Geological formation

A geological formation is the fundamental unit of lithostratigraphy. Lithostratigraphy is the scientific study of rock layers, also called strata. A formation is a specific body of rock that has a consistent set of physical characteristics. Scientists call these characteristics lithology. This lithology must distinguish the formation from any adjacent bodies of rock. Formations also occupy a specific position within a stratigraphic column. This column is the sequence of rock layers exposed in a geographic region. Understanding these units helps geologists map the Earth's surface and subsurface.

To be classified as a formation, a rock body must meet specific criteria. It must be large enough to be mapped at the surface. It must also be large enough to be traced in the subsurface. The thickness of a formation can vary significantly. Some formations are less than one meter thick. Others can reach several thousand meters in thickness. While most formations are tabular, or flat and plate-like, they are not always this shape. A formation might consist of a single lithology. It may also consist of alternating beds of different lithologies. It can even be a heterogeneous mixture of different rocks.

Lithology is the primary way scientists define these units. This includes the chemical and mineralogical composition of the rock. It also includes the texture and the color of the stone. Scientists look at primary depositional structures to identify a formation. They may also look at fossils that act as rock-forming particles. They might even look at organic materials like coal or kerogen. However, the specific taxonomy of fossils is not a valid basis for defining a formation. The boundaries of a formation are chosen to provide the greatest practical lithological consistency.

The concept of the geological formation is quite old. It dates back to the beginnings of modern scientific geology. Abraham Gottlob Werner used the term in his theories. He developed his ideas regarding the origin of the Earth between 1774 and 1817. Over time, the concept became much more formal. Today, these rules are codified in official documents. One such example is the North American Stratigraphic Code. Other regions have their own specific counterparts for these rules.

Naming a formation follows strict rules to avoid confusion. Geologists typically name them after permanent natural or artificial features. This might be a town or a geographic landmark. For example, the Morrison Formation is named after Morrison, Colorado. The Kaibab Limestone is named after the Kaibab Plateau in Arizona. A name cannot be used for two different formations. The first person to use a name has precedence. To help others recognize a formation, geologists use a stratotype. This is often a type section. A type section is an ideal exposure that shows the entire thickness of the formation.

Formations are very useful for many different scientific fields. They are the building blocks for larger units called groups. Formations can also be divided into smaller parts called members. Members do not have to be mappable at the same scale as formations. However, they must still be lithologically distinctive. Formations are especially useful for sedimentary rocks, volcanic rocks, and low-grade metamorphic rocks. They are not usually used for intrusive igneous rocks. Those are instead described as lithodemes.

Geologists use formation maps to perform important work. Structural geologists use these maps to study the Earth. They can use them to infer the tectonic history of a region. This helps them understand how the land has moved. These maps also help predict where buried mineral resources might be located. By recognizing formations, scientists can correlate rock layers across very wide distances. This allows them to connect different outcrops and exposures. This work helps us understand the complex history of our planet.