IF grains of sand can build huge dunes and little strokes fell great oaks, what could be the result of billions of tons of water drops falling on the earth for billions of years? The answer can be found in China’s karst topography, with a notable preponderance in southern China.
Karst landscape in Dacai Township of Huanjiang Maonan Autonomous County, Guangxi Zhuang Autonomous Region.
Southern China was submerged in an ocean billions of years ago, which was home to a large number of marine creatures. The deposit of their bones and other carbonate materials gradually built up a thick crust on the ocean floor, which rose up in later crustal movements to form land. Under a hot and humid climate, this region received high precipitation, which caused a chemical reaction on the carbonate rocks. The result is the marvelous landscapes we see today.
The flow of surface water carved grooves in the ground and these grooves grew deeper with the long passage of time, leaving the sections in between standing out like columns, 30-40 meters high. These columns often appear in large numbers, and are hence called stone forests.
Stone forests are well developed clints. If the “trees” of such “forests” are connected at the bottom, they form a karst peak cluster. If further erosion cuts their base apart to make freestanding peaks, they are called tower karst and cone karst. In some places the erosion continued to the point that all but one karst peak was worn away.
The many varieties of karst landscapes in southern China are vivid illustrations of the evolution of nature.
As surface water transforms the face of the earth, ground water is also reworking the space below. The ample rainfall in southern China seeps through crevices in the ground into underground caves, feeding into subterranean rivers. There are more than 2,800 such rivers with a flow rate of 50 liters per second or more in southern China. Those in Guangxi alone measure up to 13,000 km, equivalent to the length of two Yangtze Rivers. For millennia, these rivers have “drilled” a labyrinth of karst caves that number in the hundreds of thousands.
There is a gigantic cavern in the Ke’ao River Scenic Area of Ziyun County, Guizhou Province, called the Miao Hall. Its floor measures 116,000 square meters – the size of 16 football fields, and 80 meters in height. In the flood season, rainwater flushes through the hall across a graded elevation, creating a spectacular waterfall.
The cave features stalagmites and stalactites of various sizes and shapes. Among them is a 45-meter stalactite. It’s estimated that stalactites in the Miao Hall grow at a rate of 11 mm per 100 years. A formation of 45 meters is hence at least 450,000 years old.
The Diamond Hall is an affiliated cave of the Miao Hall. As its name implies, this chamber accommodates a sprawl of sparkling stalactites. But the shine comes from sparry calcite, a sedimentary stone. The irregular crystal structure of the rock deflects light in all directions, producing splendid sparks. The magic of nature often goes beyond human imagination.
Sinkholes and Natural Bridges
If the dissolution of soluble rocks goes on, a karst cave will eventually collapse, leaving a hollow in the ground called Tiankeng, or sinkhole. The largest Tiankeng in the world is in Xiaozhai Village, Fengjie County, Chongqing. It is 622 meters in diameter at the mouth, 522 meters in diameter at the bottom, and 666.2 meters deep. Looking down from the edge of the opening, one can see nothing but vertical cliffs receding into unfathomable depths. Along a narrow trail on the northeastern wall of the Tiankeng, visitors can go down to its bottom. There are two tablelands along the way, at 300 and 400 meters below the ground level respectively. The former was once home to some hermits, who left behind two huts. The latter, with lush vegetation, is a riot of color in warm seasons.
At the bottom of the Tiankeng is a maze of underground rivers and interlinked caves. Adventurers from around the world have come for spelunking, and discovered many rare species and fossils. Yet there is even more to be learnt of this site.
The erosion of elements doesn’t stop with Tiankeng. If rock dissolution keeps chipping away the walls of a Tiankeng, it will eventually be reduced into a bridge-shaped structure, called a natural bridge. In Wulong County, Chongqing, stands a trio of such bridges, named the Heavenly Dragon, Blue Dragon, and Black Dragon. They are an average of 300 meters tall and less than 1.5 kilometers apart, with two Tiankengs in between. Such a cluster of natural bridges is rare in the world.
As soft and silent as it is, water has the power to sculpt mountains and tracts of land, changing the face of the earth over millennia. The uncanny karst topography is testimony to the fascinating evolution of Earth that is still going on.