Tertiary

==Historical use of the term==
The term Tertiary was first used by [[Giovanni Arduino (geologist)|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 [[Northern Italy]].<ref>{{cite book |first=Carl O. |last=Dunbar |title=Historical Geology |edition=2nd |year=1964 |publisher=John Wiley & Sons |location=New York, NY |page=352}}</ref> Later a fourth period, the [[Quaternary]], was applied.

In the early development of the study of geology, the periods were thought by [[scriptural geologist]]s to correspond to the Biblical narrative, the rocks of the Tertiary being thought to be associated with the [[Flood myth|Great Flood]].<ref name="Rudwick">{{cite book |author-link=Martin J. S. Rudwick |author=Rudwick, M. J. S. |year=1992 |title=Scenes from Deep Time: Early Pictorial Representations of the Prehistoric World |publisher=[[University of Chicago Press]] |chapter-url=https://books.google.com/books?id=eM0xUqbjzs0C |chapter=Except |isbn=9780226731056 |via=Google Books}}</ref>

In 1833, [[Charles Lyell]] incorporated a Tertiary Period into his own, far more detailed system of classification, based on [[fossil]] [[mollusk]]s 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 [[stratum|strata]]. He used [[Greek language|Greek]] names: Eocene, Miocene, Older Pliocene, and Newer Pliocene.<ref>{{cite journal |last1=Berggren |first1=William A. |title=The Cenozoic Era: Lyellian (chrono)stratigraphy and nomenclatural reform at the millennium |journal=Geological Society, London, Special Publications |date=1998 |volume=143 |issue=1 |pages=111–132 |doi=10.1144/GSL.SP.1998.143.01.10|bibcode=1998GSLSP.143..111B |s2cid=44668464 |citeseerx=10.1.1.488.5133 }}</ref>

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. Therefore, the use of mollusks was abandoned from the definition and the epochs were renamed and redefined.{{Citation needed|date=February 2024|reason=Citation needed for the entire above section, where none are provided}}

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.{{Citation needed|date=February 2024|reason=Citation needed for the entire above section, where none are provided}}