A layer of rock with a particular composition (especially of fossils); for dating the stratum WordNet 3.0 & Open English WordNet

In geology, a horizon is either a bedding surface where there is marked change in the lithology within a sequence of sedimentary or volcanic rocks, or a distinctive layer or thin bed with a characteristic lithology or fossil content within a sequence. Wikipedia

A bedding plane or a thin bed of distinctive character within a stratigraphic sequence Wikipedia Disambiguation

Term in geology Wikidata