The region is part of the Rhenish Massif; the basement of the Apert consists of the fossil-rich Büdesheim Slate and Flinz, a rock of the lower Upper Devonian. Fossils of the following biological orders have been found (sometimes as index fossils): Buchiola, Cheiloceras, Goniatites, Clymeniida, Orthoceras and Manticoceras.