A formation is the main unit in lithostratigraphy. Lithostratigraphy is the classification of bodies of rocks based on properties of the rocks in a given strata in an area. A formation usually contains one main type of rock, but can also have occurrences of other rocks or be a recurring alternation between a few main types of rocks.
The term formations is usually applied to sedimentary rocks, but can also be used for other layered rocks such as extrusive igneous rocks. As an example one can think of a volcano that have several eruptions in a relatively short geosocial time. Each eruptions will have its own lava flows that will overlay older lava flows. Rocks formed from a particular flow will then be a layer, and all the layers from a time period may be grouped together as a formation. If the volcano have a long dormancy period between times with multiple eruptions, then there can be a lot of erosion and sedimentation separating the volcanic rocks. As a result it may be more natural to divide the igneous rocks in two formations, pre and post break.
Sedimentary formations often show the variations in the sedimentary environment, such as sea level. This is because of the formed rock, the basis of the formation, is closely connected to the sedimentary environment. The type of rock clay-, silt-, sandstone, conglomerate or limestone says something about the speed of water and ocean levels. When a river carries with it sediments to the ocean, it’s the speed of the water that determines the size of the particles it can carry. Heavy pebbles and cobbles is often left in the on river bottom to form conglomerates. Sandstone forms on earlier floodplains or close to shore. Whilst the light weight clay particles make it to deep ocean before they set on the bottom. So claystone and shale is often a sign of deep ocean. Limestone is usually formed from dead corrals that grow in shallow oceans, as corals require sunlight. You can therefore tell a lot about conditions of an area in earlier times based on the rocks in the formations from those times.
A formation can be subdivided into multiple members. Members is used when one or multiple beds/layers can be identified as substantially different from the rest, for instance due rock type or color. The smallest unit in lithostratigraphy is beds/layers. A bed is a continuous layer of deposited material, a single type of rock, with well-defined divisional planes (bedding planes) separating it from layers above and below. It can be less than 1 cm to several meter thick. There is no demand for a formation in terms number of members or beds, so a formation can be a single bed. Due their numbers bed are usually not named, but formations and members are. To recognize formations in the field, recognizable beds in the top and bottom of the formation is used.
Formations can also be grouped together in groups. A group must contain two or more formations. Groups may be defined over a larger area than the formations in it. This is because the depositional environments from area to area may vary, creating different formations from the same time period. The same goes for the groups. If extending the series either in time, adding formations vertically, or laterally gives a more complete picture then the groups are combined into supergroups. As an example the Oslofjord Supergroup, also known as the Cambrian-Silurian strata of the Oslo Graben, contains all the groups and their respective formations from the Oslo Graben. Taking its name from the Oslofjord which it surrounds, the supergroup consist of formations from all but the earliest part of the Cambrian (541-485,4 Ma) through to the end of the Silurian (443,8-419,2 Ma). The Oslofjord Supergroup makes up almost all of the sedimentary rocks around Oslo, the one exception being the those of the Asker Group. The Asker Group and the well known Kolsås Formation within it, is about 100 million year younger than the Oslofjord Supergroup youngest formations.
