Himalayas and Northern Plains: Ranges, Passes and Belts

India's physical build is usually divided into six units: the Himalayan mountains, the Northern Plains, the Peninsular Plateau, the Indian Desert, the Coastal Plains and the Islands. The first two of these form one story, because the plains are made of the material the mountains shed. This note covers the Himalayas and the Northern Plains together, in the order in which a student meets them on a map, from the snow line down to the flood plain.

How the Himalayas were formed

The Himalayas are young fold mountains. The Indian plate, drifting north, collided with the Eurasian plate, and the sediments that had collected in the shallow Tethys Sea between them were squeezed, folded and thrust upward. Because the movement has not stopped, the range is still rising and the region remains seismically active. The Himalayas run in an arc of about 2,400 kilometres from the Indus gorge in the west to the Brahmaputra gorge in the east. The belt is about 400 kilometres wide in Kashmir and narrows to about 150 kilometres in Arunachal Pradesh.

The three parallel ranges

The longitudinal valleys between the Shiwaliks and the Lesser Himalaya are called Duns. Dehra Dun, Kotli Dun and Patli Dun are the examples asked most often. The Kashmir valley, between the Pir Panjal and the Greater Himalaya, carries thick lacustrine deposits called Karewas, in which saffron is grown.

Regional divisions, west to east

Rivers, not states, divide the Himalayas for geographers. The Punjab Himalaya lies between the Indus and the Satluj, the Kumaon Himalaya between the Satluj and the Kali, the Nepal Himalaya between the Kali and the Tista, and the Assam Himalaya between the Tista and the Dihang. Beyond the eastern bend of the Brahmaputra the ranges swing south as the Purvanchal or Eastern Hills, made up of the Patkai Bum, the Naga Hills, the Manipur Hills and the Mizo or Lushai Hills.

The Trans-Himalayas, peaks and passes

North of the Greater Himalaya lie the Trans-Himalayan ranges: the Karakoram, the Ladakh and the Zaskar. The Karakoram holds K2, also called Godwin Austen, at 8,611 metres the highest peak in the Indian subcontinent, and the Siachen glacier. Mount Everest, at about 8,848 metres in Nepal, is the highest peak on earth. Kanchenjunga, 8,586 metres, in Sikkim, is the highest peak in India's undisputed territory, and Nanda Devi, 7,816 metres, in Uttarakhand, lies wholly within India. The passes are a favourite question. Zoji La leads from Srinagar towards Leh; Banihal carries the road from Jammu to Srinagar; Rohtang, Baralacha La and Shipki La lie in Himachal Pradesh; Mana, Niti and Lipulekh in Uttarakhand; Nathu La and Jelep La connect Sikkim with Tibet; and Bomdi La and Bum La lie in Arunachal Pradesh.

The Northern Plains

The Northern Plains were built by the alluvium of three river systems, the Indus, the Ganga and the Brahmaputra, together with their tributaries. The plain stretches about 2,400 kilometres from west to east and is 240 to 320 kilometres broad, covering roughly seven lakh square kilometres. Its slope is so gentle that the rivers meander and change course, and the depth of alluvium runs to hundreds of metres. Three of its rivers, the Indus, the Satluj and the Brahmaputra, are older than the mountains they cut through, which is why they flow in deep gorges.

The four belts of the plain

• Bhabar: a narrow belt of pebbles, 8 to 16 kilometres wide, laid along the foot of the Shiwaliks, where streams sink out of sight into the porous deposit.
• Terai: south of the Bhabar, a wet and marshy strip where those streams re-emerge, once thickly forested and rich in wildlife.
• Bhangar: the older alluvium, standing above the flood level in terraces and containing lime nodules called kankar.
• Khadar: the newer alluvium of the flood plains, renewed almost every year and the most fertile soil of the plain.

Regionally the plain is divided into the Punjab Plains, the Ganga Plain and the Brahmaputra Plain. The Punjab Plains were formed by the Indus and its tributaries and are cut into doabs, the strips of land between two rivers, such as the Bari, Bist, Rachna and Chaj doabs. The Ganga Plain, the widest part, runs across Uttar Pradesh, Bihar, Jharkhand and West Bengal. The Brahmaputra Plain lies mostly in Assam and holds Majuli, the largest river island in the world.

Why these two units matter

The Himalayas block the cold winds of Central Asia, force the monsoon to give up its rain over India and feed the perennial rivers from snow and glaciers. The plains, formed from what the mountains lose, carry the deepest and most fertile soils in the country and hold the densest population, which is why India's most intensive agriculture and the oldest river-based towns are found there.