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]


  1. ^ Jackson, Julia A., ed. (1997). "onlap". Glossary of geology (Fourth ed.). Alexandria, Viriginia: American Geological Institute. ISBN 0922152349.
  2. ^ Jackson 1997, "offlap".