Geologists have divided the whole of geologic history into units of varying magnitude. Together, they comprise the geologic time scale of Earth history(Figure 11.19). The major units of the time scale were delineated during the 19th century, principally by scientists working in western Europe and Great Britain. Because radiometric dating was unavailable at that time, the entire time scale was created using methods of relative dating It was only in the 20th century that radiometric dating permitted numerical dates to be added