Physically, the Himalayas forms three parallel zones: the Great Himalayas, the Middle Himalayas (also known as the Inner or Lesser Himalayas), and the Sub-Himalayas, which includes the Siw?lik Range and foothills and the Tar?i and Duars piedmont (an area of land formed or lying at the foot of a mountain or mountain range). Each of these lateral divisions exhibits certain similar topographic features. The Great Himalayas, the highest zone, consists of a huge line of snowy peaks with an average height exceeding 6,100 m (20,000 ft). 
The width of this zone, composed largely but not entirely of gneiss and granite, is about 24 km (about 15 mi). Spurs from the Great Himalayas project southwards into the Middle Himalayas in an irregular fashion. The Nepal and Sikkim (a state of northern India) portion of the Great Himalayas contains the greatest number of high peaks. The snow line on the southern slopes of the Great Himalayas varies from 4,480 m (14,700 ft) in the eastern and central Himalayas of Nepal and Sikkim to 5,180 m (17,000 ft) in the western Himalayas. To the north of the Great Himalayas are several ranges such as the Zaskar, Ladakh, and the Kailas. The Karakoram Range lies on the Tibetan side of the Great Himalayas.
The Great Himalayan region is one of the few remaining isolated and inaccessible areas in the world today. Some high valleys in the Great Himalayas are occupied by small clustered settlements. Extremely cold winters and a short growing season limit the farmers to one crop per year, most commonly potatoes or barley. The formidable mountains have limited the development of large-scale trade and commerce despite the construction of highways across the mountains linking Nepal. Older trails, which cross the mountains at high passes, also have limited trade and are open only during the summer months.
The Middle Himalayas region is a complex mosaic of forest-covered ranges and fertile valleys. While not as forbidding as the Great Himalayas to the north, this range has nonetheless served to isolate the valleys of the Himalayas from the plains. Except for the major valley centers such as Pokhara, and Kathmandu, and hill towns such as Namche Bazaar, Manang, Mustang, the region is moderately populated. Within the Middle Himalayas the intervening mountain ranges tend to separate the densely populated valleys. The numerous gorges and rugged mountains make surface travel difficult in any direction. Few roads or transport routes exist between towns, partly because it is expensive to build them over the high, rough terrain. Only major population centers are linked by air and roads with principal cities in Nepal.
