The crust of the Earth is composed of a complex mix of rock types formed in specific depositional environments. Most of your everyday experience occurs in erosional environments, places where the surface of the Earth is being eroded away by physical or chemical processes. Rocks underfoot are being broken up then transported as sediments to other areas. The Grand Canyon is a good example of a river cutting through the layers of rock, exposing those layers, and transporting the rock away—piece by piece.
A cross section model of the Grand Canyon showing the rock layers and how they are broken into recognized formations.
Where is that material going? It is transported to places where it is deposited, depositional environments, where the sediments accumulate. Given enough time and protection from further weathering, those newly deposited sediments can become the sedimentary rocks of the future. The deposition of sediment occurs in a wide variety of environments such as in oceans, rivers, ponds and the like.
If we look at the rocks on the surface of the Earth today, we can see differences in their color, texture, composition, and so on. We see layering as one rock type gives way to another. Geologists map these layers to trace out the history of the Earth, as each change in rock type represents a change in the environment that created it. So each outcropping of rock represents a time machine of sorts, transporting us back to the primeval environment of that spot.
A geologic formation is a formal unit, technically called a lithostratographic unit. That fancy word simply means that rocks are grouped by similar lithology, or rock type. For example, during the Late Cretaceous the last inland sea across the mid-continent of North America (a specific environment) deposited thick accumulations of chalk and shale. Today, we can lump specific sections of that collection of rock (lithology) into a single formation and call it the Niobrara Formation.
Each formation is described and named in a specific, formal way. There are rules that geologists should follow in designating specific formations (North American Commission on Stratigraphic Nomenclature, 2005). For example, each formation should have a type section designed. This type section is not unlike a type specimen mentioned in another post: it represents the standard for that formation to help others understand how it is unique.
In future posts we will explore more aspects of formations.
NORTH AMERICAN COMMISSION ON STRATIGRAPHIC NOMENCLATURE. 2005. North American Stratigraphic Code. American Association of Petroleum Geologists Bulletin, 89(11):1547-1591.
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February 13th, 2010 on 6:58 PM
[...] When a scientist studies an animal and determines that it is something new to science, they set up a name for it and designate a type specimen. The type specimen, or type, holds a special significance as the “name bearer” for the entire species, and subsequent investigations of that species make reference to the type. They are often kept in special collections within the museums that hold them, or at least given special protection over other specimens. For example, they often are not loaned out as other specimens in the museum collection might be, so there is less risk of damage. (For a description of geologic type sections, see formations). [...]
May 28th, 2010 on 10:04 AM
[...] Chalk by Greg under Animals, Geology < > One of the most famous formations is the Niobrara Chalk. This formation is exposed in northwest Kansas and southern Nebraska. [...]
May 28th, 2010 on 10:07 AM
[...] by E. D. Cope in 1873 as ‘Portheus’ gladius from a specimen collected from the Niobrara Chalk formation in western Kansas. The Niobrara Chalk was deposited during the Late Cretaceous period (see a [...]
May 28th, 2010 on 10:15 AM
[...] crocodilian species Sarcosuchus whose remains have been found in the modern Sahara, in the Elrhaz Formation. This Early Cretaceous (~112 million years ago) crocodile had a long, slender snout with a [...]
June 8th, 2010 on 2:37 PM
[...] collected in the Niobrara Formation for about 150 years, and while many fossils have come from this rock formation, there are always new and important discoveries to be [...]
June 9th, 2010 on 11:19 AM
[...] Apatosaurus, Diplodocus, Stegosaurus, and Camarasaurus all come from this formation. (See Formations for information about what that means.) Stratigraphic section of the Purgatoire River Canyon showing [...]
June 10th, 2010 on 9:32 AM
[...] Posts Geologic Formations Formations, geologic formation, George Sternberg, Kansas, national geographic, Niobrara Chalk, [...]
June 15th, 2010 on 5:47 PM
[...] they used in the facility to give it texture and color. Everything was there from granite from igneous rock formations, to beautiful marble, and travertine wall [...]
June 19th, 2010 on 11:24 AM
[...] Posts: Formations Niobrara Chalk My National Geographic moment Anurognathus, Hatzegopteryx, Kansas, Mesozoic, O. [...]
June 21st, 2010 on 4:08 AM
[...] of wild animals, beautiful flowers, insects, and rock formations are just some of the natural topics you can find. And having an image like this, and switching it [...]
June 21st, 2010 on 4:09 AM
[...] of the Niobrara Formation, the rocks that preserved flying reptile bones. (You can read more about formations by following this [...]
June 22nd, 2010 on 7:08 AM
[...] liked the variety of rocks used as wall and floor tiles. They came from many different rock formations and I photographed many of them. I also have used some of those images as nature wallpaper for my [...]
June 22nd, 2010 on 2:38 PM
[...] Geologists name the layers of rock that we map to help unravel geologic history. There is a whole code for the naming of rock formations. [...]
July 17th, 2010 on 2:12 PM
[...] that ocean are exposed in badlands across much of western Kansas, Nebraska, and South Dakota. These geologic formations, like the Niobrara Formation, preserve a rich record of the ocean life, and clearly show what a [...]
August 7th, 2011 on 9:23 AM
[...] name rock units so that we can describe them and communicate about them. We call these units rock formations, and each is given a formal name based upon rules that are accepted by the profession. My goal was [...]