The study and interpretation of the stratification of rocks, sediments, soils, or cultural debris, based on the principle that the lowest layer is the oldest and the uppermost in the youngest - a major tool in establishing a relative dating sequence. The sequence of deposition can be assessed by a study of the relationships of different layers. Dateable artifacts found within layers, and layers or structures which are themselves dateable, can be used to date parts of stratigraphic sequences. An archaeologist has to master the skill to recognize it - to distinguish one deposit from another by its color, texture, smell, or contents; to understand it - to explain how each layer came to be added, whether by natural accumulation, deliberate fill, or collapse of higher-standing buildings; and to record it in measured drawings of the section. There can be problems where a feature filled with one type of material cuts into layers of the same material. Unless the later feature is recognized, objects of two different phases may appear to be stratified together. The underlying principles are: law of superposition, law of cross-cutting relationships, included fragments, and correlation by fossil inclusions. The stratigraphy principle was adopted from geology and is the basis of reconstructing the history of an archaeological site.