Where the Southeast Asian knot of mountains is narrowest, between the low plains of Assam in the west and Szechwan in the east, the Minya Konka massif rises to over 20,000 feet. Toward the south it slopes off and divides into four mountain systems which stretch out from each other like the arms of a polyp--the Arakan Chain along the western edge of Burma; the Central Cordillera, extending into the Malay Peninsula; the Cordillera of Annam; and the South Chinese mountains. Between these mountain systems lie three low plains. The most westerly is occupied by the Irrawaddy River system, the most easterly by the Gulf of Tongking and the plains of lower Tongking (North Vietnam). In the middle, between the Annam Cordillera and the Central Cordillera, lies a similar but wider and deeper depression which contains the two most important landscapes of Thailand, the alluvial plains of the èhaophraya and the expanse of Khorat.
The lower part of the Čhaophraya basin, or great Central Valley of Thailand, contains the Bangkok Plain, a continuation of the Gulf of Thailand. The plain lies just above sea level. The adjacent Khorat region to the east is on the average 300 to 500 feet higher. Mountain systems on the west and on the east unite in northern Thailand and then, with increasing elevations, combine with the mountain complex of Yunnan. The Central Cordillera, or Western Mountains region of Thailand, which bounds the western side of the Central Valley, continues southward the length of the Peninsula and on into the Malayan Federation.
While Thailand contains a great variety of topography, there are, in general, three physiographic types: the highlands, the plains of thin alluvium over rock, and the low alluvial plains. Highlands extend along the western side of the country and occupy much of the North. Down the Peninsula mountains stand en echelon, and there are more mountains in the Southeast. Most of Khorat is shallow alluvium over bedrock. The Central Valley is deltaic alluvium, and narrow alluvial plains border the entire coast of the Peninsula and the Southeast between the mountains and the sea.
Thailand may be divided into five physiographic regions: The Central Valley, subdivided into the Bangkok Plain, the Upper Plain, and the Marginal Plains; the Continental Highlands, subdivided into the Northern Hills and Valleys and the Western Mountains; Khorat; Southeast Coast; and the Peninsula, subdivided into the East Coast region and the West Coast region. While these regions are based upon similar internal landform types, the internal similarity does not end there. This regional division of Thailand also is applicable to a considerable degree to climate, mineral resources, landuse and agriculture, historical development, language, ethnic grouping, and other physical and cultural features.