Landforms and their Evolution — Practice

Question 1

Explain the formation of an ox-bow lake from a meander. Include: (a) the process that creates a meander, (b) how the neck narrows, and (c) how the ox-bow lake is finally isolated.

Accepted Answer

(a) **Meander formation**: Water flows faster on the outer bank of a river bend (longer path, same volume = higher velocity). Faster water erodes the outer bank (river cliff). On the inner bank, slower water deposits material (point bar). This makes the bend more pronounced — a meander loop.

(b) **Neck narrowing**: As erosion continues on both outer banks of adjacent meander loops, the narrow land between them (the neck) gets progressively thinner. During floods, higher water levels cover the neck temporarily.

(c) **Ox-bow lake formation**: During a major flood, the river takes the shorter, straight path across the neck — cutting off the old meander loop. The new straight channel carries the river; the old loop's ends gradually silt up. The isolated loop becomes an ox-bow lake — shaped like an ox's neck yoke. In India, ox-bow lakes in the Ganga floodplain of Bihar are called 'jheel' and are important fish habitats.

Question 2

Distinguish between a V-shaped valley and a U-shaped valley. What agent formed each? For each type of valley, describe TWO associated landforms found nearby.

Accepted Answer

**V-shaped valley (Fluvial)**:
- Agent: River (fluvial erosion)
- Formation: River flowing downslope cuts vertically into bedrock. Vertical erosion > lateral erosion in mountain stage. Side slopes erode by mass movement and slope wash, creating the V-shape.
- Profile: Narrow, steep-sided, V in cross-section. River occupies full valley floor.
- Associated landforms: (i) **Waterfall** — river drops over resistant rock ledge; (ii) **Gorge** — very steep, narrow V-valley cut through hard rock (e.g., Beas Kund gorge in Himachal).

**U-shaped valley (Glacial)**:
- Agent: Glacier (glacial erosion — plucking + abrasion)
- Formation: Glacier fills a pre-existing river valley, erodes all three surfaces (floor + two walls) equally by abrasion and plucking. Creates characteristic flat floor + steep, straight sides = U-shape.
- Profile: Wide, flat-floored, steep-walled. Glacier fills entire valley; after glacial retreat, a ribbon lake or river occupies the floor.
- Associated landforms: (i) **Hanging valley** — a smaller tributary glacier occupied a higher valley; when glaciers retreat, the tributary valley hangs above the main floor, creating a waterfall (Roopkund area, Uttarakhand); (ii) **Moraines** — unsorted glacial debris deposited along the valley sides (lateral moraine) and at the glacier's snout (terminal moraine).

Question 3

Explain the sequence of coastal erosional landforms from a wave-cut cliff to a stump. Draw a labelled sketch showing this sequence. What determines how quickly this sequence progresses?

Accepted Answer

**Coastal Erosion Sequence**:

1. **Wave-cut notch**: Waves attack the base of a coastal cliff, excavating a notch. Process: hydraulic action (water pressure in cracks) + corrasion (pebbles used as cutting tools) + solution (chemical dissolution of limestone cliffs).

2. **Cliff**: The overhanging rock above the notch collapses. The exposed, near-vertical face = wave-cut cliff. The flat platform cut by waves at the cliff base = wave-cut platform.

3. **Cave**: Weakness zones (joints, fault lines) in the cliff face are preferentially attacked by waves, forming a cave — deeper than wide, usually at sea level.

4. **Arch**: When caves form on BOTH sides of a headland and break through, the roof remains → natural arch. The sea passes UNDER the arch.

5. **Stack**: The arch roof collapses from wave erosion and weathering. The outer portion of the headland is now isolated as a column = sea stack. Example: Old Man of Hoy (Scotland); similar features on India's Konkan coast.

6. **Stump**: The isolated stack continues to be eroded at its base by waves. It collapses to sea level or just below = stump (visible only at low tide).

**[Sketch description: horizontal sea on left → stack (isolated column) → arch (tunnel with rock above) → cave (hollow in cliff face) → notch at base → cliff with hatching → wave-cut platform at sea level]**

**Factors affecting rate**:
(i) Rock hardness: soft rock (sandstone, chalk) erodes in decades; hard rock (granite, basalt) takes millennia.
(ii) Wave energy: exposed coasts facing prevailing winds (India's west coast during SW monsoon) erode faster.
(iii) Joint/fault density: heavily fractured rock gives waves more attack points.
(iv) Sea level change: rising sea brings waves to higher cliff faces, accelerating erosion.

Practice — Landforms and their Evolution

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