A thick deposit of continental sandstone and conglomerate covers most of eastern Thailand and forms the bedrock of Khorat. Similar beds occupy parts of intermontane valleys in the northwest and extend south along the west coast of the Thai Gulf. In the extreme southern part of the Peninsula are found quartzite, phyllite, and crumpled black shale from the northern extensions of beds recently mapped in Malaya as Triassic. Such an age assignment seems probable for part of the Khorat series.
The top of the beds is marked by a widespread erosional surface on which lie Tertiary and Quaternary alluvial deposits. The Khorat series includes beds of gray-green sandy shale and beds of marly, thin limestone, nodular and concretionary, as well as red and violet sandstone, conglomerate, and shale, which form the bulk of the sedimentary rocks. The conglomerate locally contains limestone pebbles and cobbles derived from Ratchaburi limestone. Wells and springs of salty water in Khorat suggest that salt deposits may be found at depth in the series.
In the vicinity of Maesot, a formation referred to as the Kamawkala is composed of dense, gray, impure limestone, quartzitic red sandstone, and conglomerate, and contains fossils of early middle Jurassic age. Limestone beds are interbedded with brown and red quartzitic sandstone and conglomerates and apparently are conformable with the underlying and overlying continental beds. These sediments appear to be a marine tongue intercalated in the Khorat series. The younger sandstones and conglomerates in the Khorat area are flat-lying or gently folded.