Stratigraphy examines the origin, sequence, structure, fossils and lithology, age of rocks, age of formations and layers of the earth's crust. Stratigraphic studies are not only based on knowing the sequence of sedimentary layers in different geological times in order to understand the history of earth events and the evolution of organisms, but also examines the lateral changes of facies in different places. In other words, stratigraphy examines the determination of changes in the earth's layers and their relative age, which is based on the following three principles: 1- The principle of stacking of layers: In this case, the layers may have changed due to tectonic factors, in this condition, the science of stratigraphy is related to the science of structural geology. 2- The principle of continuity in a certain sedimentary layer in terms of petrological properties: in this case, stratigraphy is related to petrology. 3- The principle of similitude in the paleontology of strata: in this case, palaeontology is considered the basis of the science of stratigraphy.