UKPSC Prelims 2025 General Studies Paper-I · Question 84 of 149

Correct — C, Unconsolidated sediments.

The Shiwalik is not a slice of old crust lifted into a ridge. It is the waste of the Himalaya itself. As the mountains rose, rivers stripped them and dumped gravel, sand, silt and clay in a trough along the mountain front.

Later uplift folded that pile of river debris into low hills. So the material of the range is Unconsolidated sediments — young, loosely packed, laid down grain by grain rather than crystallised or recrystallised.

The idea to carry away is that the Shiwalik is built from the Himalaya's own eroded debris. That single fact explains its position at the very front, its youth, and the ease with which its slopes gully and slide.

The older, pre-rationalisation NCERT physiography chapter put it in these words: "These ranges are composed of unconsolidated sediments brought down by rivers from the main Himalayan ranges located farther north."

• (a)
  Igneous rocks — Igneous rock forms where magma or lava cools and crystallises. Shiwalik beds were transported by water and deposited — a depositional origin, not a molten one.

  This label fits the Deccan Trap basalts of peninsular India, poured out as lava flows long before the Shiwalik beds were laid down.

• (b)
  Consolidated rocks — This is the near-miss. Shiwalik beds do carry rock names — sandstone, conglomerate — but they are the youngest sequence in the Himalayan system and are poorly cemented, so the range behaves as loose material.

  Hard, compacted and folded rock is what the Lesser Himalaya is made of: the limestone, slate and quartzite of ranges such as the Mussoorie range north of Dehra Dun.

• (d)
  Metamorphic rocks — Metamorphic rock has been recrystallised into new minerals by heat and pressure at depth. Shiwalik material has been tilted and folded, but it has not been cooked in that way.

  Gneiss, schist and migmatite describe the crystalline core of the Great Himalaya, the belt that carries the highest peaks.

A mountain range can be classified by what it is made of, and that answer usually records how the range was built.

Ranges raised by squeezing and lifting older crust expose hard material — crystalline, metamorphic, or well-cemented sedimentary rock.

Ranges built out of erosional waste expose the opposite: gravel, sand, silt and clay, shed by a rising chain, carried out by its rivers, piled into a basin at its foot and only partly compacted before being uplifted again.

The Shiwalik is the second kind. It is a molasse pile — the debris apron of the Himalaya, later folded into hills.

• The Shiwalik is the southernmost and youngest of the Himalayan divisions, lying between the Lesser Himalaya and the Indo-Gangetic plain.
• Its material is detritus eroded from the rising Himalaya and deposited by rivers in a foreland trough along the mountain front.
• The beds range broadly from Miocene to Pleistocene in age, which is why they have had little time and burial to harden.
• The Main Boundary Thrust marks the northern limit of the Shiwalik, against the Lesser Himalaya; the Himalayan Frontal Thrust marks its southern edge against the plain.
• Longitudinal valleys between the Shiwalik and the Lesser Himalaya are called duns, and their floors are filled with thick gravel and alluvium; Dehra Dun is one.
• The Shiwalik belt is 10 to 50 km wide.
• The Bhabar belt at the foot of the Shiwalik is a porous mass of pebbles and boulders, through which surface streams percolate and disappear.
• Shiwalik beds are known for a rich record of fossil vertebrates.

• Reading 'Shiwalik' as shorthand for 'Himalaya' and reaching for hard rock; the Shiwalik is the debris apron in front of the mountains, not their crystalline core.
• Taking 'consolidated rocks' as a safe middle answer because Shiwalik beds carry rock names such as sandstone and conglomerate; those beds are young and poorly cemented.
• Swapping the Shiwalik with the Lesser Himalaya ranges immediately north of it, which are built of limestone, slate and quartzite.
• Putting Bhabar and Terai on the wrong sides: Bhabar is the pebbly belt at the Shiwalik foot, Terai the wet belt further south.
• Assuming the range must be old because it is low; it is the youngest part of the Himalayan system, and its height reflects young, weak material.

• practice — not a real PYQ

  The longitudinal valleys lying between the Shiwalik range and the Lesser Himalaya are known as

  • (a)Duns
  • (b)Bhabar
  • (c)Terai
  • (d)Khadar
  Answera — Duns are the flat, gravel-filled longitudinal valleys behind the Shiwalik, Dehra Dun being the best known.

  Bhabar is the porous pebble belt at the southern foot of the Shiwalik, not a valley behind it. Terai is the marshy, spring-line belt south of the Bhabar. Khadar is newer floodplain alluvium out on the plains — a soil term, not a valley type.

• practice — not a real PYQ

  Streams descending from the Shiwalik disappear in the Bhabar belt mainly because

  • (a)the belt is floored by impermeable clay that ponds the water
  • (b)the belt is a porous mass of pebbles and boulders through which water percolates
  • (c)evaporation in the belt exceeds the flow of the streams
  • (d)the streams are diverted underground along a line of hot springs
  Answerb — The Bhabar is a coarse apron of pebbles and boulders with wide pore spaces, so surface flow sinks into it and re-emerges further south in the Terai.

  Option (a) reverses the property: impermeable clay holds water at the surface, which is the Terai's character rather than the Bhabar's. Option (c) blames evaporation, which does not account for a loss on this scale. Option (d) invents a hot-spring route in place of ordinary percolation.

• practice — not a real PYQ

  The tectonic boundary separating the Shiwalik range from the Lesser Himalaya to its north is

  • (a)Main Central Thrust
  • (b)Main Boundary Thrust
  • (c)Indus-Tsangpo Suture Zone
  • (d)Himalayan Frontal Thrust
  Answerb — The Main Boundary Thrust is the contact along which the Lesser Himalaya rides over the Shiwalik.

  The Main Central Thrust lies further north, between the Great Himalaya and the Lesser Himalaya. The Indus-Tsangpo Suture Zone marks the join between the Indian and Asian plates, beyond the Great Himalaya. The Himalayan Frontal Thrust is the Shiwalik's other edge, facing the plain.

• practice — not a real PYQ

  Which one of the following best describes the origin of the material forming the Shiwalik range?

  • (a)Lava flows erupted through fissures along the mountain front
  • (b)Marine limestone deposited in the Tethys before the collision
  • (c)Debris eroded from the rising Himalaya and deposited by rivers in a basin at its foot
  • (d)Older crustal rock recrystallised under heat and pressure at depth
  Answerc — Shiwalik beds are the erosional waste of the growing Himalaya, carried out by its rivers, laid down in a foreland trough and later folded into hills.

  Option (a) gives a volcanic origin, which fits the Deccan Traps rather than these river deposits. Option (b) describes the older Tethyan sedimentary belt north of the Great Himalaya. Option (d) describes metamorphism, which produced the crystalline core of the Great Himalaya.