Stratigraphy is a branch of geology concerned with the systematic study of bedded rock layers and their relations in time and the study of fossils and their locations in a sequence of bedded rocks. A stratum is a bed or layer of sedimentary rock having approximately the same composition throughout (plural is strata).
James Hutton also contributed to a theories about rock formations. A rock formation is the primary unit of stratigraphy, consisting of a succession of strata useful for mapping or description. A rock formation typically consists of a unique lithology (rock type) that has a relatively defined geologic age and is considered mapable (occurs throughout area or region, both on the surface and in the subsurface).
Rock formations preserved information about what conditions were like when the original sediments were deposited, such as on a river delta, a coastal beach environment, a ocean setting, or a massive dune field. Rock formations can also consist of igneous rocks, such as ancient lava flows or massive volcanic ash deposits. Rock formations typically represent materials that accumulated over period of hundreds of thousands, to many millions of years.