Regional and detailed gravity surveys over Carboniferous rocks in northeast Wales have revealed an elongated Bouguer anomaly low over part of the outcrop of the Cefn-y-Fedw Sandstone Group. The anomaly has a pronounced gradient over the boundary of these sediments with the limestones of the underlying VisCan Series, but downdip, to the east, the anomaly merges with the general decrease of Bouguer anomaly values. The anomaly can be explained if the thickness of the lower density Cefn-yFedw Sandstone Group increases abruptly along the margin of the outcrop of these rocks. Various explanations for this are considered including faulting, a syncline. and channel-like deposits. The interpretation is further complicated by the presence of sandstones with densities varying between 2.2 Mg m ~' and 2.6 Mg m+.Although a full explanation is not possible without further evidence, the existence of the Bouguer anomaly low necessitates a review of the nature of the Cefn-y-Fedw Sandstone Group.