The Continental Highlands consist of the entire mountainous area north, northwest, and west of the Central Valley, continuous to the border of Burma and Laos. This region may be divided into two subprovinces: the Northern Hills and Valleys (or "North Thailand"), which is all the area north of the Central Valley, above about 18° north latitude, from Maehongson to Nan čhangwat; and the Western Mountains, which is all the area west of the Central Valley, below about 18° north latitude, including Tak, Kančhanaburi, and Ratchaburi čhangwat and the western sections of several Central Valley čhangwat.
Northern Hills and Valleys
This is a region of parallel mountain ridges between which the Ping, Wang, Yom, and Nan Rivers drain southward into the Čhaophraya. These rivers flow in entrenched meanders in elongated, level-floored basins, separated by gorges which suggests a drainage superimposed from former peneplaned surfaces. To the north of the Phiphannam divide are wide, level, and often swampy basins draining northward to the Maekhong.
The many intermontane plains of this region lie at very different elevations. In the east in the basin of the Nan River from the headwaters southwards, important plains include Mu'ang Lae (1,000 feet altitude), Mu'ang Pua (750 feet), Mu'ang Rim (700 feet), and the wider basin of Mu'ang Nan (650 feet) which stretches more than 20 miles from north to south. In the drainage basin of the Yom River to the west of the Nan are the plains of Ban Oi, Mu'ang Pong (1,150 feet altitude), Mu'ang Ngao (950 feet), and Mu'ang Phrae (525 feet). There is also a series of basins in the drainage of the Wang, but above Lampang the valley widenings are too small to be of much significance, and contain few towns or market settlements. In the drainage of the Ping and its tributaries are the basins of Chiang Dao, Mu'ang Phrao, and the larger Chiangmai (7,000 feet altitude).
Along the rivers draining into the Maekhong to the north lie the intermontane basins of Mu'ang Phan, Mu'ang Phayao (about 1,500 feet), Mu'ang Thoeng (1,150 feet), and Chiangkham in the river basin of the Ing, and Mu'ang Fang (1,475 feet) and Chiangrai (1,150 feet) in the basin of the Maekok.
In all these basins the bedrock is not far below the surface of the alluvium and is often exposed in the stream beds. The low hills on the margins of the plains, which are often scattered far out into the plain, give further indication of this condition. Some of the alluvial plains have rocky thresholds at their outlet. Apparently, the resistant rock has preserved such plains, and acted as a local base level.
The development of the relief of the mountains of Northern Thailand has taken place under the influence of two different bases of erosion. The Maekhong forms the base level for the more northern part while the catchment area of the Čhaophraya serves this purpose in the south.
In the north, two main rivers are tributary to the Maekhong. The first is the Maekok, which flows from the north out of the Shan States ( Burma) and breaks through the Daen Lao Range. Soon after it crosses the Thai border, the Maekok receives the Fang from the south and later, below Chiangrai, the Lao. In the drainage system of the Fang there is an especially wide basin at Mu'ang Fang which is about 30 miles from north to south. The second of the principal tributaries of the Maekhong is the Ing, which flows in an irregular course, first southward through the broad valleys of Mu'ang Phan and Ban Maechai, then eastward through the valley of Mu'ang Phayao, and finally northeastward through the basins of Mu'ang San and Thoeng to the Maekhong. These various rivers all exhibit the same characteristic of flowing alternately through wide basins in entrenched meanders and through narrow, deeply incised gorges. Both the Maekok and the Ing, with their tributaries, have their base level controlled by the Maekhong. Between the several rivers flowing to the Maekhong stand formidable mountain ranges with peaks often exceeding 5,000 feet above sea level, and in several places 6,000 feet. West of the Fang River near the Burma border one peak reaches 7,532 feet.
Through the greater part of the Northern Hills and Valleys region flow the four principal tributaries of the Čhaophraya, with their tributaries. From west to east these are the Ping, the Wang, the Yom, and the Nan. The valley systems with their intermontane basins are separated from one another by mountain ranges which run generally north and south. Between the Wang and the Ping there is a large continuous range, rising northeast of Chiangmai to a height of 6,601 feet. Between the Ping and its tributary, the Chom, is a similarly continuous high range. Its greatest peak, Doi Inthanon, 8,452 feet, is probably the highest in all Thailand. Eastward, in the drainage basins of the Wang and the Yom, elevations are lower, continuous ranges of resistant granite are lacking, and the highest peaks are of Permocarboniferous limestone which, forming characteristic steep peaks, may seem to tower over the otherwise weak residual rocks of the ranges. While dramatic in appearance, these peaks seldom exceed 3,500 feet altitude. A typical example of such a limestone peak is Khao Phadung, east of Phrae, which is 3,556 feet. The entire mountainous region of the Čhaophraya watershed and the intervening intermontane plains is called the Phiphannam or "the ghost of a thousand waters."
Farther to the east in the watershed between the Nan and the Maekhong, there are again higher elevations. Northeast of Mu'ang Nan, peaks rise to 5,500 feet. And the range on the boundary between Thailand and Laos rises to more than 6,500 feet. Its highest known peak is recorded as 7,546 feet. In this most northeasterly part of Northern Thailand there are no intermontane basins. This is a young mountain landscape of strong relief, and the valleys of the larger rivers have cut down to depths of 1,300 to 1,475 feet. They are steep-sided and narrow and contain very few widenings.
While we can follow the complexities of the Central Cordillera en echelon far south into the Peninsula, those of the other ranges of the northern Thai mountains are much less clear, for the region has been insufficiently mapped.
Also in North Thailand is part of the watershed of the Salween. This area is principally within Mahongson čhangwat and includes the valleys of the Maerai, Yuam, and Thauguin Rivers, all minor tributaries of the Salween. The Thauguin marks the Thailand-Burma border for some distance, as does the Salween itself. These valleys are within ranges of the Central Cordillera and are generally narrow and steep-sided, without intermontane plains. They are more or less isolated from the rest of North Thailand.