stratigraphy, branch of geology specifically concerned with the arrangement of layered rocks (see stratification). Stratigraphy is based on the law of superposition, which states that in a normal sequence of rock layers the youngest is on top and the oldest on the bottom. Local sequences are studied, and after considering such factors as the average rate of deposition of the different rocks, their composition, the width and extent of the strata, the fossils contained, and the periods of uplift and erosion, the geological history of the sequence is reconstructed. These sequences are then correlated to those of similar age in other regions with the ultimate aim of establishing a consistent geochronology for the entire earth. Statigraphy is therefore important in the relative dating of all types of rock. In areas where the strata have undergone folding, faulting, and erosion, stratigraphic techniques are used to determine their correct sequence. The principle of included fragments in stratigraphy states that any rock fragment included in another rock must be older than the surrounding rock. Fossils have been the most important means of correlation because, as a result of evolution, rock strata of approximately equal age exhibit similar flora and fauna. Dating and correlation of stratified rocks by means of fossils is called stratigraphic paleontology. See also dating.
See B. Kummel, History of the Earth (1961); E. W. Spencer, Basic Concepts of Historical Geology (1962); R. K. Matthews, Dynamic Stratigraphy (1974); P. C. Cattermole and P. Moore, The Story of the Earth (1985).