A fundamental geological unit used in the description and interpretation of layered sediments, sedimentary rocks, and extrusive igneous rocks. A formation is defined on the basis of lithic characteristics and position within a stratigraphic succession. It is usually tabular or sheetlike, and is mappable at the Earth's surface or traceable in the subsurface (for example, between boreholes or in mines). Examples are readily recognized in the walls of the Grand Canyon of northern Arizona (see illus.). Each formation is referred to a section or locality where it is well developed (a type section), and assigned an appropriate geographic name combined with the word formation or a descriptive lithic term such as limestone, sandstone, or shale (for example, Temple Butte Formation, Hermit Shale). This usage of “formation” by geologists differs from its informal lay usage for stalactites, stalagmites, and other mineral buildups in caves.