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A formation or geological formation is the fundamental unit of lithostratigraphy. 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. Formations can be divided into members and are themselves frequently parcelled together in groups. ==Usefulness of formations== The definition and recognition of formations allow geologists to correlate geologic strata across wide distances between outcrops and exposures of rock strata. Formations were initially described to be the essential geologic time markers based on relative ages and the law of superposition. The divisions of the geological time scale were the formations described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries. 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. For example, the Hamersley Basin of Western Australia 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. Geologic formations are usually sedimentary rock layers, but may also be metamorphic rocks and volcanic flows. Igneous intrusive rocks are generally not divided into formations. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「geological formation」の詳細全文を読む スポンサード リンク
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