A geologic cross section of the Grand Canyon. Black numbers correspond to groups of formations and white numbers correspond to formations (click on picture for more information).

Strata of the Grand Canyon

A formation, or rock formation, is the fundamental unit of lithostratigraphy.

A formation consists of a certain number of rock strata. They have similar lithology (rocks), sedimentary facies (appearance) or other properties. Formations are not defined on the thickness of the rock strata, and the thickness of different formations can therefore vary widely.

The concept of formally defined layers or strata is central to stratigraphy. A formation can be divided into 'members' and are themselves packed together in 'groups'.