landforms):

Erosional Landforms:

1. Lapies: Irregular surfaces with sharp pinnacles, grooves, and ridges formed by differential solution of limestone.
2. Sinkholes/Dolines: Small to medium-sized circular depressions formed by solution or collapse of the roof of underground caves.
3. Uvalas (Valley Sinks): Larger depressions formed by the coalescence of several sinkholes.
4. Poljes: Very large flat-floored depressions formed by the merging of uvalas.
5. Blind Valleys: Valleys that end abruptly where a surface stream disappears underground through a swallow hole.
6. Caves and Caverns: Large underground hollow spaces formed by the dissolution of limestone along joints and bedding planes.

Depositional Landforms (inside caves):

1. Stalactites: Icicle-like deposits of calcium carbonate hanging from the roof of caves, formed by dripping water.
2. Stalagmites: Pillar-like deposits rising from the floor of caves, formed by dripping water.
3. Pillars/Columns: Formed when stalactites and stalagmites join together.

Conclusion:
Thus, the availability of water is the key factor determining how limestone behaves, and in humid regions, karstification produces a unique and spectacular landscape of sinkholes, caves, and karst towers.

Given: The role of glaciers in reducing high mountains to low hills and plains.

Introduction:
Glaciers are large masses of ice that move slowly under the influence of gravity. They are powerful geomorphic agents that erode, transport, and deposit enormous quantities of rock material, gradually reducing high mountains.

1. Glacial Erosion:
Glaciers erode mountains through the following processes:

• Plucking (Quarrying): The glacier freezes onto the bedrock and as it moves, it plucks out large blocks and fragments of rock from the valley floor and walls. This deepens and widens the valley.
• Abrasion: Rock fragments embedded in the base and sides of the glacier act like sandpaper and scratch, grind, and polish the underlying bedrock, wearing it down. This produces glacial striations and smoothed rock surfaces.
• Attrition: Rock fragments carried by the glacier collide with each other and are broken into smaller and smaller pieces.

Through these processes, glaciers carve:

• Cirques: Deep, armchair-shaped basins at the heads of glaciers, formed by intense erosion.
• Arêtes: Sharp, knife-edged ridges formed between two cirques eroding from opposite sides.
• Horns (Pyramidal Peaks): Sharp, pointed mountain peaks formed when three or more cirques erode a mountain from different sides (e.g., the Matterhorn).
• Glacial Troughs (U-shaped valleys): Deep, wide, flat-floored valleys carved by glaciers, replacing the original V-shaped river valleys.
• Hanging Valleys: Tributary glacial valleys left high above the main glacial trough.
• Fjords: Glacial troughs that are flooded by the sea after the glacier retreats.

2. Glacial Transportation:
The eroded material (called till or moraine) is transported by the glacier:

• Material falls onto the glacier from valley walls (supraglacial transport).
• Material is carried within the ice (englacial transport).
• Material is dragged along the base (subglacial transport).

3. Glacial Deposition:
When the glacier melts or its velocity decreases, it deposits the transported material:

• Moraines (lateral, medial, terminal, ground): Ridges and sheets of till deposited at the sides, middle, and front of the glacier.
• Drumlins: Streamlined hills of till deposited beneath the glacier.
• Eskers: Long, winding ridges of sand and gravel deposited by meltwater streams.
• Outwash Plains: Broad, flat plains of sorted sand and gravel deposited by meltwater beyond the glacier's snout.

Conclusion:
Through the combined action of intense erosion (plucking and abrasion) that wears down mountain peaks and ridges, and deposition that fills valleys and plains with debris, glaciers systematically reduce high, rugged mountains into lower, smoother hills and eventually into plains over millions of years. The sharp peaks are reduced, the valleys are widened and deepened, and the eroded material is spread over lowland areas as glacial deposits.