Grottes et karsts de Chine
Sur les traces de Xu Xiake

SAN GUI SHUI (LAO PUNG) TIANKENG AND A LESSON IN DESTRUCTIVE LANDFORMS

By Ged Campion

Lao Pung is a hill area located on the Tian’e / Fengshan county boundary. The road from Tian’e winds up in a southerly direction past sites we had visited behind the Long Tong Gorge. The village of Lao Pung is very small but services the isolated road between the two towns. A small friendly café provides excellent all day noodle breakfasts served with that extra bit of chilli!

The hills just beyond Lao Pung, however, were to reveal something quite special in terms of cave discovery - “a new tiankeng”.

Tiankengs are a relatively new phenomena in karst nomenclature. Prof Zhu Xuewen and his colleagues at the Institute of Karst Geology in Guilin have been major exponents of this new karst classification.

Tiankengs are a unique geological feature found throughout China but especially in South West China. They have often been called “great” dolines or “giant” dolines in the past. These immense structures dominate landscapes in Leye, Haolong, Jialoe in Guangxi and Xingwen in Sichuan province. There has been much heated debate about what constitutes a tiankeng as opposed to a doline, and our encounter with San Gui Shui gave us an opportunity to ponder further on this debate. Basically, the chief characteristic of a tiangkeng is that it is of a very large size. Our experience of exploring Dashiwei Tiangkeng in Leye and Haolong in Bama has helped inform our knowledge of these spectacular features and it is difficult not to be in awe of their incredible sizes. My recent visit to Haolong Tiangkeng in Bama, one of the three biggest dolines in the world, made me aware of just how big these structures can get. Its entrance diameter is a staggering 800m long from east to west and 600m wide north to south, the maximum depth being 509.3m; the total volume is 110 million m3. These types seem to form a super tiankeng category of their own, which Chinese geologists call ‘oversize’ tiankengs.

In contrast most dolines look like shallow basins where the width is greater than the depth. Tiankengs have a clear division between their walls and their floors, where dolines generally do not. Tiankengs have steep walls at least 100 metres high and significant evidence of collapse on their floors, while dolines typically have gentle slopes, thick soil and sediment on their base, and a small sinkhole. The development of karst dolines is closely related to - and positively enhanced by - surface landforms, for example dry valleys, arches or cone karst, whereas tiankeng development is known as “negative” or “destructive” to surface features, in that the surface area collapses and takes everything with it! Indeed tiankeng formation dictates that there is likely to be no relationship between the tiankeng’s entrance position and the surrounding surface landforms. Tiankengs generally develop through deep saturated zones influenced by the formation of vigorous underground rivers, with a humid, rainy climate providing the right conditions for dissolution and river flow on the surface.

The development of tiankengs is therefore likely to be abrupt and result in some cases in massive collapse. In geological terms they are the youngest of the large negative karst landforms formed generally since the later Pleistocene, making them more than 128,000 years old.

Tiankengs of different sizes or at different stages of development will have distinctive features. The smaller tiankengs for example, will look like inverted dolines as the process of collapse begins, the entrance then broadening out with additional erosion; eventually the oversize tiankengs may, paradoxically, have profiles more like giant dolines, eg Haolong Tiankeng in Bama.

Our new tiankeng, with an entrance perimeter of 292 square metres, could be classified as a youthful tiankeng yet having the vital statistics to allow its inclusion in the very exclusive tiankeng club: 100m perimeter cliffs (at their highest point), separate walls and floor, evidence of considerable collapse at the bottom, a location bearing no relation to surrounding surface landforms, and a volume weighing in at 457,300cu m. The Chinese name for the tiankeng is San Gui Shui meaning ancient water cave, but as we peered over its precipitous entrance we could see no sign of water. The Chinese had never been able to descend into this feature so it held much awe and mystery for them, and we appeared to have the whole village assembled around the top of the tiankeng as we prepared our gear, rigging off the biggest tree we could find to descend this monster. I was nominated as the rigger, Bruce would be occupied with the filming, and the two Daves were happy for me to have the pleasure of touching down first. As I dropped down to the tree that hung over the void, the trees hanging over the hole swayed in the wind and golden leaves spiralled down the shaft in a beam of light. As I embraced the tree trunk, struggling to get the 2m tape sling round its girth, the rope bag with its 200m innards decided to move off the grass ledge where I had persuaded it to sit while I fixed the belay. The weight of the bag pulled heavily on my harness, unearthing vegetation and a few rocks which crashed down the drop below me reminding me of the seriousness of my position. I hung the bag on the tree and attached the rope to the sling. I began abseiling slowly, spiralling in the free hanging void in awe of the size of this tiankeng.

Stillness and light gave me an opportunity to survey the architecture of the walls below the entrance portal. The tiankeng’s entrance was severely undercut, hence increasing the volume of the void considerably. Quite remarkably the tree gave us a perfect 85 metres free-hang all the way to the bottom. As the noise of the circus above melted away into confused echoes, I struggled to feed the weight of the rope through my stop. I released the remainder of the rope from the bag to assess the distance to the bottom. The rope whizzed down and landed haphazardly on the canopy below. Eventually I descended through the trees and landed in a deep, dusty, brown coloured soil, undisturbed by man or beast until now. The verdant forest canopy which formed the floor of the tiankeng disappeared away down a 100m steep loose slope to a low arch that guarded the access to the cave below. I moved well away from the fall line of the pitch to safety below the tiankeng’s arching walls; to have been hit at this depth would be painful to say the least! Incredible large plants were in abundance giving the place an eerie feel indeed, it wouldn’t have been a surprise to see a small dinosaur rush out of the undergrowth! Instead, I was greeted by a grumbling Bruce who dangled like giant spider above the canopy. It took a long time for the others to join us, each one in awe of the tiankeng’s dimensions.

San Gui Shui was a real gem, beautifully decorated and quite spectacular in size. Big galleries led off up and down, but there was no sign of a river or a pitch that would reward us with depth. In terms of tiankeng formation this was an example of a feature where the river had already found an alternative route, leaving in its wake vast, undisturbed fossil passages. We surveyed the huge galleries and chambers and took photographs before we made our way back to the tiankeng, and on the way met Zhang Hai and Mr Yel Ho who were marvelling at the well decorated passages. They turned around and ascended the pitch first. Mr Yel Ho managed to get stuck on the tree belay and took a record 50 minutes to climb the rope! By the time we had our turn we were rewarded by a prussik up to starlit sky and only a few spectators. The circus had long since gone but had left us the glowing embers of a burnt out fire and a small boy with an irritable, yapping, little dog, which ran the risk of disappearing into the tiankeng’s forest 100m below. Zhang Hai was delighted by the discovery of a new tiankeng to add to the Karst Institute’s growing list of big limestone features.