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The Sterk­fontein Caves

A lime­stone cave or cav­ern is a nat­ur­al cav­i­ty that is formed under­neath the Earth’s sur­face that can range from a few metres to many kilo­me­tres in length and depth.

Most of the world’s caves, includ­ing those at the Cra­dle of Humankind, are formed in porous lime­stone. Over mil­lions of years, acidic ground­wa­ter or under­ground rivers dis­solve away the lime­stone, leav­ing cav­i­ties which grow over time.

Ear­ly life forms appeared in the oceans about 3.8-billion years ago. These were sin­gle-celled, blue-green algae, called cyanobac­te­ria, which made their own food through pho­to­syn­the­sis, releas­ing oxy­gen into the atmos­phere in the process.

Dolomitic lime­stone, a sed­i­men­ta­ry rock, was formed over mil­lions of years through chem­i­cal reac­tions gen­er­at­ed by these ear­ly organisms.

With move­ments with­in the Earth’s crust, the sed­i­men­ta­ry dolomitic lime­stone even­tu­al­ly became exposed on dry land.

As time passed the lime­stone, which is per­me­able and sol­u­ble, was erod­ed by water. Weak car­bon­ic acid in rain­wa­ter, react­ing with the chem­i­cals in the rock, dis­solved and erod­ed away the lime­stone as the water fil­tered into the under­ly­ing depths of sed­i­ments. Large hol­low solu­tion cav­i­ties were formed in the lime­stone in this way.

Many cav­i­ties occur at var­i­ous depths in a cave sys­tem due to the con­tin­u­al seep­age and flow of the mild­ly acidic water through the deposits, while under­ground rivers may even­tu­al­ly carve their way through a moun­tain­side, cre­at­ing open­ings and entrances to the outside.

Oth­er cave entrances include pit and depres­sion areas that are locat­ed at the tops of caves.

Strange, beau­ti­ful structures

Many beau­ti­ful struc­tures – includ­ing sta­lag­mites and sta­lac­tites – form inside caves as car­bon­ic acid, car­ry­ing lime­stone, drips through cave roofs and onto their floors. Struc­tures inside a cave may require mil­lions of years to develop.

Some of the geo­log­i­cal struc­tures that may devel­op inside a cave include:

Speleothem

Speleothem” is a gen­er­al geo­log­i­cal term for a deposit of cal­ci­um car­bon­ate in a cave, includ­ing for­ma­tions such as sta­lac­tites, sta­lag­mites and flowstones.

Flow­stone

Flow­stones are speleothems (deposits of cal­ci­um car­bon­ate) on the walls or floor of a cave formed from a grad­ual flow of water over a rel­a­tive­ly broad area.

Sta­lac­tite

The term sta­lac­tite comes from the Greek work sta­lak­tos, which means drip­ping”, because these oth­er-world­ly for­ma­tions drip” from the roofs of lime­stone caves. Essen­tial­ly, water reacts with car­bon-diox­ide to form car­bon­ic acid. It then seeps slow­ly through the roof of the cave, deposit­ing cal­ci­um car­bon­ate, which hard­ens and builds up over time to form a stalactite.

Sta­lag­mite

Sta­lag­mites are cor­re­spond­ing for­ma­tions on the floor of caves to sta­lac­tites. Sta­lag­mites rise from the floor in a build-up of cal­ci­um car­bon­ate over time, from min­er­al-bear­ing water dropped from the roof of the cave. The term sta­lag­mite comes from the Greek word, sta­lag­ma, to drop”.

Col­umn

Some­times, sta­lac­tites and sta­lag­mites meet, form­ing a pil­lar or col­umn of rock-hard cal­ci­um carbonate.

Helic­tite

A for­ma­tion of cal­ci­um car­bon­ate in a cave that grows in a twist­ed, curled fash­ion, like a helix (hence the name), seem­ing­ly defy­ing the laws of gravity.

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