Madagascar Has a Forest Where Every Tree Is a Stone Needle

Picture a lemur springing through the air in western Madagascar. It is not jumping from branch to branch. It is jumping from one grey stone spire to the next, across a forest in which the trees are made of rock. This is the Tsingy de Bemaraha, a plateau near the coast where limestone stands in thousands of pale needles, packed together like the bristles of a giant brush.

The needles are called tsingy, and the tallest rise about 100 metres, according to the International Union of Geological Sciences, which lists the site among the world's geological heritage. Between them run narrow, shadowy corridors. Seen from a NASA satellite, the whole plateau looks cross-hatched, as if somebody had scored the rock with a knife in several directions. UNESCO made the reserve a World Heritage Site in 1990.

How a lagoon became a field of needles

The story begins under water. NASA's Earth Observatory dates the start to about 200 million years ago, in the Jurassic period, when layers of calcite settled on the bed of a lagoon. The geologists' union adds that the rock grew from old coral deposits, and that the bed ended up 300 to 500 metres thick. Later, movements in the Earth's crust lifted that bed up, and when sea levels dropped during the ice ages, still more of it came into the open air.

Then the rain went to work. Limestone dissolves, very slowly, in water. Monsoon rains washed away the softer rock and left the harder rock standing. The plateau is crossed by cracks running in several directions, and what was left standing between them became blades and spires.

• 100 m height of the tallest pinnacles
• 300–500 m thickness of the limestone bed they are cut from
• ≈ 300 m depth of the canyon the Manambolo River has cut through it

What people often get wrong

It is natural to assume the needles were whittled from the top down, the way a sculptor carves a block. A good part of the work was done from below, where nobody could see it. In 2008 a team led by Márton Veress surveyed the deep clefts of Bemaraha, measured their directions, and reported in the International Journal of Speleology that most of them began as caves, dissolved out beneath the water table. Only later did they open to the sky, perhaps as the dissolving worked its way downwards, perhaps as cave roofs fell in. The authors offer both possibilities and do not pick one. So many of the corridors between the spires are old caves that have lost their ceilings.

Who lives in a forest of stone

The second surprise is that the place is not bare. The hollows that water has dissolved in the rock shelter pockets of green vegetation, and the reserve protects habitat for lemurs and birds. The sharp rock that makes walking so slow for people kept visitors away for a very long time, which suited the wildlife nicely. The sifaka lemurs do not walk at all. They jump from spire to spire.

The plateau also does a quiet practical job. The geologists describe the tsingy as a water tower for the nearby Antsalova region. Two rivers cross the limestone, and one of them, the Manambolo, has cut a canyon about 300 metres deep on its way through.

Why it is worth knowing

The stone forest is really a portrait of water. Water laid the rock down in a lagoon, hollowed it out from the inside, sharpened it from above, and still runs through the middle of it. Nothing here was done quickly, and nothing was done by anything stronger than rain. The lemurs, for their part, seem to regard the result as a perfectly ordinary climbing frame.