The correct option is 2 and 3 only.
Explanation
Valleys are significant erosional landforms created by the action of running water. Their evolution follows a specific developmental sequence, and their final morphology is determined by the geological structure and the intensity of erosion.
Statement 1 is Incorrect:
The evolution of a valley does not begin with a canyon. Instead, the process starts with small, narrow channels called rills. These rills gradually develop into larger and wider gullies. Over time, gullies deepen, widen, and lengthen to give rise to valleys. A canyon is a specific type of deep valley that forms at a later stage or under specific geological conditions, not at the beginning of the cycle.
Statement 2 is Correct:
Valleys are classified based on their dimensions and cross-sectional shape. Common types include:
- V-shaped valleys: Characterized by steep sides, typically formed in the youthful stage of a river due to vertical erosion.
- Gorges: Deep valleys with very steep to straight sides, where the width at the top and bottom is nearly equal.
- Canyons: Deep valleys with steep, step-like side slopes, usually wider at the top than at the bottom.
Statement 3 is Correct:
The specific type of valley formed is heavily dependent on the type and structure of the underlying rocks. For example, canyons typically form in horizontally bedded sedimentary rocks where resistant and soft rocks alternate, creating step-like slopes. Gorges are more likely to form in hard, resistant rocks where vertical erosion dominates over lateral erosion.
Key Takeaway:
Valley formation follows the sequence: Rills → Gullies → Valleys. The distinction between a gorge and a canyon is primarily based on the slope characteristics and the geological structure of the bedrock.