The caves at Sterk­fontein were formed in dolomitic lime­stone over mil­lions of years. They began to form as ear­ly as 20-mil­lion years ago. Most caves includ­ing Sterk­fontein – are formed by the dis­solv­ing action of weak­ly acidic rain­wa­ter, which seeps into the sol­u­ble rock through the soil.

In ancient Neoar­chaen times, 2.58-billion years ago, dolomite formed in the bed of a shal­low sea. With­in the last 20-mil­lion years, the Sterk­fontein Caves were formed with­in the dolomitic rock. Slight­ly acidic ground­wa­ter dis­solved solu­tion cav­i­ties beneath the water table. This zone beneath the ground­wa­ter sur­face or water table is known as the phreat­ic zone. Over time, the water table dropped and the caves became air-filled voids in which sta­lac­tites and sta­lag­mites could form. Water from the sur­face per­co­lat­ing through cracks in the dolomite absorbed cal­ci­um car­bon­ate. On reach­ing the cav­ern ceil­ing, car­bon diox­ide was released, and a thin film of cal­ci­um car­bon­ate was deposit­ed. The deposits built up over time to form sta­lac­tites and oth­er formations.

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Under­ground lake in the Sterk­fontein Caves

Sta­lac­tites and stalagmites

The term sta­lac­tite comes from the Greek word sta­lak­tos, which means drip­ping”, because these oth­er-world­ly for­ma­tions are pro­duced by drips” from the ceil­ings of lime­stone caves. Water reacts with chem­i­cal ele­ments in the ground and rock, and seeps slow­ly through the roof of the cave, deposit­ing cal­ci­um car­bon­ate, which hard­ens and builds up over time to form a stalactite.

Sta­lag­mites are cor­re­spond­ing for­ma­tions on the floors of caves, ris­ing up over time as drops of water, rich in cal­ci­um car­bon­ate, fall from the roof of the cave. The term sta­lag­mite comes from the Greek word, sta­lag­ma, to drop”.

How to remem­ber the difference

Sta­laC­tite has a c” for ceil­ing”
Sta­laG­mite has a g” for ground”
Sta­lac­tites hang tite” to the ceil­ing above
Sta­lag­mites mite” grow all the way to the ceiling

Bones and brec­cia in caves

Solu­tion and roof col­lapse cre­at­ed entrances to the caves in the form of ver­ti­cal shafts. Soil, rocks, bones and veg­e­ta­tion fell in from the sur­face – the ani­mals and plants from which fos­sils formed did not actu­al­ly live in the caves.