noun
GeologyA progressive increase in the lateral extent of conformable strata in passing upwards from older to younger strata, so that each stratum is hidden by the one above; a set of strata exhibiting this.
Origin
1940s. From on- + lap, after offlap, overlap.
verb
GeologyTo cover (a stratum) so as to form an onlap.
Origin
1950s. From onlap.
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