Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates. A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties. Formations are not defined on the thickness of the rock strata they consist of and the thickness of different formations can therefore vary widely. The concept of formally defined layers or strata is central to the geologic discipline of stratigraphy. A formation can be divided into members and are themselves grouped together in groups. In geology and related fields, a stratum (plural: strata) is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers. Each layer is generally one of a number of parallel layers that lie one upon another, laid down by natural forces. Strata are typically seen as bands of different colored or differently structured material exposed in cliffs, road cuts, quarries, and river banks. Each band represents a specific mode of deposition: river silt, beach sand, coal swamp, sand dune, lava beds, just to name a few. Originally captioned: The earths crust reveals its history.