Why Krabi looks like this: the limestone karst

Krabi looks the way it does because of one rock: limestone, laid down on the floor of a shallow sea in the Permian period, roughly 250 to 300 million years ago. Rainwater has been dissolving it ever since it rose above the sea, and the towers you see are what is left. When the sea rose again after the last ice age, it flooded the plain between them, and the tops became islands.

What follows explains each part of that story in turn, the towers, the bay full of islands, the groove along the waterline, the caves and the hidden lagoons, and then says plainly where the science stops being settled. Some of what you will hear on a boat is folklore; we say which parts.

A sea floor from the Permian

Geologists call the rock the Ratburi Limestone. It formed in shallow, warm sea water, on a wide carbonate platform: a flat shelf where lime mud and the debris of shelled creatures built up, layer on layer, over millions of years. The same limestone runs down the Andaman coast from Phang Nga to Satun and on to Langkawi in Malaysia.

Sources agree that it is Permian, but they place the start and end of the deposit a little differently, so treat any figure to the nearest million years with suspicion. The lower beds are dark grey, well bedded and studded with nodules of chert; the upper beds are paler and massive, and they are the ones that make the sheer cliffs.

You will sometimes hear that the cliffs are an ancient coral reef. The geological sources we have read describe a platform on a shallow sea floor, not a reef.

How rock becomes towers

Rain picks up carbon dioxide from the air, and much more from tropical soil, and becomes a weak acid. That acid dissolves limestone where it can get in, along cracks and joints. Over a very long time the cracks widen into valleys and pits, and the rock between them is left standing.

Around Krabi this has produced two shapes. Some towers stand alone, with sheer sides; others rise in clusters from a shared rocky base. A study of Krabi's towers argued that the shape depends more on the rock itself, how massive and porous its beds are, than on the climate.

The towers were not built by the sea. A geologist who studied Phang Nga Bay described the landscape as formed mainly on land, by rain, and wrote that the sea had probably steepened the sheerest walls. We have found no measurement of how long Krabi's towers took to form; the figures in millions of years that circulate online come from studies in China.

Why the bay is full of islands

During the ice ages a great deal of the world's water was locked up in ice, and the sea stood much lower than today. What is now Phang Nga Bay was dry land, a limestone plain with towers on it. When the ice melted, the sea rose and flooded the plain, and the towers became islands. That is why geologists describe the bay as a drowned karst.

Seen this way, the islands off Ao Nang and the towers on the mainland, at Railay or behind Krabi Town, are the same landscape, one with its feet in the sea and the other on dry ground.

Krabi islands: which day, which boat

The groove at the waterline

Look at almost any island from the boat and you will see a deep groove cut into the rock at sea level, with the cliff overhanging it. This is a tidal notch. It is cut in the band between low and high tide, by a mix of dissolving sea water, tiny organisms that bore into the rock, grazing animals that scrape it, and the waves.

It is slow work. A ten-year study that measured notches in this same limestone in Phang Nga Bay found the rock wearing back by about a quarter of a millimetre a year on average, a little faster on the floor of the notch than on its roof.

Higher up some cliffs you can see older notches, now well above the water. They mark a time when the sea stood higher. Rock oysters dated in notches along the Thai coast put the sea about 2.5 metres above today's level some 7,000 to 6,000 years ago, and a separate study of Phang Nga Bay and Phuket reached a similar figure. Notches and cave mouths higher still, up to 10 or 15 metres, probably date either from the last warm period between ice ages or from the last ten thousand years or so; which of the two is not settled.

Caves, hongs and stalactites

Because limestone dissolves from the inside as well as the outside, the towers are riddled with caves. Some open high up the cliffs and are dry, left from older water levels. Others sit at sea level, and some are now under water.

The most striking are the hongs, from the Thai word for a room. A hong is a hollow inside an island, open to the sky and ringed by cliffs, with a tidal lagoon at the bottom. You reach it through a cave at sea level, and often only for a short time around low tide: at high tide the entrance can be under water. Mangroves grow at the water's edge inside, and plants cling to the walls.

Stalactites form where water seeps through the rock into an air-filled cave. The water gives up its carbon dioxide to the cave air and leaves a little calcite behind; drop after drop, the calcite builds downwards into a stalactite, and what falls to the floor builds a stalagmite. You can see them in the cave inside Khao Khanap Nam, the pair of towers beside the river at Krabi Town.

One cave researcher attributes the streaks of red, orange and black down the cliff faces to algae and lichens.

Where to see it around Krabi

From a boat among the islands. The four-island day off Ao Nang and the Hong Islands day both run among towers standing in the sea, with notches along their bases. The Hong Islands day usually includes a lagoon.

At Railay. The peninsula is cut off by limestone cliffs, which is why there is no road. The same walls, eroded and full of pockets, are what bring climbers here.

At Krabi Town. Khao Khanap Nam, the two towers on either side of the river, is reached by a short longtail ride and has a cave with stalactites and stalagmites.

In the mangroves of Ao Luek. Tham Lot is a cave you pass through by boat or kayak, at low tide only.

Kayaking around Krabi

Rock climbing at Railay

What is still debated

Geology is surer of some of this story than of the rest. These are the points where the sources hedge or disagree, and where we would rather tell you so than pick one.

The exact age of the rock. Permian, everyone agrees, but the sources place the start and end of the deposit differently. Roughly 250 to 300 million years is as precise as we will be.

How much the sea shaped the towers. The geologist who studied Phang Nga Bay saw mainly the work of rain, with the sea steepening the walls. Tourist material often gives the waves all the credit.

How the notches are cut. Studies of tropical coasts stress the organisms that bore and scrape the rock; a study elsewhere put more weight on salt and on the rock drying out between tides. On Krabi's hard limestone, the balance is not fully worked out.

How old the high notches are. The ones a few metres up are dated to the last 7,000 years or so. Those up to 10 or 15 metres may date from the last warm period between ice ages or from the last ten thousand years or so, and the researcher who described them wrote only "probably".

How the hongs began. Whether each hong was once a cave with a roof that fell in, and whether the sea caves came before the hollows or after them, is left open by the researchers who have described them.

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Sources

The geology here comes from published research and from a geological database. Several papers were read in abstract only; where an author hedged, we have kept the hedge.

• Age and setting of the Ratburi Limestone: Thailex, Ratburi Limestone Formation
• Origin of the Phang Nga tower karst: K. Kiernan, The coastal tower karst of Phang Nga Bay (1994)
• Hongs, sea caves and the drowned karst: L. Price, Hongs of Phang Nga Bay, Thailand, Tourism and Karst Areas (2011)
• Erosion rates in tidal notches: C. Moses et al., Earth Surface Processes and Landforms (2015)
• Higher sea levels in the mid-Holocene: G. Oliver and J. Terry, Palaeogeography, Palaeoclimatology, Palaeoecology (2019)
• How stalactites form: US National Park Service, How stalactites and stalagmites form