A body of rock that is distinguishable from other rock bodies by significant differences in lithology (and therefore is mappable). Its boundaries may be either sharp or arbitrarily specified in a gradational sequence. Formations are subdivided into members, which in turn are subdivided into beds. Members and beds have lithological characteristics that distinguish them from other members and beds, but which are insufficient to classify them as separate formations or members. Several adjacent formations may form a group, whose boundaries are marked by major changes in lithology and/or depositional environments, or by a major break in sedimentation. The divisions are somewhat arbitrary but are usually specified by Stratigraphic Commissions; see Hedberg (1976) and Salvador (1994). Nonsedimentary rock units that do not conform to the law of superposition are called lithodemes (Greek for stone bodies). See stratigraphic classification.