Weathering and alteration of the rocks are deep. There are three topographic provinces: the drainage basin of MeNam River, the Korat Plateau, and Siamese Malaya.

The Paleozoic rocks of the MeNam basin and Siamese Malaya are metamorphic. In the Korat Plateau of northeastern Siam and locally in northern Siam, there are thick beds of Triassic sandstone, gently folded. In northern Siam and Siamese Malaya there are limited basin deposits of Pleistocene beds. These beds are steeply folded in some places but are not metamorphic. The principal igneous rocks are granite laccoliths; but andesite and other intrusive rocks are present, and there are some small basaltic flows.

In the Paleozoic rocks the structures are closely folded. As a general rule the Triassic beds rarely have dips exceeding 10 or 12 degrees. The Pleistocene beds are in many places much disturbed and dips as great as forty degrees are not uncommon.

Tar seeps in the narrow MeFang Valley in the extreme north occur on the margin of a synclinal basin between Paleozoic metamorphic rocks on the west and Triassic red sandstones and granite on the east. The seeps issue through arkose outwash deposits. There are no known source beds in any of the rocks exposed, and the source of the oil is problematical. Structural conditions are favorable in the Triassic beds in a part of the Korat Plateau; but no source beds are known, and there are no indications of oil in the Triassic. No Tertiary beds are known to be present in Siam.