The period began with the extinction of the non-avian dinosaurs in the Cretaceous–Paleogene extinction event, at the start of the Cenozoic Era, and extended to the beginning of the Quaternary glaciation at the end of the Pliocene Epoch.
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 Great Flood.
[3] In 1833, Charles Lyell incorporated a Tertiary Period into his own, far more detailed system of classification, based on fossil mollusks 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 strata.
[4] Although these divisions seemed adequate for the region to which the designations were originally applied (parts of the Alps and plains of Italy), when the same system was later extended to other parts of Europe and to America, it proved to be inapplicable.