Origins and Development of Limestone

Where limestone caves are found, the first wonder is not the caves themselves.  The first wonder is the rock in which they are found - limestone. Limestone was not one of the primary rocks of the earth's crust.  It did not exist when dry land first appeared.

Limestone is a sedimentary rock that contains more than 50% calcium carbonate (CaCO3).  It consists of the cemented remains of marine organisms, including corals, brachiopods, stromatoporoids and crinoids. You may find many fossils in limestone, or very few. At Jenolan, the limestone includes the remains of a coral reef, though it is composed mainly of fine grained calcite (lime mud). 

Over millions of years, these sediments become buried under layers of volcanic rocks, formed when solid matter from explosive volcanic eruptions fell into the sea. This eventually compressed the sediments (lithification) into the hard rock known as limestone.

Also, over millions of years, Earth movements lift both the limestone and the covering volcanics from the sea, and expose it to the atmosphere. During the uplifting process, the rock is often turned, twisted and deformed.  You may see the beds of limestone at various angles, even completely overturned.

Once exposed to surface conditions, the overlying volcanics are open to the elements.  During long periods of time, they are often eroded to expose the limestone.  Then, cave formation may begin, although it may begin even before the overlying volcanics have been completely eroded.

Some limestones undergo further heat, pressure and mechanical stress, and are transformed into marble. During this process (metamorphism), the limestone is re-crystallised. This type of rock, marble, can be seen at Wombeyan Caves.

Limestone and marble have many commercial uses, including the production of cement, lime, decorative rock, including tiles, tables and statues, toothpaste, ladies face powder and, believe it or not, even toilet paper.