The Lahaul-Spiti valley in Himachal Pradesh remained cut off from the rest of India for nearly six months every year before a major tunnel was constructed by the Border Roads Organisation. (i) What geographic factor caused this seasonal isolation? (ii) How does tunnel construction address this problem better than a surface road through the same mountain pass?
Generated by claude-sonnet-4-6 · 2026-06-26 13:36 · grounding rag
Model Answer
(i) The Lahaul-Spiti valley was cut off due to heavy snowfall in the Pir Panjal range of the Himalayas. Snow blocked the mountain passes (like Rohtang Pass), making surface roads impassable for about six months every year.
(ii) A tunnel passes through the mountain at a lower altitude (3,000 m MSL), avoiding the high-altitude snow-covered passes entirely. Unlike a surface road over a pass, the tunnel is sheltered from snowfall and avalanches, making it an all-weather route that keeps the valley connected throughout the year. The Atal Tunnel (9.02 km), built by BRO, achieves exactly this.
Source: Border Roads, Chapter 7 (Roadways section)
---
Explanation
- The passage explicitly states "heavy snowfall" as the cause — use this exact phrase.
- For part (ii), the key contrast is surface road over a pass vs. tunnel through the mountain — the tunnel avoids exposure to snow. Mentioning "all-weather" is important as that is the textbook's implied benefit.
- Mentioning the Atal Tunnel's name, length, and BRO adds precision and often earns full credit.
- Don't over-explain; 2 marks' worth of content is enough for part (ii).