onlap noun 1verb 1forms 3 noun1 •The phenomenon of successively younger rock strata extending progressively further across an erosion surface cut in older rocks.Displacement on F3 was followed by continued accumulation and onlap of the fine-grained sands of unit 3 on the footwall and development of soil Qb2. — 2001 verb1 •To form an onlap.Elsewhere, many of the youngest volcanic plains units onlap tesserae at crustal plateau boundaries. — 1998 formsonlapped · onlapping · onlaps
