The northern mountains, the southeastern continuation of the uplift process that formed the Himalayas, extend southward along the Thai-Myanmar border and reach as far south as northern Malaysia. Long granitic ridges were formed when great masses of molten rock forced their way upward through the older sedimentary strata. Peaks average about 5,200 feet (l,585 metres) above sea level. Mount Inthanon, at 8,481 feet (2,585 metres) the highest in the country, is in northwestern Thailand, near the ancient city of Chiang Mai (Chiengmai); the city is overshadowed by Mount Suthep, a tourist attraction and site of the royal summer palace. Some of the rugged limestone hills contain caves from which remains of prehistoric humans have been excavated.
The Khorat Plateau is a vast tableland bounded by the Mekong River on the north and east. It was formed by uplifting along two perpendicularly arranged crustal faults—one trending north-south in the west and the other east-west in the south. As a result, the underlying sedimentary rocks were tilted rather than uniformly uplifted. This tilting created ranges of low hills and mountains along the western and southern edges of the plateau: the Phetchabun and Dangrek (Thai: Dong Rak) mountains, respectively. The escarpments of these uplands overlook the plain of the Chao Phraya basin to the west and the Cambodian plain to the south. Surface elevations on the Khorat Plateau range from about 650 feet in the northwest to about 300 feet in the southeast: the terrain is rolling, and the hilltops generally slope to the southeast in conformity with the tilt of the land.
Situated between the northern and western mountain ranges and the Khorat Plateau is the extensive Chao Phraya River basin, which is the
cultural and economic heartland of Thailand. The region, sometimes called the Central Plain, consists of two portions: heavily dissected, rolling plains in the north and the flat, low-lying floodplain and delta of the Chao Phraya in the south. It was formed by the outwash of immense quantities of sediment brought down from the mountains by the Chao Phraya's tributaries, which produced vast fan-shaped alluvial deposits.
The generally rolling countryside of the southeast has high hills in the centre and along the eastern boundary with Cambodia. Notable peaks are Mount Khieo (2,614 feet), visible from the top of Mount Phu Thong (the Golden Mount) in Bangkok, and Mount Soi Dao, which attains a height of 5,471 feet. The hills, reaching nearly to the sea, create a markedly indented coastline fringed with many islands. With their long stretches of sandy beach, such coastal towns as Chon Buri and Rayong and some of the islands have become popular year-round tourist resorts.
The topography of the peninsula is rolling to mountainous, with little flat land. A gently sloping sandy coastline, including such resort communities as Hua Hin, borders the
Gulf of Thailand on the east. Higher mountains reaching about 4,900 feet line the peninsula on the west and contain narrow passes linking Thailand and Myanmar. These ranges separate the Andaman and South China seas as the peninsula narrows near the Malaysian border. Off the rugged and much indented west coast lie numerous large islands, including tin-rich Phuket Island. - Britannica Encyclopedia
