Shiwalik

Also called: Outer Himalaya, Siwalik, Shivalik · Topic: Structure and Physiography of India: Himalayas, Plains, Peninsula, Coasts and Islands · NCERT: Class 7, Ch 1 "Geographical Diversity of India"; Class 11, Ch 2 "Structure and Physiography"; Class 11, Ch 3 "Drainage System"

Meaning

The Shiwalik (also called the Outer Himalaya, Sub-Himalaya, Siwalik or Shivalik) is the southernmost and youngest range of the Himalaya. It is about 900-1,100 m high and 10-50 km wide, and it is made of unconsolidated (loose, not cemented) river gravel and boulders. It rises straight up from the Indo-Gangetic plain.

It matters because its loose rock wears away very easily. This causes landslides, chos and badlands. Long, flat valleys called duns, such as Dehra Dun, lie behind it, and cities have grown in them.

Explanation

Place in the Himalayan system

• The Himalaya is a young fold mountain system. A fold mountain forms when two land masses push together and the rock layers between them crumple upward.
• It rose in the Tertiary Himalayan orogeny (a period of mountain building), when the Indian plate pushed into the Eurasian plate.

• It runs about 2,500 km from east to west [3] (NCERT gives about 2,400 km) and is 200-400 km wide [3].

• The ranges run side by side. From north to south they are the Trans-Himalaya, the Greater Himalaya (Himadri), the Lesser Himalaya (Himachal) and the Shiwalik [3].

• Height falls as you go south:
• Greater Himalaya: about 6,000 m
• Lesser Himalaya: 3,700-4,500 m [2]

• Shiwalik: 900-1,100 m. Crests reach 900-1,500 m in places but seldom go above 2,000 m [3].

• Age: the Shiwalik rocks were laid down from the middle Miocene to the late Pleistocene [4]. This makes them the youngest rocks in the Himalaya.

Boundaries: the thrust faults

• A thrust fault is a crack in the Earth's crust where one block of rock is pushed up and over another. All the main Himalayan thrusts slope down towards the north [4].
• Southern edge: the Himalayan Frontal Thrust (HFT, also written MFT) separates the Shiwalik from the Indo-Gangetic plain [4].
• Northern edge: the Main Boundary Thrust (MBT) is where the Lesser Himalaya meets the Shiwalik [4].
• The rocks still move.
• India keeps pushing north.
• Along the Main Himalayan Thrust (the boundary between the two plates), rocks slip by about 12-21 mm a year [4].
• Strain builds up and is released as earthquakes.

How it formed and why it erodes so fast

• Cause: Himalayan rivers carried gravel and boulders down from the higher ranges and dropped them at the mountain front.
• Process: later these loose layers were lifted up into a range. They were never hardened into solid rock.
• Result: heavy monsoon rain on bare slopes washes them away easily. Soil erodes fast and landslides are common.

Landforms of the Shiwalik

• Duns:
• A dun is a long, flat valley between the Shiwalik and the Lesser Himalaya.
• It runs parallel to the ranges, so it is a longitudinal valley (a valley that runs along the length of the mountains).

• How duns formed:

  • When the Shiwalik rose, it blocked rivers flowing south.
  • Lakes formed behind it and filled with sediment.
  • Later the rivers cut through the Shiwalik, and the lakes drained away. Flat valley floors were left behind.

• Chos:

• These are seasonal streams (streams that flow only in the rainy season) that cut the bare southern slopes of the Shiwalik in Punjab and Himachal Pradesh.
• Forests here were cleared, so rainwater runs off fast.
• The chos carve deep gullies and turn the land into badlands (land so cut up by gullies that it cannot be farmed).

Where it occurs

• India: it is the outermost belt of the Himalaya, rising straight up from the Indo-Gangetic plain along the HFT [4].
• Duns:
• Dehra Dun is in Uttarakhand.

• Kotli Dun and Patli Dun are other examples.

• Chos and badlands: found on the southern Shiwalik slopes in Punjab and Himachal Pradesh.

• Next to it:
• To the north is the Lesser Himalaya (Pir Panjal, Dhauladhar and Mussoorie ranges in India, and the Mahabharat range in Nepal), beyond the MBT [4].
• To the south is the Indo-Gangetic plain, beyond the HFT [4].

Don't confuse with

• Lesser Himalaya (Himachal): it is higher (3,700-4,500 m [2]) and older, and it has the hill stations (Shimla, Mussoorie, Nainital, Darjeeling). The Shiwalik is lower (900-1,100 m) and is made of loose river gravel.
• Dun vs Kashmir valley: both are longitudinal valleys. Duns lie between the Shiwalik and the Lesser Himalaya. The Kashmir valley lies between the Pir Panjal and the Greater Himalaya.
• HFT vs MBT: the HFT is the southern edge of the Shiwalik (against the plain). The MBT is its northern edge (against the Lesser Himalaya) [4].
• Bhabar: this is a belt of pebbles in the plains at the foot of the Shiwalik, where rivers drop their load. It is part of the plain, not a mountain range. The Shiwalik is gravel that has been lifted up into a range.

Prelims Hooks

• Order from north to south: Trans-Himalaya → Greater Himalaya (Himadri) → Lesser Himalaya (Himachal) → Shiwalik (Outer Himalaya). The Shiwalik is the southernmost and youngest range.
• Order of thrusts from south to north: HFT (plain/Shiwalik) → MBT (Shiwalik/Lesser) → MCT (Lesser/Greater) → South Tibetan Detachment (Greater Himalaya/Indus-Tsangpo Suture Zone) [4].
• Rock and age: unconsolidated river gravel and boulders, laid down from the middle Miocene to the late Pleistocene [4].
• Size: about 900-1,100 m high and 10-50 km wide. Crests seldom go above 2,000 m [3].
• Trap: duns lie between the Shiwalik and the Lesser Himalaya, not between the Lesser and the Greater Himalaya. Examples are Dehra Dun (Uttarakhand), Kotli Dun and Patli Dun.
• Chos: seasonal streams on the Shiwalik in Punjab and Himachal Pradesh that create badlands.

Mains Points

• Loose rock makes this belt hazard-prone (GS-I/GS-III):
• Unconsolidated gravel and heavy monsoon rain cause fast soil erosion and landslides.

• Cutting the forests has let chos carve the land into badlands in Punjab and Himachal Pradesh. Planting trees again and checking gullies are the main ways to fix this.

• Earthquake risk at the mountain front:

• The Shiwalik lies between the HFT and the MBT.
• The plate boundary slips about 12-21 mm a year, so strain builds up for great earthquakes [4].

• Building codes and land-use zoning in the duns and foothill towns must follow these fault belts.

• Settlement in the duns:

• Flat, fertile dun floors attract farming and cities such as Dehradun.
• Unplanned building on the weak Shiwalik slopes nearby makes landslide and earthquake damage worse. Planned, hazard-aware growth is needed.

Related concepts

• Greater Himalaya
• Lesser Himalaya
• Ladakh and Zaskar ranges
• Kailash range
• Mount Everest
• Kanchenjunga
• Namcha Barwa
• Pir Panjal range

Read more

• The parallel ranges: Trans-Himalaya, Greater Himalaya, Lesser Himalaya and Shiwalik