Floian

The Floian is the second stage of the Ordovician. It succeeds the Tremadocian with which it forms the Lower Ordovician epoch. It precedes the Dapingian stage of the Middle Ordovician. The Floian extended from 477.7 to 470 million years ago. The lower boundary is defined as the first appearance of the graptolite species Tetragraptus approximatus.

The Floian stage is named after Flo, a village in Västergötland, southern Sweden. The name "Floan" was proposed in 2004, but the International Commission on Stratigraphy adapted Floian as the official name of the stage.

The GSSP of the Floian is the Diabasbrottet Quarry (58°21′32″N 12°30′09″E / 58.3589°N 12.5024°E / 58.3589; 12.5024) which is an outcrop of a shale-dominated stratigraphic succession. The lower boundary of the Floian is defined as the first appearance of Tetragraptus approximatus which is above the base of the Tøyen Shale. Radiometric dating has set the Tremadocian-Floian boundary at 477.7million years ago.