The term geologic column is applied to a composite columnar section in which there is an attempt to superimpose rocks representing every period of time for the world as a whole. In this way it is thought that the column can be used somewhat like a calendar for dating rock formations. Time units used in the geologic column include eras, the major divisions such as Paleozoic, Mesozoic and Cenozoic; periods, the first subdivisions of eras including Cambrian, Ordovician, Silurian, etc.; epochs, the subdivisions of periods such as Lower, Middle and Upper Cambrian, or Paleocene, Eocene and so on in the Tertiary period; and stages, divisions of the epochs used primarily by specialists. For each time division there is a unit of strata equivalent.