The other scale measures absolute time. Both scales help describe the geologic record. Relative time places events in a sequence but does not identify their actual date of occurrence. It indicates age in comparison with other events. The relative time scale is based on the sequence of layering of the rocks and the evolution of life.
Absolute time identifies the actual date of an event.
Geologists use the radiometric time scale which is based on the natural radioactivity of chemical elements in some of the rocks. Such values make it possible to determine the rate at which a geologic process such as mountain building occurs. Absolute time might seem more important than relative time because absolute time gives an actual date, but absolute dates are difficult and expensive to obtain. Most geologic work is done using relative time. The information is accessible from many different sources.