Broadly speaking, the line of latitude 17°N separates the north of the country from the rest of Thailand. Characteristic of this region are the foothills of the south-east Asian mountain ranges, the highest point being Doi Inthanon, some 50 km (30 mi.) from Chiang Mai; at 2565 m (8415 ft) it is the highest mountain in Thailand.
In the valleys of northern Thailand the rivers Nan, Yom, Ping and Wang flow southwards. Protected from the wind, large and small basins snuggle between the mountains, providing favorable conditions for a successful agricultural economy, even though large parts of the paddy-fields have to be artificially irrigated because of the varying amounts of rainfall. For many years now reservoirs and dams have been built in an attempt to guarantee an even supply of water right through the year.
Two different geological structures determine the relief, and these in turn are the result of various mountain formations. The underlying rock of northern Thailand dates from the Palaeozoic era, and there are large areas of Palaeozoic limestone and slate; mountains of limestone are characterized by precipitous rocks with numerous caves and caverns.
These old mountains were again subjected to pressure when the Himalayas were formed during the Tertiary period and molten magma (which solidified into granite) was thrown up to form flat-capped mountains. Doi Inthanon is one such mountain.
The main centers of population are the plains and basins between the mountains. The economic structure hinges mainly on agriculture, as is witnessed by the fact that most of the population live in villages rather than towns. An exception to this are the 3000 or so crude settlements which are home to the Akha, Lisu, Yao, Meo and Karen mountain tribes. Only a few of these tribesmen agreed to be housed under government resettlement programs; the vast majority have remained faithful to their age-old nomadic traditions, moving on to pastures new when the soil becomes exhausted after years of relentless cultivation. They also practice stubble burning which has tended to denude the soil.