Geological formation

A formation or geological formation is the fundamental unit of lithostratigraphy. A formation consists of a certain amount of rock strata that have a comparable lithology, facies or other similar properties. Formations are not defined by the thickness of their rock strata; therefore the thickness of different formations can vary widely.

The concept of formally defined layers or strata is central to the geologic discipline of stratigraphy. Groups of strata are divided into formations, which are divided into members.

Grand Canyon geologic column
A geologic cross section of the Grand Canyon. Black numbers correspond to groups of formations and white numbers correspond to formations (click on picture for more information).

Usefulness of formations

The definition and recognition of formations allow geologists to correlate geologic strata across wide distances between outcrops and exposures of rock strata. Formations were at first described as the essential geologic time markers, based on their relative ages and the law of superposition. The divisions of the geological time scale were described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries.

The lithology of a rock is a description of its readily visible physical characteristics. Modern geology prefers to use lithology, that it an examination of the visible features of the component rocks, to identify discrete formations.

Geologic formations are divided into the broad categories of: sedimentary rock layers; metamorphic rocks; and volcanic rocks. Intrusive igneous rocks are generally not considered to be formations.

Defining lithostratigraphic formations

The Permian 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.

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 geologic 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 the geographic area in which they were first described.

Strictly, formations cannot be defined by any criteria other than primary lithology. Nevertheless, 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.

Other uses of the term

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 odd shapes (forms) that rocks acquire through erosional or depositional processes. Such a formation is abandoned when it is no longer affected by the geologic agent that produced it. Some well-known cave formations include stalactites and stalagmites.

See also

Further reading

  • Brookfield, Michael E. (2008). Principles of Stratigraphy. John Wiley & Sons. pp. 111–114, 200. ISBN 9780470693223.
  • Rey, Jacques; Simone Galeotti, eds. (2008). Stratigraphy : terminology and practice. Paris, France: Editions OPHRYS. ISBN 9782710809104.
Azapa Formation

Azapa Formation (Spanish: Formación Azapa) is a geological formation in northern Chile made up of gravels of fluvial origin. It is conformably overlain by Oxaya Formation. Azapa Formation is deformed by the Oxaya anticline.

Camotes Sea

The Camotes Sea is a small sea within the Philippine archipelago, situated between the Eastern Visayan and the Central Visayan regions. It is bordered by the islands of Leyte to the north and east, Bohol to the south, and Cebu to the west. It contains the Camotes Islands, Lapinig Island, Olango Island, Mactan Island, and various other small islets.

The sea is connected to the Visayan Sea to the northwest. It is connected to the Bohol Sea (also called the Mindanao Sea) in two ways: to the SW by the Cebu Strait (and its 3 channels, the Mactan, the Olango, & the Hilutangan), and to the SE by the Canigao Channel.

The Camotes Sea also contains the Danajon Bank, which is a double barrier reef in the Philippines, which is a very rare geological formation, and there are only 6 double barrier reefs in the world. It comprises two sets of large coral reefs that formed offshore on a submarine ridge due to a combination of favorable tidal currents and coral growth in the area.

Campanario Formation

Campanario Formation (Spanish: Formación Campanario) is a Miocene geological formation in the high Andes of Central Chile and northern Neuquén Province in Argentina. The formation crops out around the upper parts of the basins of Maule and Ñuble rivers.

Chimney Rock National Historic Site

Chimney Rock is a prominent geological rock formation in Morrill County in western Nebraska. Rising nearly 300 feet (91 m) above the surrounding North Platte River valley, the peak of Chimney Rock is 4,228 feet (1,289 m) above sea level. The formation served as a landmark along the Oregon Trail, the California Trail, and the Mormon Trail during the mid-19th century. The trails ran along the north side of the rock, which remains a visible landmark for modern travelers along U.S. Route 26 and Nebraska Highway 92.

Cholchol Formation

Cholchol Formation (Spanish: Formación Cholchol) is a geological formation composed of sediments that were deposited during the Miocene in the Temuco Basin of south–central Chile. The sediments were deposited in a marine environment.

Drill stem test

A drill stem test (DST) is a procedure for isolating and testing the pressure, permeability and productive capacity of a geological formation during the drilling of a well. The test is an important measurement of pressure behaviour at the drill stem and is a valuable way of obtaining information on the formation fluid and establishing whether a well has found a commercial hydrocarbon reservoir.


The Gault Formation is a geological formation of stiff blue clay deposited in a calm, fairly deep-water marine environment during the Lower Cretaceous Period (Upper and Middle Albian). It is well exposed in the coastal cliffs at Copt Point in Folkestone, Kent, England, where it overlays the Lower Greensand formation, and underlies the Upper Greensand Formation. These represent different facies, with the sandier parts probably being deposited close to the shore and the clay in quieter water further from the source of sediment; both are believed to be shallow-water deposits.The etymology of the name is uncertain and probably of local origin.

Granite Dells, Arizona

Granite Dells is a populated place situated in Yavapai County, Arizona, named after the contiguous Granite Dells geological formation. It has an estimated elevation of 5,089 feet (1,551 m) above sea level.


Gravel is a loose aggregation of rock fragments. Gravel is classified by particle size range and includes size classes from granule- to boulder-sized fragments. In the Udden-Wentworth scale gravel is categorized into granular gravel (2 to 4 mm or 0.079 to 0.157 in) and pebble gravel (4 to 64 mm or 0.2 to 2.5 in). ISO 14688 grades gravels as fine, medium, and coarse with ranges 2 mm to 6.3 mm to 20 mm to 63 mm. One cubic metre of gravel typically weighs about 1,800 kg (or a cubic yard weighs about 3,000 pounds).

Gravel is an important commercial product, with a number of applications. Many roadways are surfaced with gravel, especially in rural areas where there is little traffic. Globally, far more roads are surfaced with gravel than with concrete or asphalt; Russia alone has over 400,000 km (250,000 mi) of gravel roads. Both sand and small gravel are also important for the manufacture of concrete.

Lesser Antilles

The Lesser Antilles is a group of islands in the Caribbean Sea. Most form a long, partly volcanic island arc between the Greater Antilles to the north-west and the continent of South America. The islands form the eastern boundary of the Caribbean Sea with the Atlantic Ocean. Together, the Lesser Antilles and the Greater Antilles compose the Antilles (or the Caribbean in its narrowest definition). When combined with the Lucayan Archipelago, all three are known as the West Indies.


A member is a person who belongs to a social group or organization. By extension it can refer to any part of a whole. The relationship between members is that of peers.

Member may also refer to:

Military jury, referred to as "Members" in military jargon

Element (mathematics), an object that belongs to a mathematical set

In object-oriented programming, a member of a class

Field (computer science), entries in a database

Member variable, a variable that is associated with a specific object

Limb (anatomy), an appendage of the human or animal body

Euphemism for penis

Structural component of a bridge

User (computing), a person making use of a computing service, especially on the Internet

Member (geology), a component of a geological formation

Member of parliament

The Members, a British punk rock band

Meronymy, a semantic relationship in linguistics

Church membership begins at initiation (baptism)

Mininco Formation

Mininco Formation (Spanish: Formación Mininco) is a geological formation composed of sediments that deposited during the Pliocene in central Chile. Near Angol the formation reaches thicknesses of up to 300 m. The upper strata of the formation contain tuff layers and coal beds that are rich in leaf fossils. Other fossils that have been found in the formation include fresh-water diatoms and bivalves.

Niagara Escarpment

The Niagara Escarpment is a long escarpment, or cuesta, in the United States and Canada that runs predominantly east/west from New York, through Ontario, Michigan, Wisconsin, and Illinois. The escarpment is most famous as the cliff over which the Niagara River plunges at Niagara Falls, for which it is named.

The Escarpment is a UNESCO World Biosphere Reserve. It has the oldest forest ecosystem and trees in eastern North America.The Escarpment is composed of the Lockport Formation of Silurian age, and is similar to the Onondaga Formation, which runs parallel to it and just to the south, through western New York and southern Ontario. The Escarpment is the most prominent of several escarpments formed in the bedrock of the Great Lakes Basin. From its easternmost point near Watertown, New York, the escarpment shapes in part the individual basins and landforms of Lakes Ontario, Huron, and Michigan. In Rochester, New York, three waterfalls over the escarpment are where the Genesee River flows through the city. The escarpment thence runs westward to the Niagara River, forming a deep gorge north of Niagara Falls, which itself cascades over the escarpment. In southern Ontario, it spans the Niagara Peninsula, closely following the Lake Ontario shore through the cities of St. Catharines, Hamilton, and Dundas, where it takes a sharp turn north in the town of Milton toward Georgian Bay. It then follows the Georgian Bay shore northwestwards to form the spine of the Bruce Peninsula and Manitoulin Island, as well as several smaller islands in northern Lake Huron, where it turns westwards into the Upper Peninsula of northern Michigan, south of Sault Ste. Marie. It then extends southwards into Wisconsin following the Door Peninsula through the Bayshore Blufflands and then more inland from the western coast of Lake Michigan and Milwaukee, ending northwest of Chicago near the Wisconsin–Illinois border.

Onondaga Limestone

The Onondaga Limestone is a group of hard limestones and dolomites of Devonian age that form an important geographic feature in some areas in which it outcrops, in others; especially its Southern Ontario portion, the formation can be less prominent as a local surface feature.In upstate New York and southern Ontario the sedimentary rocks tend to slope slightly southward, and the Onondaga outcrops in a line that usually forms an escarpment (the steep face of a cuesta), because of its resistance to erosion. The outcrop can be traced from the Hudson River valley westward along the southern rim of the Mohawk River valley, passing just south of Syracuse, and along the northern heads of the major Finger Lakes to Buffalo, New York. From Fort Erie, Ontario it runs to Windsor just north of the Lake Erie shoreline, becoming less prominent as one travels westward. It is not distinct west of Windsor, but begins to become noticeable as a steep hill just northwest of Leamington, as it forms a low ridge/escarpment along much of the Lake Erie shoreline.

In several spots it is breached by geologically young streams and spectacular waterfalls are formed, such as at Chittenango Falls just east of Syracuse, Buttermilk Falls at Le Roy, New York and Indian Falls west of Batavia.

A few other breaches occur in older valleys, which likely once had waterfalls, but erosion eventually obliterated them. Such breaches occur at the Tully valley, the Genesee River valley near Avon, New York, and at Port Colborne, Ontario, where the old valley forms a harbor on Lake Erie.

The formation is broken by the only major fault line in western New York, the Linden Fault just east of Batavia, where the eastern side of the fault has dropped down and the ledge moved southward relative to the western side. On the western side of the fault in Genesee County the escarpment achieves its greatest prominence. The New York State Thruway has a rock cut at Batavia which clearly shows the fault and is a popular point for geology class field trips. The fault, which runs from Attica, New York northward to Lake Ontario, is still active and periodically causes minor earthquakes in the area.

The Onondaga Limestone also can be found in other areas where rocks of the same age outcrop, such as in western Pennsylvania and Michigan but they do not form prominent geographic features.

A similar and more prominent outcrop known as the Niagara Escarpment runs parallel and about 25 miles (40 kilometers) to the north through upstate New York, but curves northwestward in southern Ontario toward Lake Huron and eventually into Michigan's Upper Peninsula and Wisconsin's Door Peninsula.

Another smaller outcrop known as the Portage Escarpment lies about 35 miles (56 km) to the south, running along the southern ends of the Finger Lakes and forming Cascadilla, Ithaca and Buttermilk Falls in Ithaca.

The Onondaga Escarpment contains significant outcrops of flint (a type of chert) which bears the escarpment's name. This variety of chert was of great importance to First Nations peoples throughout Southern Ontario, who used it to make stone tools (lithics) such as projectile points and hide scrapers. This variety of chert, which is of reasonably high-quality and which was highly valued by First Nations peoples, is often a common variety of chert recovered archaeologically from sites relatively adjacent to outcrops; for example, Onondaga-variety chert comprises 95% of all of the flint material from some sites in Milton, Ontario. The material has also been found as well at some distance from its original source; Onondaga chert has been recovered at the late archaic Duck Lake archaeological site in northern Michigan, circa 400 kilometers from the nearest outcropping of the material. This wide distribution implies either a very large seasonal migration of ancient peoples or long-distance trade routes, with both likely being the case at different times throughout the prehistory of the Great Lakes region.

Oxaya Formation

Oxaya Formation (Spanish: Formación Oxaya) is a geological formation in northern Chile made up of ignimbrite sheets. The formation formed about 25 to 19 million years ago in the Late Oligocene and Early Miocene. Oxaya Formation is deformed by the Oxaya anticline.

Quiriquina Formation

The Quiriquina Formation is a geological formation in Chile whose strata date back to the Late Cretaceous. Dinosaur remains are among the fossils that have been recovered from the formation. The glauconitic sandstones and conglomerates of the formation were deposited in a marine environment.

San Cristóbal Canton

San Cristóbal Canton is a canton of Ecuador in the Galápagos Islands archipelago and one of the oldest in this insular geological formation. It consists of Española, Floreana, Genovesa, San Cristóbal, and Santa Fe Islands.

Viñita Formation

Not to be confused with the Vinita member of the Carnian Doswell Formation of Virginia, which is sometimes elevated to formation status.The Viñita Formation is a geological formation in Coquimbo, Chile, whose strata date back to the Late Cretaceous (Santonian to Maastrichtian). Dinosaur remains are among the fossils that have been recovered from the formation.


Šipovo (Serbian Cyrillic: Шипово) is a town and municipality located in Republika Srpska, an entity of Bosnia and Herzegovina. It is situated in the southern part of the Bosanska Krajina region. As of 2013, it has a population of 10,293 inhabitants.

The municipality covers an area of 510 km2 (197 sq mi), much of which is forested.


This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.