STRATIGRAPHY

scientific discipline concerned with the description of rock successions and their interpretation in terms of a general time scale. It provides a basis for historical geology, and its principles and methods have found application in such fields as petroleum geology and archaeology. Stratigraphic studies deal primarily with sedimentary rocks but may also encompass layered igneous rocks (e.g., those resulting from successive lava flows) or metamorphic rocks formed either from such extrusive igneous material or from sedimentary rocks. A common goal of stratigraphic studies is the subdivision of a sequence of rock strata into mappable units, determining the time relationships that are involved, and correlating units of the sequenceor the entire sequencewith rock strata elsewhere. Following the failed attempts during the last half of the 19th century of the International Geological Congress (IGC; founded 1878) to standardize a stratigraphic scale, the International Union of Geological Sciences (IUGS; founded 1961) established a Commission on Stratigraphy to work toward that end. Traditional stratigraphic schemes rely on two scales: (1) a time scale (using eons, eras, periods, epochs, ages, and chrons), for which each unit is defined by its beginning and ending points, and (2) a correlated scale of rock sequences (using systems, series, stages, and chronozones). These schemes, when used in conjunction with other dating methodssuch as radiometric dating (the measurement of radioactive decay), paleoclimatic dating, and paleomagnetic determinationsthat, in general, have been developed within the last half of the 20th century, have led to somewhat less confusion of nomenclature and to ever more reliable information on which to base conclusions about Earth history. Because oil and natural gas almost always occur in stratified sedimentary rocks, the process of locating petroleum reservoir traps has been facilitated significantly by the use of stratigraphic concepts and data. An important principle in the application of stratigraphy to archaeology is the law of superpositionthe principle that in any undisturbed deposit the oldest layers are normally located at the lowest level. Accordingly, it is presumed that the remains of each succeeding generation are left on the debris of the last.