AbstractThree gravity traverses across the eastern half of the South Wales Coalfield indicate a local, elongate, negative Bouguer anomaly of a few milligals amplitude bordering the southern rim. The anomaly has a maximum amplitude of 4 mgal near Maesteg and attenuates eastward, dying out in the vicinity of the East Crop. Two possible geological causes of the anomaly are examined, namely a local thick sequence of normal Upper Carboniferous strata and a discrete, low‐density body of Carboniferous age with no apparent surface expression. The former, uncontroversial explanation requires rock densities at variance with measured values. In the case of the hypothetical low density body which can explain the anomaly, a variety of possible ages and configurations are discusse