Geologic formation

From Freepedia

A geologic formation is a formally named rock stratum or unit. The concept of formally defined layers or strata is central to the geologic discipline of stratigraphy. The formations are defined based on lithologic (rock type) and paleontologic criteria. Formations allow geologists to correlate geologic strata across wide distances. Formations were 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 17th and 18th centuries. Geologic formations are typically named for the geographic area in which they were first described. A widely recognized example of a formation is the Jurassic dinosaur bearing Morrison Formation of the western United States which was named for an exposure near Morrison, Colorado.

Geologic formations are usually sedimentary rock layers, but may also be metamorphic rocks and volcanic flows or even igneous intrusive bodies.

The term "formation" is often used informally to refer to a specific grouping of rocks, such as those encountered within a certain depth range in an oil well.

"Formation" is also used informally to describe the sometimes odd shapes (forms) that rocks acquire through erosional or depositional processes. Some well known cave formations include stalactites and stalagmites.