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Onlap or ''overlap'' is the
geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Ear ...
phenomenon of successively wedge-shaped younger rock strata extending progressively further across an
erosion surface In geology and geomorphology, an erosion surface is a surface of rock or regolith that was formed by erosion and not by construction (e.g. lava flows, sediment deposition) nor fault displacement. Erosional surfaces within the stratigraphic reco ...
cut in older rocks. It is generally associated with a
marine transgression A marine transgression is a geologic event during which sea level rises relative to the land and the shoreline moves toward higher ground, which results in flooding. Transgressions can be caused by the land sinking or by the ocean basins filling ...
. It is a more general term than '' overstep'', in which the younger beds overlap onto successively older beds. 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 A marine regression is a geological process occurring when areas of submerged seafloor are exposed above the sea level. The opposite event, marine transgression, occurs when flooding from the sea covers previously-exposed land. Evidence of mari ...
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References

Stratigraphy {{Geology-stub