Geological formation

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'.
Formations were initially described as time markers, based on relative dating and the law of superposition. The divisions of the history of the Earth were the formations described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries. 
Rock formations are formed by sedimentary deposition in environments which may persist for hundreds of millions of years. For example, the Hammersley Basin in Pilbara, Western Australia, is a Proterozoic sedimentary basin where up to 1200 million years of sedimentation is preserved intact. Here, up to 300 million years is represented by a single unit of banded iron formation and shale.