Onlap

the bedding of layers of sedimentary rock that results from the advance of the sea onto land, a process called transgression. In onlap, the upper, younger layers are more widely distributed than the lower horizons, called the base horizons, which were deposited earlier. The latter overlie more ancient rocks with a stratigraphic break. The presence of such a break indicates that dry land existed for a more or less protracted period after deposition of the rock in the given area, after which it subsided. The advance of the sea onto land is accompanied by the abrasion and leveling of the land surface, on which the sediments of the transgressing marine basins lie horizontally. Onlap is often combined with angular unconformities. The identification of onlaps is very important in paleogeographic investigations and in studying the history of the oscillatory movements of the earth’s crust.