The dramatic little mountains you see more an more of as you head South on the Thai Southern Line are called ‘karst’, and they’re pretty weird.
These Karst mountains are made of limestone – a sedimentary rock composed mainly of shells or skeletons of marine organisms such as corals and moluscs and microscopic marine creatures. They leave their shells behind when they die, which build and build over generations to create mounds and reefs in shallow waters (like that of the Gulf of Thailand). Then sometimes, techtonic plates – large surface chunks of the Earth’s crust – push against each other which can force these reefs and limestone seabeds up out of the water over millions of years to become mountains and hills, like the Tennaserim range we see inland.
But when up pushed up into the dry air, limestone is dissolved by the weak acids in rainfall or rivers and lakes. Over millions years more, these acids find weaknesses in the limestone and filter through to form subterranean sinkholes, caverns and drainage systems underneath the surface rock. This then weakens and cracks the surface rock, which widens by further rainfall, vegetation, and wind.
Eventually, all of the limestone rock erodes down into a coastal plain such as that which we’re travelling through. The peaks we see here are the few holdouts in this area – yet to complete the process of breaking down.
When we think about Thailand and Southeast Asia in general, these limestone karst mountains are frequenly what comes to mind. But they are found throughout Asia: They are the quintessential image of Guilin in China, and Ha Long Bay in Vietnam, for example.
The more spectacular formations in Thailand are found further south in the Krabi region. But you will invariably find spectacular little worlds within almost every limestone karst mountain, even up here. When the acids break down the primary skeletons and shells stacked in the mountains, it forms solutions that trickle through into subterranean caves and drainage systems, where they often deposit as stallactites and stallacmites, and a menagerie of other structures based purely on the positions, angles, and distances of regular dripping and trickling. These caves are often inhabited by strange creatures that are oddly adapted to living in them – particularly by losing their sight and acquiring incredible organs that are sensitive to vibrations and smell. Their little ecosystems can create further layers of weirdness in this surreal world. Often these little ecosystems are so contained and isolated from each other, that they could contain their own endemic species. This is also true for the ecosystems covering the outside of karst mountains.
A final layer of weirdness can come from the way humans interract with these places – such as bird nest harvesting, and erecting artefacts for religious worship.
So even if nature, and particularly caves aren’t your thing, I really think you have to visit one of these surreal landscapes at least once in a good Thai holiday. Amidst the karst mountains of Southern Thailand, good options are plentiful.
Get the audio guide
Download the Window Seater app - your offbeat guide to the world outside your window