Visitors to Jenolan Caves often want to know how old things are. In order to provide these answers someone firstly has to do the dating. A number of techniques are available with which to date rocks.

A large Silurian coral Ref : Geology of NSW - Sussmilch, 1914 Angus & Robertson

1. Use of Fossils. It is possible to examine a rock to see what types of fossils it contains. The presence of certain fossils can be used to indicate roughly what age the rock dates from.

2. Law of Superposition. In many instances it may not be possible to find fossils to date a rock, nor is it possible to use other techniques used to extract a date. If though it is possible to identify what rock strata lie above and below a rock bed, and provided their ages are known, an approcimate age can be applied to the strata in question. If a bed of rock overlays another then it can be surmised that the overlying rock formed after the rock below it.

3. Radiometric Dating. This technique is useful for dating calcite deposits in caves. It relies on the fact that most water on earth contains a small amount of uranium. Uranium is unstable and over time it decays to produce thorium and lead. The older the calcite is, the more uranium that is trapped within it will have decayed to produce more thorium. The ratio of thorium to uranium can be measured and this ratio is then used to produce an approximate date. This technique can be used to date sample sback to about 400,000 years ago.