Blackdown pericline is the western pericline of the Mendip Hills, which were formed during the Variscan Orogeny at the end of the Carboniferous. Unlike the more easterly periclines, Blackdown pericline has no associated magnetic anomaly, indicating the absence of the Silurian volcanic rocks responsible for the anomaly in eastern Mendip. The gravity field of Blackdown pericline cannot be adequately modelled by the known geology of the Mendip Hills and requires the presence of an anomalously dense body at shallow depth. The geometry of the body is inferred from a seismic reflection profile and is interpreted as a wedge of Precambrian basement rocks. Its high density and negligible susceptibility may indicate an amphibolitic composition. Three models involving Variscan thrusting and post-Variscan listric normal faulting are presented in explanation of the presence of the wedge. The anomalous body is shown to be laterally continuous to west and east. A linear negative gravity anomaly to the north of Blackdown pericline is interpreted to be caused by a small occurrence of Coal Measures, which have not before been described from this area.