Other forms: Tertiaries
Tertiary is the term formerly used to describe the geologic period that lasted from about 66 to 2.6 million years ago. Today, Tertiary is an obsolete term, having been split into the Paleogene and Neogene in 1989.
The name Tertiary, meaning "third," was coined by 18th-century geologist Giovanni Arduino, who classified rocks into four distinct, chronological layers: Primary for the deepest, oldest rocks, followed by Secondary, Tertiary, and finally Quaternary, the newest, top layer of rock. As scientists learned more about geologic history, this naming system was abandoned. While the Tertiary was eventually split into two distinct periods, the term is sometimes still used informally.