Rocks can also be classified according to the period when they were formed. As with rock types, geologists use a tree strucutre for classification, identifying four or five major eras, and then subdividing each of these into periods. The main Eras, with their sub-divisions, are listed below. The Cambrian is usually regarded as the period when large numbers of life forms become abundant. The characteristics of the fossil record have a major role in determining the later divisions.
(1) The Tertiary and Quaternary are frequently regarded as a single era, the Cenozoic.
(2) An alternative division of the Tertiary is between the earlier 'Paleogene' (comprising Eocene and Oligocene) and the later 'Neogene' (covering the Miocene and Pilocene).