Broadly stratigraphy deals with the succession of geologic events and / or rock layers from the beginning of the crustal formation up to the present time. It aims at establishing and describing the correct order of superposition of rock-units on the earth's surface.Thus, it actually establishes the correct succession of rock formations / layers. It thereby unfolds the history of geological events on the earth from the geologic past to the present time and hence it is also referred to as historical geology as a synonym.