In the vicinity of St. Jerome, southern Quebec, platform rocks that abut against the Precambrian Shield margin belong to the Lower Ordovician Beekmantown Group. There, the basal member of the platform succession — the Upper Cambrian Potsdam Formation — that normally underlies the Beekmantown rocks does not appear at the surface. However, a gravity profile across the shield margin in this locality indicates the presence of a minimum of ~320 m of Potsdam below the Beekmantown rocks. The gravity profile also shows that at the shield margin, the Precambrian surface descends abruptly under the platform cover and the amplitude of the drop is about 600 m or more. This abrupt drop and the absence of outcropping Potsdam can be explained as due to post-Beekmantown faulting, which may actually have taken place in the Mesozoic Era.