As you stroll through the caves in Guangxi province, you might ponder on their formation and try to spot some of their intriguing features.

Formation

The life-span of a solution cave can be split into three phases:

(i) Initiation phase: groundwater seeps slowly through cracks and fractures in the limestone bedding planes. Over 3,000 to 10,000 years are needed to turn one such fracture to a pencil-sized opening of 5 to 10 mm wide.

(ii) Enlargement phase: with a pencil-sized opening the water dynamics change. Instead of slowly percolating, water travels turbulently through the small conduit; its flow pattern equivalent to that of a pipe. With increased velocities at work, a cave may be enlarged from pencil-sized to a three metre diameter in 10,000 to 100,000 years.

(iii) Stagnation and Decay phases: after abandonment by the lowering water table, caves can remain abandoned for millions of years. Surface erosion dissects the landscape, lowering hilltops and plateaus, until denudation of the land surface destroys the last vestiges of the cave.

Cave Features

Cross-sectional shape

If the cave was completely filled with water during its formation, it will be elliptical in shape; the floor, walls and ceiling having been eroded to the same degree. However, should the water level have dropped during the formation of the cave, the roof – out of reach of the water erosion – will be shallower in form. The resulting shape, known as a canyon, produces a thin and tall chamber, with elliptical floor and walls. Other cave passages may be irregular resulting from the competition between the hydraulics of flowing water that work to create smooth stream-lined forms and the structural arrangement of joints, fractures and bedding-planes that initiated the passageway.

Scuplturings

· Ax-blade shapes: these form where water seeping through the cave wall is mixed with water already in the passage. If the two sets of water have the right chemistry they can form a corosive acid which will dissolve the joint-controlled wall and ceiling pockets.

· Floor channels: small secondary channels carved into the floors provide evidence of small later-stage streams that occupied the cave passage after it had been drained of the original flow.

· Scallops: small scooplike depressions, which vary in size from several centimetres to more than one metre. They have a steep wall on the upstream side and a gentler slops on the downstream side. Thus they can indicate the original flow of water through the cave passage. In addition the size of a scallop indicates the speed of the water flow – the smaller the scallop, the faster the water flow.

· Cave mineral deposits or speleothems : the shapes of speleothems depend on whether they were formed by dripping or flowing water. Dripping water produces stalactites and stalagmites, the former growing from the ceiling and the latter from the floor. Stalactites can become so large that they break, and sometimes fragments of a stalactite may be seen on a cave floor. Water flowing along ledges and down walls leaves behind sheets of calcite, which build up a deposit known as flowstone. These are densely packed and coloured various shades of tan, orange and brown.

· Crystals: some speleothems have shapes controlled by crystal growth rather than by the constraints of dripping and flowing water. Helictites are like stalactites in that they have a central canal down which solution flows to the tips. However, they twist and turn in all directions and are not guided by the gravitational pull of water drops.