Basic Guidelines For English Spellings
READ THESE ARTICLES
A 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.
1940s. From on- + lap, after offlap, overlap.
To cover (a stratum) so as to form an onlap.
1950s. From onlap.