What is 'Karst' Limestone Landscape?
It’s the technical term for a highly distinctive form of weathered limestone dominated by rounded conelike hills or steep or vertical sided ‘towers’. Although fengling (cone) and fengcong (tower) karst are different in appearance, they are sequential stages of a long process of formation. Fengcong towers developed from fengling cones that were steepened by water table undercutting.

The entire process required three key elements to coexist over millions of years:

  • a very deep layer of high-quality limestone (the limestone beds in Ha Long Bay are well over a kilometre thick)
  • continuous warm wet weather
  • slow, steady tectonic uplift – around a millimetre a year
The only area in the world where these requirements were fully satisfied was Southeast Asia, and particularly in what is now the Guangxi province of southern China and in Vietnam.

The layers of sediment from which the limestone was formed was laid down in horizontal layers, or strata. Steady tectonic pressure from underneath caused the limestone strata to fracture. If the uplift was completely even and the limestone pure and free from impurities, the bedrock fractured along regular lines at right angles, creating the familiar ‘limestone pavements’ found in cool latitudes.

As the limestone rose, the fractures became hair cracks wide enough for moisture to enter. Rain fell through the atmosphere, picking up carbon dioxide, which dissolved in the droplets. When the rain hit the ground, it percolated through the soil to form a weak solution of carbonic acid (H2O+CO2=H2CO3). The weak acid infiltrated the narrow cracks and, over a long period, dissolved the limestone.

Gradually the cracks expanded to become caves and a lower drainage system developed as the rivers and lakes left the surface via ‘sink holes’ to flow through underground channels eventually emerging at a lower level sometimes far away. This is the early stage of a karst area. These features are common in moderate climate zones but the climatic and tectonic conditions were such that the process didn't progress to the eye-catching mature stage.

As the process continued, the limestone caves expanded and those near the surface collapsed to become ‘dolines’ – deep, steep-sided valleys, usually with a rivers through the middle.

In karst areas, the erosion process followed the fracture lines so, from an aerial view cave systems were visible as ‘daisy chains’ of dolines, thus creating a checkerboard pattern of conical hills separated by dolines. This is the fengling stage of the karst process.

The fengling stage is the more common and less spectacular form of karst landscape with steep conical limestone hills, typically covered by dense forest.

As the erosion process continued, the former limestone plateau was cut into isolated limestone islands, the sides of which became steeper forming the unique rectangular towers of mature fengcong karst landscape

Fengcong karst is a dramatic landform with high towers reaching from 30m to 300m with vertical or overhanging sides. The walls are typically bare rock, as they are too steep for vegetation. Another feature of fengcong karst is an abundance of cave openings where doline collapses have exposed caves at higher levels. In many areas, the towers are full of inactive caves at (multiple) higher levels, and with active caves through their bases.

Vietnam’s Ha Long Bay is unique, a large area of nature fengcong karst that has been overwhelmed by the sea, creating an archipelago of small islands. There is also a considerable area of submerged mature karst towers on the coast of Thailand and in the Chinese Sea.