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10 Incredible Geological Formations
10 Incredible Geological Formations
Published: 2014/03/15
Channel: Alltime10s
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Geology
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Published: 2015/08/31
Channel: Bozeman Science
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How Was the Grand Canyon Formed?
Published: 2016/04/29
Channel: Smithsonian Channel
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Top 10 Most Amazing Geological Wonders In The World
Published: 2015/05/24
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National Geographic The Story of Earth HD
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Channel: Beauty Geography
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The Geological History of Earth
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Channel: GeoScience Videos
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Geologic History of North America
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Channel: Christopher Bross
3 Types of Rocks and the Rock Cycle: Igneous, Sedimentary, Metamorphic - FreeSchool
3 Types of Rocks and the Rock Cycle: Igneous, Sedimentary, Metamorphic - FreeSchool
Published: 2016/03/15
Channel: Free School
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Geologic History of Southern California.mov
Published: 2011/08/22
Channel: Wendy Van Norden
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Basics of Geology All About Rocks and Minerals
Published: 2014/03/09
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Living Rock An Introduction to Earths Geology
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Geological Isostatic Adjustment and The Great Lakes Region
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Channel: snowproductionsBI
The Formation of the Mediterranean Sea
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Channel: ZorakGoesOn
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Channel: Close Encounters UFO
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Channel: Hectic Express
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Channel: Universe TV
Subnautica 22 Geological Formation Base
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Channel: SnakeNACake _
How the Rockies Were Made - History Channel Documentary
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Channel: Mountain Springs
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Published: 2013/05/30
Channel: hugefloods
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Channel: River 1
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Channel: American Museum of Natural History
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Geology of Virginia 2014
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How North America got its shape - Peter J. Haproff
Published: 2016/07/05
Channel: TED-Ed
Geological Formations -- Mrs. Feist
Geological Formations -- Mrs. Feist's Science Class
Published: 2016/05/27
Channel: Rita Vondracek
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WIKIPEDIA ARTICLE

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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).

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.

Usefulness of formations[edit]

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.[clarification needed]

Modern revision of the geologic sciences has restricted formations to lithologies,[clarification needed] because lithological units are formed by depositional environments, some of which may persist for hundreds of millions of years and will transgress chronostratigraphic intervals or fossil-based methods of correlating rocks. For example, the Hamersley Basin of Western Australia is a Proterozoic sedimentary basin where up to 1200 million years of sedimentation is preserved within the intact sedimentary stratigraphy, with up to 300 million years represented by a single lithological unit of banded iron formation and shale.

Geologic formations are usually sedimentary rock layers, but may also be metamorphic rocks and volcanic flows.  Igneous intrusive rocks are generally not divided into formations.

Defining lithostratigraphic formations[edit]

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.

Formations are the only formal lithostratigraphic units into which the stratigraphic column everywhere should be divided completely on the basis of lithology.

The contrast in lithology between formations required to justify their establishment varies with the complexity of the geology of a region and the detail needed for geologic mapping and to work out its geologic history.

Formations must be able to be delineated at the scale of geologic mapping 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.

Sequence stratigraphy is a concept which challenges the idea of strict lithostratigraphic units by defining units based on events in sedimentary basins such as oceanic regressions and transgressions. These sequences are a combination of chronostratigraphic units, linked by time, and depositional environment linked by the geologic events which occurred at that time, regardless of the grain size of the sediments.

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.

Other uses of the term[edit]

"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[edit]

Further reading[edit]

  • 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. 


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