overlap [stratig] . A general term referring to the extension of marine, lacustrine, or terrestrial strata beyond underlying rocks whose edges are thereby concealed or "overlapped", and to the unconformity that commonly accompanies such a relation; esp. the relationship among conformable strata such that each successively younger stratum extends beyond the boundaries of the stratum lying immediately beneath. The term is often used in the sense of onlap, and sometimes in the sense of overstep (as by De la Beche, 1832); because of such conflicting usage, Melton (1947, p.1869) and Swain (1949, p.634) urged that the term be abandoned. See also: replacing overlap.