I found this information on the internet:
These limestone karsts were formed 260 million years ago when a shallow sea, running the entire length of south Asia, slowly built up deep deposits of coral and shells which were later buried under sediments washing in from the land.
Compressed deep in the earth these calcium carbonate remains formed limestone. This was thrust up above the surface when the Indian subcontinent collided with mainland Asia some 30 million years ago. From Gualin in China, to Sarawak in Borneo, the limestone created craggy islands on land and in water. In the South of Thailand, the provinces of Surat Thani, Phang Nga, Krabi and Trang have the most spectacular examples of this type of scenery.

