Onlap or overlap is the geological phenomenon of successively wedge-shaped younger rock strata extending progressively further across an erosion surface cut in older rocks. It is generally associated with a marine transgression. It is a more general term than overstep, in which the younger beds overlap onto successively older beds.^[1] The opposite is offlap, in which each younger rock bed pinches out short of the full extent of the underlying older bed, typically due to a marine regression.^[2]