Moodustis

Moodustis is a geological term used in Estonian stratigraphy to refer to a formal unit of rock that can be mapped over a region. It corresponds to the English term formation and represents a coherent body of rock that shares distinctive lithology, fossil content, or depositional history.

Moodustis boundaries are defined by changes in rock type or other distinguishing features at the top and base, such as unconformities, combined with lateral continuity across the mapped area. A moodustis is intended to be large enough to be traced on geological maps and to represent a defined interval of geological time, though its exact age may vary regionally.

Most moodustis are named after a geographic locality or a characteristic rock type, for example, a limestone moodustis near a town or a sandstone moodustis named for a landmark. Within a moodustis, subdivisions called members may occur, representing distinct subunits with particular lithologies or fossil assemblages.

Moodustises are used to describe depositional environments and the geological history of an area, and they support correlation of rock units between outcrops and boreholes. They are applied in fields such as hydrogeology, mineral exploration, and paleogeography. Dating can be achieved through index fossils, radiometric methods, or correlation with other dated formations.