Landforms and their Evolution

(a) Youth stage
Reason: In the youth stage, streams flow with high velocity on steep slopes, causing intense vertical erosion (down-cutting), leading to V-shaped valleys and gorges.

(d) Canyon
Reason: While a gorge has steep, often vertical walls, a canyon typically features step-like slopes due to the differential erosion of horizontal rock layers (e.g., Grand Canyon).

(b) Limestone region
Reason: In limestone (Karst) regions, the chemical action of water (solution/carbonation) dissolves the rock, creating landforms like sinkholes and caves.

(d) An irregular surface with sharp pinnacles, grooves and ridges
Reason: Lapies are karst landforms created by the solution activity of rainwater on exposed limestone, leaving behind sharp ridges and furrows.

(a) Cirque
Reason: A cirque is an armchair-shaped hollow formed by glacial erosion at the head of a valley. [Image of glacial cirque landform]

• Incised Meanders (in rocks): Indicate that the river was meandering on a gentler slope initially, but rapid upliftment of the land or lowering of the base level forced the river to cut vertically deep into the bedrock (Rejuvenation).
• Meanders in Plains (Alluvium): Indicate lateral erosion in the mature/old stage where the river flows slowly on a gentle slope, eroding banks and depositing sediments.

In karst regions, solution action creates small funnel-shaped depressions called Sinkholes. When multiple sinkholes enlarge and merge together due to continuous dissolution and collapse of roofs, they form larger, compound depressions known as Valley Sinks or Uvalas.

Limestone is highly permeable and soluble. Surface water quickly widens cracks and joints through chemical solution (carbonation). This creates vertical swallow holes through which the entire surface river may disappear underground, flowing through caves and fissures instead of on the surface.

These forms are called Moraines.

• Terminal Moraines: Ridges of debris at the end (snout) of the glacier.
• Lateral Moraines: Ridges running parallel along the sides of the glacial valley.
• Medial Moraines: Formed in the center of the valley when two lateral moraines merge.
• Eskers: Long, winding ridges of sand and gravel located on the valley floor (formed by sub-glacial streams).

Wind acts by Deflation (lifting loose particles), Abrasion (sandblasting rocks), and Attrition.
Is it the only agent? No. While wind is prominent, rain/sheet floods are also crucial. Though rain is rare in deserts, when it occurs, torrential downpours cause significant erosion (badlands) because there is no vegetation to check the flow.

Running water dominates landform shaping globally:

• In Humid Climates: Rainfall is heavy and regular. Rivers act constantly, cutting valleys (V-shaped), forming gorges, meanders, and vast floodplains. The entire landscape is degraded by overland flow and linear flow (streams).
• In Arid Climates: Though rain is scarce, it is torrential. The lack of vegetation exposes the soil. When it rains, sheet floods move large amounts of debris. Ephemeral rivers shape wadis and pediments. Wind modifies these features, but the primary landscape sculpting (like canyons or badlands) is often initiated by water.

Why different? Limestone is a weak rock in humid climates because it dissolves in water ($CaCO_3$ reacts with Carbonic acid). In arid climates, devoid of moisture, limestone is resistant and stands out as strong ridges.
Dominant Process: Solution (Chemical Weathering/Carbonation).
Results (Landforms):

• Erosional: Sinkholes, Lapies, Caves, Uvalas.
• Depositional: Stalactites (hanging from roof), Stalagmites (rising from floor), and Pillars in caves.

Glaciers act as massive “ploughs” through the following sequence:

• Plucking and Abrasion: Glaciers pluck rocks from the sides and floor and use this debris to abrade (scrape) the valley, creating steep U-shaped valleys.
• Headward Erosion: Cirques cut backward into the mountain peaks. When cirques from opposite sides converge, they form sharp peaks (Horns) and knife-edge ridges (Arêtes).
• Degradation: Over thousands of years, this continuous grinding lowers the mountain heights, transforming rugged peaks into rounded hills (Roche Moutonnée) and eventually depositing vast sheets of debris (Outwash Plains), flattening the topography.

(Guidance for Student Activity)
Observe your surroundings:

• If in Mountains: Look for V-shaped valleys, waterfalls, or landslides (Process: Running Water/Gravity).
• If in Plains: Look for river meanders, floodplains, or alluvial soil (Process: Deposition by River).
• If near Coast: Look for beaches or cliffs (Process: Waves).
• If in Desert: Look for sand dunes or mushroom rocks (Process: Wind).