A formation consists of a certain number of rock strata. They have similar lithology (rocks), sedimentary facies (appearance) or other properties. Formations are not defined on the thickness of the rock strata, and the thickness of different formations can therefore vary widely.
The concept of formally defined layers or strata is central to stratigraphy. A formation can be divided into 'members' and are themselves packed together in 'groups'.
Formations were initially described as time markers, based on relative ages and the law of superposition. The divisions of the Earth history#geologic time scale were the formations described and put in chronological order by the geologists and stratigraphers of the 18th and 19th centuries.
Rock formations are formed by sedimentary deposition in environments which may persist for hundreds of millions of years. For example, the Hammersley Basin in Pilbara, Western Australia, is a Proterozoic sedimentary basin where up to 1200 million years of sedimentation is preserved intact. Here, up to 300 million years is represented by a single unit of banded iron formation and shale.
- Stratigraphic groups
- Stratum (the smallest unit)
Images for kids
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.
Geological formation Facts for Kids. Kiddle Encyclopedia.