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The Tertiary ( ) is an obsolete geologic period spanning 66 million to 2.6 or 1.8 million years ago. The period began with the extinction of the non- in the Cretaceous–Paleogene extinction event, at the start of the , and extended to the beginning of the Quaternary glaciation at the end of the . The Tertiary has not been recognised by the International Commission on Stratigraphy (ICS) since the late 1980s, with the timespan of the Tertiary now being split in to the earlier and the more recent periods, though the Tertiary continues to be used in some scientific publications.


Historical use of the term
The term Tertiary was first used by Giovanni Arduino during the mid-18th century. He classified geologic time into primitive (or primary), secondary, and tertiary periods based on observations of geology in . Later a fourth period, the , was applied.

In the early development of the study of geology, the periods were thought by scriptural geologists to correspond to the Biblical narrative, the rocks of the Tertiary being thought to be associated with the .

(1992). 9780226731056, University of Chicago Press.

In 1833, incorporated a Tertiary Period into his own, far more detailed system of classification, based on he collected in Italy and Sicily in 1828–1829. He subdivided the Tertiary Period into four epochs according to the percentage of fossil mollusks resembling modern species found in those . He used names: Eocene, Miocene, Older Pliocene, and Newer Pliocene.

Although these divisions seemed adequate for the region to which the designations were originally applied (parts of the and plains of Italy), when the same system was later extended to other parts of Europe and to America, it proved to be inapplicable. Therefore, the use of mollusks was abandoned from the definition and the epochs were renamed and redefined.

For much of the time during which the term 'Tertiary' was in formal use, it referred to the span of time between 65 and 1.8 million years ago. The end date of the Cretaceous and the start date of the Quaternary were subsequently redefined at c. 66 and 2.6 million years ago respectively.

In 1989, the International Commission on Stratigraphy (ICS) eliminated the use of "Tertiary" on their stratigraphic charts, instead dividing this timespan into the and periods (terms which had been coined over a century earlier in the 1850s by Austrian geologist Moritz Hörnes), which in previous stratigraphic charts were treated as subdivisions of the Tertiary. In 2004, the ICS considered the Neogene to also include the entire period, thus making the Neogene span until the present, but this was not widely followed and the ICS later restored the Quaternary to full period status by 2009. In 2006 it was remarked that despite the ICS elimination of the term over a decade earlier, at that time, "Tertiary" was still more common than Paleogene and Neogene.


Modern equivalents
The Tertiary period lies between the and the , although it is no longer recognized as a formal unit by the International Commission on Stratigraphy.

The span of the Tertiary is subdivided into the (66–56 million years ), the (56–33.9 million years BP), the (33–23.9 million years BP), the (23–5.3 million years BP) and the (5.3–2.6 million years BP), extending to the first stage of the , the Stage.

(2025). 9780521781428, Cambridge University Press. .
(2025). 9780444594259, Elsevier. .


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