In Krabi the towers do not rise: the ground falls

The first impression on arriving in Krabi is that something rose out of the ground. Grey stone towers of a hundred, two hundred, three hundred metres, with walls vertical or leaning outwards, planted in the middle of a perfectly flat green plain.

The impression is exactly the reverse of what happened. Nothing went up. What the landscape did was lower everything around them.

01 A limestone two hundred and fifty million years old

The raw material is a massive limestone with dolomitic stretches that geologists group under the name Ratburi Group. It was deposited in the Permian, of the order of two hundred and fifty million years ago, when this was a warm, shallow sea floor.

The package is enormous: in the peninsula it measures between seven hundred and fifty and nine hundred metres thick. All the height you see in a tower is contained within that package, which is why the tallest towers exceed three hundred metres without exhausting it.

Limestone has a property that separates it from almost every other rock: it dissolves. It does not break or wear down, it leaves in the water. And the water that dissolves it is rain, which on passing through soil and vegetation takes up carbon dioxide and turns slightly acidic.

02 The water attacks at the foot

Here is the whole mechanism. The most acidic water is not what falls on the tower's bare summit: it is what filters through the soil of the plain, where there is decomposing organic matter and roots respiring. That water attacks the base.

Meanwhile the exposed walls do the opposite. Water running down them evaporates at the surface and leaves the carbonate it carried dissolved, forming a crust harder than the rock beneath. Geologists call it case hardening.

Combine the two and you get the result: the plain goes down and the tower does not. The extreme verticality of these walls, overhanging along many stretches, is not a whim of the landscape, it is the signature of that uneven distribution of chemical attack.

The process needs specific conditions to work: at least some twelve hundred millimetres of rain a year and around eighteen degrees of average temperature. The Thai peninsula gives at least thirteen hundred millimetres and runs around twenty-eight degrees, well above the threshold.

03 Two centimetres every thousand years

The obvious question is how long this takes. There is an estimate, obtained from palaeomagnetic data on a Chinese tower: development advances at less than two and a half centimetres every thousand years.

Translated to a specific tower, a thirty-metre one — a modest height for what is here — needs of the order of one million three hundred thousand years to stand out like that. The three-hundred-metre ones dominating the Krabi plain are an order of magnitude older in their present form.

That slowness has a consequence visible at a glance: the landscape you are looking at is not a final state, it is a snapshot of a process. Low rounded towers and sharp tall ones are not different types: they are different phases of the same job.

04 The visor that marks where the plain was

There is a detail that lets you read the history in the stone itself. At the base of many towers, and above all those rising out of water, you can see a horizontal notch: a cut that bites inwards and leaves the rock above flying out like a visor.

Those visors can reach six metres high and twenty wide. They form when the level of the alluvium, or of the sea, stays still long enough for dissolution to work always at the same height.

That is: each notch is the record of a pause. When the plain stopped lowering, the tower stopped growing upwards and began narrowing at the foot. Looking at the height of the visor is looking at where the ground was a long time ago.

05 A hundred and fifty-four islands are the same thing

The province has 154 islands, and almost all of them are the same thing you see inland. When the sea rises it floods the plain and leaves the towers sticking up; when it falls it reconnects them with continuous ground.

That is why Krabi's map is so strange. There is no coastline separating land from water with an orderly line: there is a field of towers that in some places has alluvium between them, in others mangrove and in others sea, and the three situations alternate within a few kilometres.

And it explains something practical: here the boat is not an excursion, it is transport. Beaches two kilometres in a straight line from a road have no land access, because between the road and the beach there are three hundred metres of wall.

06 And the plain below is no longer jungle

One important correction to the postcard remains. The green plain the towers rise from is not tropical forest almost anywhere: it is plantation. Oil palm occupies some 1,568 square kilometres, 52 per cent of the province's agricultural land.

Added to rubber, the two plants account for 95 per cent of all cultivated land. The forest that remains is some 915 square kilometres, 17.2 per cent of the territory, concentrated above all on the slopes where nothing can be planted.

That is the honest way to look at Krabi. A marine rock two hundred and fifty million years old, a plain that has spent another million dissolving beneath it, and on top of that plain, for a few decades now, rows of palm. All three layers are visible from any road.