The revision of the geological map Aye - Marche-en-Famenne has emphasized an important deformation structure that we have named the Marenne Fault. This fault is now clearly visible at the southeast face of the Marenne quarry. The dating by rugose corals has allowed us to define with precision the Givetian lithologic units which have been put in contact by this normal fault with a later, right-lateral strike-slip component. To the east, the Marenne Fault may be traced over a distance of 4 km, as far as the Ourthe valley, to the south of Hotton where its stratigraphic displacement is about 226 m. More generally, the Marenne Fault is probably late Hercynian, of the same type and origin as the Lamsoul Fault known to the south of Marche-en-Famenne. It seems to form part of a family of faults which are regularly observed on the south side of the Dinant Synclinorium.