Résumé : Within, the framework of the revision of the geological map of Wallonia (Belgium) and France, our attention was drawn by a stratigraphical unit of shales and siltstones which did not seem yet to be described. The importance of this detrital level for the comprehension of the side passage, within Eifelian, of the Couvin Formation, limestone (in the west) to the Jemelle Formation, terrigenous (in the east) was pointed out by a regional geological survey. The cartography between the localities of Chimay, to the west and of Wellin, to the east, (more than 60 kilometers apart) made it possible to characterize this variation. So, the "Couvin Limestone" recognized on nearly 400 meters thickness in the area of the same name quickly loses its thickness at the meridian of Nismes, 6 kilometers in the east of the stratotype. This reduction of thickness is made in favour of terrigenous sediments. This argillaceous-silty level is identified as a new member of the Jemelle Formation: the "Vieux Moulin" Member, in reference to the old mill located between the localities of Treignes and Najauge where representative outcrops are easily accessible.