A Geologic Time Scale A geologic time scale is a framework for deciphering the history of the Earth since its birth. It is a two fold scale that subdivides all the rocks formed since the Earth came. It is believed that Earth is about 4.6 billion years old. A time scale consists of geologic time units known as geochronologic unit. Each unit comprising a number of units of shorter time interval. It include eon, era, period, epoch, age, subage or chronozone. Eon is divided into eras and era is further divided into periods and period is divided into epochs. In geologic time scale there are two major eons- the Precambrian eon and Phanerozoic eon. Each unit is characterised by specific event which took place on Earth in past geologic time. A Geologic Time Scale The branch of geology that deals with the age relations of rocks is known as Chronostratigraphy. Each geologic time unit coincides with a particular chronostratigraphic unit.