NatGeo’s Missing Info on China’s Mysterious Mountains

towerkarstNational Geographic magazine has some good articles, but it has some bad articles too – either containing misleading information, or containing a lack thereof. Regarding the latter, the latest issue of NatGeo has an article on the famous and mysterious tower-like mountains of southern China and the associated caves, near the city of Guilin.

The article mainly discusses the authors’ and rock climbers’ experiences there. As always, there are good photographs. And there’s a cool diagram of the caves. But if you want to know how the tower-like mountains were formed, you’re in for a letdown. That should be the first question in any discerning reader’s mind. Apart from a couple of comments about erosion over the eons – just in the captions no less – nary a word is written about how the geology of that region came to be.

So allow me to plug in some of the gaps in the NatGeo article, borrowing from this web page. The type of geologic feature is called tower karst formations. In addition to the southern China region, they’re also found in Thailand, Malaysia, Indonesia, Papua New Guinea, Australia, Honduras, Cuba, Jamaica and Puerto Rico. It forms through the erosion/dissolution of limestone, of which the whole region consists.

The limestone base was formed when the area was at the bottom of the ocean, when calcium carbonate in the water settled to the bottom, building up layer upon layer over millions of years. When the seas recede or when the rock is uplifted, water easily percolates through the limestone and dissolves it, forming caves and other features.

Tower karst only develops in humid, tropical areas with a lot of rainfall. That water reacts with the vegetation to erode the limestone. But at the beginning of the process, certain spots are resistant to erosion. So those spots or mounds remain intact while the area immediately around them erode away. There is much less soil or vegetation on the slopes, which means less acidity on those slopes when it rains, making the slopes also resistant to erosion. They form into steep, erosion-resistant surfaces, while the base of the structure erodes away. So the landscape is peppered with these steep mounds, while the flat area erodes. Over millions of years the flat area keeps eroding to a lower and lower elevation, while relative to the surrounding countryside, the mounds turn into tall, thousand-foot-high towers.

Meanwhile elaborate caves develop within the towers, as the limestone dissolves inside.