A light-gray crystalline limestone, capping many hills and forming high cliffs, is widely distributed through the country. Many of the limestone massifs are isolated buttes or hills resting on older rocks, but limestone terrain extends over large areas in west central Thailand along the west side of Khorat. In the Peninsula, and to a lesser extent in the North, the limestone regions are long, narrow isolated belts following the lineation of mountain chains. The limestone has seemingly been folded and somewhat recrystallized, but generally to a lesser extent than the underlying metamorphic and sedimentary rocks. The thicknesses measured vary from less than 3,000 feet to as much as 13,000 feet.
The lower part of the limestone contains many thin layers and bits of chert carrying fossils of Permian age. Recrystallization of the limestone is widespread, at places forming marble. Most of the limestone ranges in color from light gray to nearly white, with many pinkish stringers. Locally, and perhaps in extensive beds, it is dolomitic. However, the Permian sequence is not everywhere limestone or marble.