By comparing the proportion of the two, one can work out how old the deposit is.
With the discovery of radiometric dating, it became possible for the first time to attempt precise figures.
Radiometric dating works on the principle that certain atoms and isotopes are unstable.
The Christian Creationists have criticized it on the grounds that it is inaccurate.
But these inaccuracies are the result of variation in the level of Carbon 14 in the atmosphere, and when this is worked out (through calibration with tree rings of the bristlecone pine, the oldest living organism) precise dates can be had.
However, the error range increases drastically once you pass 50,000 years.