through
Jurassic strata of the
Colorado Plateau area of southeastern
Utah demonstrate the principles of
stratigraphy. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as
Capitol Reef National Park and
Canyonlands National Park. From top to bottom: Rounded tan domes of the
Navajo Sandstone, layered red
Kayenta Formation, cliff-forming, vertically jointed, red
Wingate Sandstone, slope-forming, purplish
Chinle Formation, layered, lighter-red
Moenkopi Formation, and white, layered
Cutler Formation sandstone. Picture from
Glen Canyon National Recreation Area, Utah. (Ayers Rock) in
Australia is underlain by the Mutitjulu Arkose, a formation composed almost entirely of a single lithology (arkosic sandstone). is composed of alternating thin beds of two lithologies, mudstone and sandstone, penetrated by veins of a third lithology, gypsum. The boundaries of a formation are chosen to give it the greatest practical lithological consistency. Formations should not be defined by any criteria other than lithology. The lithology of a formation includes characteristics such as chemical and mineralogical composition, texture, color, primary
depositional structures,
fossils regarded as rock-forming particles, or other organic materials such as
coal or
kerogen. The
taxonomy of fossils is
not a valid lithological basis for defining a formation. The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region. Formations must be able to be delineated at the scale of geological mapping normally practiced in the region; the thickness of formations may range from less than a meter to several thousand meters. Geologic formations are typically named after a permanent natural or artificial feature of the geographic area in which they were first described. The name consists of the geographic name plus either "Formation" or a descriptive name. Examples include the
Morrison Formation, named for the town of
Morrison, Colorado, and the
Kaibab Limestone, named after the
Kaibab Plateau of Arizona. The names must not duplicate previous formation names, so, for example, a newly designated formation could not be named the Kaibab Formation, since the Kaibab Limestone is already established as a formation name. The first use of a name has precedence over all others, as does the first name applied to a particular formation. As with other stratigraphic units, the formal designation of a formation includes a
stratotype which is usually a
type section. A type section is ideally a good exposure of the formation that shows its entire thickness. If the formation is nowhere entirely exposed, or if it shows considerably lateral variation, additional
reference sections may be defined. Long-established formations dating to before the modern codification of stratigraphy, or which lack tabular form (such as volcanic formations), may substitute a type locality for a type section as their stratotype. The geologist defining the formation is expected to describe the stratotype in sufficient detail that other geologists can unequivocally recognize the formation. Although formations should not be defined by any criteria other than primary lithology, it is often useful to define
biostratigraphic units on
paleontological criteria,
chronostratigraphic units on the age of the rocks, and
chemostratigraphic units on geochemical criteria, and these are included in stratigraphic codes. ==Usefulness of formations==