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VOL. 10.8

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Secrets of the Karst and Caves

By Thom Henley

Distinctive limestone formations are found all the way from the Tarutao Marine National Park, in Thailand’s extreme south, to the dramatic outcrops of Chiang Mai and Chiang Rai provinces in the north. Karst geology underlies the spectacular scenery of Krabi and Phang Nga, in the forested mountains on the peninsula between Koh Samui and Phuket, and among the islands of Ang Thong Marine National Park that neighbours Samui. Limestone islands also provide the caves and hongs that have made sea-canoeing so popular in areas such as Phang Nga Bay.

If there is a single feature of South Thailand’s landscape that commands one’s attention right from the start, it’s the dramatic crags that rise from the water’s edge and the valleys to sheer vertical heights of more than 900 metres.

These butt-like crags are 260-million-year-old limestone karst formations. Long ago, a shallow sea running the entire length of Southern Asia slowly built up deposits of shells and corals that were later buried under sediments washing in from the land. Compressed deep in the earth, these calcium carbonate remains formed limestone, which was then thrust up above the surface when the Indian subcontinent collided with mainland Asia about 60 million years ago. The collision of these two titanic plates rolled the whole of Southern Thailand and the Malay Peninsula clockwise and created ruptures along the 5,000km ancient coral reef line.

From Gualin, in China, to Borneo, the limestone was thrust up above the surface at key pressure points – much like discs slipping out of place along a terribly abused human spine. Phang Nga, Krabi, Trang and Surat Thani Provinces are the areas in Southern Thailand where karst intrusions are most pronounced.

Fluctuating sea levels during the ice ages allowed for extensive wave erosion of this soft sedimentary rock. For ages, monsoon rains have also helped to dissolve the karsts. Decomposing leaves from karst vegetation create a slightly acidic water that readily dissolves the calcium carbonate of the limestone as the water percolates down through the tiniest cracks and crevices in the rock. Rivers too have cut courses through the karsts, resulting in a labyrinth of cave chambers and passageways.

Sometimes the roof of a huge cave chamber collapses, and the ensuing sunlight allows colonizing plants to flourish in the hidden Garden of Eden, or Hong. These hongs (Thai for “room”) are one of the most fascinating features of karst topology. If the former cave floor was at or below sea level, then the hong may become a lagoon at high tide. If the cave site is well above sea level, then forest may ultimately grow here.

Karsts support an intriguing flora and fauna uniquely adapted to a demanding habitat. Vegetation on karsts has evolved the ability to withstand long periods of dryness and almost a complete lack of soil. Plants find a tenacious foothold in small crevices where moisture and some semblance of nutrients from decomposing leaves are available. Among the many plant species that colonize karsts are cycads, bonsai-like palms which look like miniature coconut trees, and endemic species of palm like the fan-shaped palm pralahoo. There are even some species of figs that are able to colonize karst environments, thereby providing an abundance of seasonal fruits for many species of birds and primates.

Long-tailed macaques feed here on everything from fruit and insects at higher elevations to crabs and crustacea where the karsts reach the waterline of rivers, lakes or the sea. Gibbons may be found on the larger karst complexes, of continuous forest cover of approximately 20-50 hectares is available to support their need to forage widely for pulpy ripe fruits. Dusky langurs, or leaf monkeys, are more commonly found living on karsts, as their exclusive leaf diet allows them to hold smaller ranges than that required by gibbons.

The pied hornbill is a common resident of many karst islands and is frequently sighted. This wide-ranging and amazingly adaptable bird eats everything from fruits, to insects, reptiles and small mammals.

The “little heron” is another bird commonly seen standing motionless on lower rock ledges where karsts meet the river or sea.

Raptors such as Brahminy kites, white-bellied sea eagles and peregrine falcons often use overhanging karst ledges to build their substantial nest platforms, to shelter their young from inclement weather, and to protect their progeny from predators. The blue rock thrush, streaked wren babbler and dusky crag martin are other birds that prefer limestone karsts for their homes.

Another animal that seeks protection from predators by restricting itself to this inhospitable, near-vertical terrain is the serow. This ungulate, a sort of goat or antelope, has evolved a short body with very long legs. The sure-footed serow can leap from ledge to ledge over frighteningly deep chasms where few predators can follow. The animal almost everyone associates with caves are bats. An evening exodus of thousands of bats from the dark honeycombed chambers of a cave can be one of the greatest spectacles in nature. Often, a small scouting group of bats will exit the cave first, then return to signal clear weather to the throngs of bats still awaiting departure. Once the flow of bats from the cave mouth has begun, a continuous drone of wings drowns out all other sound and a ribbon of bats may wend its way across the sky for up to an hour. The karst seems incapable of holding such multitudes, and, to add to the chaos, swiftlets are often entering the cave chamber at the same time the bats are exiting.

The edible nest swiftlet and the black-nest swiftlet patrol the skys for insects by day, while bats slumber in the cave chambers. Dawn and dusk are the time clocks signaling shift changes between swiftlets and the many species of insectivorous bats. The poor insects never seem to get a break.

Within the cave ecosystem, where darkness precludes primary producing plant life, bat guano is the fundamental nutrient base for many forms of cave life. Cave crickets feed on the bacterial molds that break down the guano, while roving cave centipedes and cave toads prey on the crickets. Flying insects hatching out of the rich guano get caught in the webs of cave spiders or fall prey to cave frogs or even fish living in the underground waterways.

Where rivers flow through cave chambers, aquatic life can be especially rich. Crabs, shrimp, crayfish and blind fish which find their prey in the dark with the help of long whisker-like sensors are but a few of the hidden secrets of the darkest recesses.

Of course, one does not immediately encounter all cave species just by entering the first chamber. The transition from light to total darkness is a gradual one, as is the presence of certain species. Deer, bear, porcupine, giant pittas and even elephants are known to frequent cave entrances from time to time, presumably to seek shelter from monsoon rains or to find mineral salts.

The Asian reticulated python, the worlds longest snake at 10 metres, often seeks out deeper cave recesses where it may rest for weeks between large meals of deer or wild boar. Another snake, only found in caves, is the striped cave racer. This sleek 1.5 metre reptile has a cream-coloured body with dramatic dark stripes ending in a dark tail. The cave racer has the uncanny ability to scale sheer cave walls, sneak up on a slumbering bat, crush it and swallow it whole. It does all this in a matter of minutes. While a snake swallowing a bat in total darkness may resemble some people’s worst nightmares, it is an amazing drama to watch from within the intimacy of a small torch beam. Then again, one reason that there are still so many undiscovered secrets of the karst caves is that phobias for darkness, small spaces, spiders, bats and snakes are so common that most people are just as happy to have them remain secrets.