In geology and related fields, a stratum (plural: strata) is a layer of sedimentary rock or soil, or igneous rock that was formed at the Earth's surface,[1] with internally consistent characteristics that distinguish it from other layers. The "stratum" is the fundamental unit in a stratigraphic column and forms the basis of the study of stratigraphy. A stratum can be seen in almost every single country in the world.

Strata in Salta (Argentina).
Goldenville strata in quarry in Bedford, Canada. These are Middle Cambrian marine sediments. This formation covers over half of Nova Scotia and is recorded as being
8,800 m (29,000 ft) thick in some areas.