Tertiary (historical geological term)

Time span and main characteristics

Informally, the interval called the Tertiary extended from the beginning of the Cenozoic, about 66 million years ago, until roughly 2.6 million years ago. During this interval the world recovered from the end-Cretaceous extinction, mammal lineages diversified and occupied many ecological roles, modern orders of birds and flowering plants became widespread, and continents continued to drift toward their present positions. Climate trends included a warm early phase followed by longer-term cooling and the development of polar ice by the end of the interval.

History of the term and formal replacement

The label "Tertiary" originated in the 18th–19th century practice of dividing sedimentary rocks into broad groups (primary, secondary, tertiary, quaternary) based on relative age. Advances in stratigraphy, radiometric dating and international coordination of geologic time scales led to a redefinition of many traditional names. Modern stratigraphic authorities recommended splitting the old Tertiary interval into two formal periods: the Palaeogene and the Neogene, which together cover roughly the same time span that the Tertiary once denoted. Decisions about these names and boundaries are overseen by international bodies responsible for standardizing the geologic time scale.

Uses, examples and where the name persists

Although not preferred in formal chronologies, the word Tertiary still appears in many contexts: older scientific papers and books, regional geological maps and reports, petroleum exploration literature, museum labels, and general-audience materials. In such contexts it is often used as a convenient shorthand for the interval encompassing the Palaeogene and Neogene. Readers consulting modern stratigraphic charts should instead look for the Palaeogene (earlier) and Neogene (later) subdivisions when precise ages or stage names are required.

Distinctions and notable facts

• The persistent use of Tertiary explains why the next-youngest interval is called the Quaternary: the name originally followed the Tertiary in the older fourfold scheme.
• When precision matters, use the formal period and epoch names adopted by international stratigraphic commissions rather than the umbrella term Tertiary.
• Evolutionary milestones often associated with this interval include rapid mammalian diversification, the spread of grasslands in the Neogene, and the earliest appearances of hominin ancestors in the later part of the Neogene.

Guidance for readers

For historical reading or broad-stroke descriptions, "Tertiary" remains intelligible and useful. For scientific work, mapping, or age determination, consult current stratigraphic charts and use the formal subdivisions: the Palaeogene and Neogene, and refer to standards set by international organizations concerned with stratigraphy such as those noted by sources linked under the historical label Tertiary (historical).