Knowledge of the Geologic Time Scale is important in the interpretation of geologic maps. The ages of the rocks on the surface allow us to determine the sequences of geologic events in the developement of a region and give us clues as to what's happening underneath. The ages of the rocks are given on the geologic map. On most maps the geologic periods are used in labeling the map.
For convenience, geologists have grouped certain periods into Eras. The Eras can be grouped into Eons. Some geologic maps of very large areas, for example, may lump all of the rocks of the Paleozoic into one "rock unit" on the map. Consequently, a familiarity with the geologic time scale from the Eons to the periods is essential for the interpretation of maps.