Geologic formations are typically named for the geographic area in which they were first described. The thickness of formations may range from less than a meter to several thousand meters. The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region and the detail needed for geologic mapping and to work out its geologic history.ДЇormations must be able to be delineated at the scale of geologic mapping practiced in the region. Igneous intrusive rocks are generally not divided into Formations.Дefining lithostratigraphic formations ДЇormations are the only formal lithostratigraphic units into which the stratigraphic column everywhere should be divided completely on the basis of lithology. Geologic formations are usually sedimentary rock layers, but may also be metamorphic rocks and volcanic flows. For example, the Hammersley Basin is a Proterozoic sedimentary basin where up to 1200 million years of sedimentation is preserved within the intact sedimentary stratigraphy, with up to 300 million years represented by a single lithological unit of banded iron formation and shale. Modern revision of the geologic sciences has restricted Formations to lithologies, because lithological units are formed by depositional environments, some of which may persist for hundreds of millions of years and will transgress chronostratigraphic intervals or fossil-based methods of correlating rocks. The divisions of the geological time scale were the formations described and put in chronological order by the geologists and stratigraphers of the 17th and 18th centuries. A formation can be divided into members and are themselves grouped together in groups.ДЇormations allow geologists to correlate geologic strata across wide distances between outcrops and exposures of rock strata.ДЇormations were initially described to be the essential geologic time markers based on relative ages and the law of superposition. The concept of formally defined layers or strata is central to the geologic discipline of stratigraphy. Formations are not defined on the thickness of the rock strata they consist of and the thickness of different formations can therefore vary widely.

A formation consists of a certain number of rock strata that have a comparable lithology, facies or other similar properties.

Black numbers correspond to groups of formations and white numbers correspond to formations (click on picture for more information).ДЄ formation or geological formation is the fundamental unit of lithostratigraphy. These gaps are called unconformities in geological jargon.A geologic cross section of the Grand Canyon. This causes gaps in the sequence of strata, which may have risen and sunk many times in Earth history. When strata rise above sea level they get worn down by erosion, such as weather. Then it requires research to work out what has happened to the strata. In the long passage of time, sedimentary rocks may get deformed by huge forces in the Earth: volcanism, orogeny (mountain building) or other causes. In normal strata, the later strata are laid down on earlier strata in horizontal layers. For instance, chalk was laid down in the Upper Cretaceous period, and consists mainly of the remains of microscopic algae called coccoliths. The strata are often typical of a particular time and place, and allow geologists to relate rocks in different places. These layers are laid down as sediment, often in the sea, and are slowly changed by pressure, heat and chemical action into rocks. The study of strata is called stratigraphy. It is a term in sedimentary and historical geology the singular is stratum. Strata are layers of rock, or sometimes soil. This aerial photo shows the sloping strata in the cliff quite well

Rock strata definition