Beacon Hill pericline is the most easterly pericline of the Mendip Hills, whose folds and thrusts were formed during the Variscan Orogeny by earth movements directed from the south. The Silurian volcanic rocks making up the core of the pericline are shown to be responsible for the large, linear magnetic anomalies present over eastern Mendip. Magnetic anomalies within the Radstock Coalfield to the north, however, are related to volcanic horizons in the Carboniferous Limestone. A three-dimensional model shows the presence of Silurian rocks beneath the spine of the eastern Mendip Hills.
Seismic reflection sections are used to control two-dimensional interpretations of potential field data from Beacon Hill pericline. Although the magnetic anomalies are adequately explained by the seismic model, the gravity anomalies require the presence of an additional low density body beneath the pericline's southern flank. This body is interpreted as a thrust wedge of Old Red Sandstone emplaced in its present position by a previously unrecognized post-Variscan normal fault. The considerable lateral extent of the structure is demonstrated by the trend of the maxima of the horizontal gradient of the gravity anomalies.