Eonothem

In practice, the rock column is discontinuous: Technically, a complete geologic record doesn't occur anywhere.

[1]Eonothems, despite discontinuities (locally missing strata or unconformities), can be compared to others where the rock record is more complete and, by correlation of points of correspondence, be fixed appropriately within the eon.

Eonothems are subdivided into erathems and their smaller subdivisions within geology and paleobiology and their sub-fields, and a whole system of cross-disciplinary classification by strata is in place with oversight by the International Commission on Stratigraphy.

Global Standard Stratigraphic Ages (GSSAs) are defined by the International Commission on Stratigraphy and are used primarily for time-dating rock layers older than 630 million years ago (mya), before a good fossil record exists.

Eonothem derives from eon, “age”, a Latin transliteration from the koine Greek word ὁ αἰών (ho aion) from the archaic αἰϝών (aiwon), and thema, "that which is placed or laid down", "subject of a discourse".

Horseshoe Canyon Formations exposed in Horseshoe Canyon near Drumheller , Alberta .
Oxfordian (Upper Jurassic) cyclic sediments at Péry-Reuchenette, near Tavannes, kanton Bern, Switzerland. Alternating layers are limestone (light, more competent ) and marl / clay ; dominant cycle is the 200000 year-cycle.