Stratigraphy is the study of the formation, composition, sequence and correlation of layered rocks. It comprises three sub-disciplines: lithostratigraphy deals with rock units of broadly uniform composition, which are readily mappable units, the basic one of which is the formation; chronostratigraphy involves rock units deposited during specific intervals of geological time; and biostratigraphy comprises rock units characterised by particular fossils or assemblages of fossils. Since the 1950s there has been growing agreement on standardisation of nomenclature on an international scale, which has facilitated correlation of rock strata globally, and has helped to recognise synchronous events over wide areas of the Earth.