This area consists of a sparsely inhabited strip of the Central Cordillera along the Burma boundary from about 18° north latitude to about 12° east to the plain of the Čhaophraya. It includes high rugged mountains in which streams have cut deep canyons and narrow valleys. The Thanon Thongchai Range coming from the Northern Hills and Valleys region ends at about 16° north latitude and divides into three distinct sub-parallel ridges. The westernmost ridge, the Tanaosi or Tenasserim, begins at Three Pagodas Pass (15° 18′ north) along the Thailand-Burma border and extends south along the western boundary of Thailand into the Peninsula. A second ridge of bare limestone crags, the Maeklong Mountains, lies between the Khwae Noi and Khwae Yai River Valley. The third ridge lies between the Khwae Yai and the Central Valley. A number of peaks rise above 5,000 feet, and a smaller number above 6,000.
The Western Mountains are the watershed of a number of tributaries of the Čhaophraya and the Salween, as well as the Maeklong. The streams generally flow through precipitous canyons or restricted valleys rarely more than a mile or a mile and a half wide. The single exception is in the Maesot basin, which is a considerable widening in the Valley of the Moei, a tributary of the Salween. Two other main rivers of the Western Mountains, the Khwae Noi and the Khwae Yai, join near Kančhanaburi in the Central Valley to form the Maeklong.
The Maeklong Mountains graphically exhibit the morphological relationships of the Western Mountains to their eastern piedmont regions, which are part of the marginal area of the Central Valley. As the drainage basins of the Khwae Yai and Khwae Noi unite to form the Maeklong, the mountains completely disappear. The high lands are spatially limited by the expansion of the surrounding plains--the true alluvial plains. In these plains, a long series of mountain stocks and inselberg-type hills follows the strikes of the folded structures. The alluvial plains gradually become narrower and narrower as one proceeds up the Khwae Yai and the Khwae Noi. Along the eastern tributary, the Khwae Yai, the valley is wider than along the western tributary, the Khwae Noi. And along this smaller, western tributary the fall of the stream is steeper. These differences correspond with the greater height of the Tanaosi Range to the west, which culminates in a peak of 4,300 feet. Within the Maeklong Mountains a relic of old flat topography with low relief lies between Sisawat and Sankhlaburi. Older, mature hill land, at an elevation of 2,000 feet, is surrounded by mountains about 3,000 feet high, which are mostly stocks of limestone.
Farther to the west in the schistose zone of the Tanaosi and other mountains along the boundary with Burma, no relics of old mature topography have been observed. There are no limestones through which water can percolate. The range is exposed on the west to the southwest monsoon which brings great quantities of rain for erosion and denudation. And the powerful tributaries of the Khwae Noi, because of the steep slopes and the large quantities of water, have cut their channels headward most intensively and closest to the highest elevations. As a result, in the most elevated portions only young landscape forms occur, and a long row of high peaks characterize the mountains along the boundary with Burma.