Geological formation

A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics (lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exposed in a geographical region (the stratigraphic column). It is the fundamental unit of lithostratigraphy, the study of strata or rock layers.

A formation must be large enough that it can be mapped at the surface or traced in the subsurface. Formations are otherwise not defined by the thickness of their rock strata, which can vary widely. They are usually, but not universally, tabular in form.

The concept of a geologic formation goes back to the beginnings of modern scientific geology. The term was used by Abraham Gottlob Werner in his theory of the origin of the Earth, which was developed over the period from 1774 to his death in 1817. The concept became increasingly formalized over time.

Geological maps showing where various formations are exposed at the surface are fundamental to such fields as structural geology, allowing geologists to infer the tectonic history of a region or predict likely locations for buried mineral resources.

Defining formations

The boundaries of a formation are chosen to give it consistent characteristics. These characteristics include the composition, texture, color, and materials like coal found in the rocks. Each formation must be large enough to be mapped out clearly.

Geologic formations are often named after a place where they were first found, like the Morrison Formation named after a town in Colorado. These names help scientists identify and study different layers of rock around the world.

Usefulness of formations

The idea of formally defined layers of rock is very important in the study of stratigraphy. A formation is the basic unit in this field of study. Formations can be grouped together into larger units called groups or divided into smaller units called members.

Recognizing formations helps geologists match rock layers across large areas, even between different places where rocks are exposed at the surface, known as outcrops. These formations were originally used as markers of geologic time, based on their order and the law of superposition. Geologic formations apply to sedimentary rock layers, some metamorphic rocks, and volcanic rocks.

Other uses of the term

The word "formation" can also describe the interesting shapes that rocks take due to wind, water, or other natural forces. Once these shapes are no longer changed by nature, they are called abandoned formations. Some famous rock shapes found in caves are stalactites and stalagmites.