Interpretation of shallow seismic reflection profiles indicates a thick Permo-Triassic to Jurassic sequence, overlain in the west of the region by at least 400 m of upper Cretaceous. Both sequences are folded along E.–W., westerly-plunging axes. Cores obtained from an area 15–20 km NW. of Lundy have given ages ranging from Permo-Trias to ?upper Jurassic. Deposits of postulated Palaeogene occupy a shallow, partially faulted basin in the SW. of the region. The Tertiary NW.–SE. wrench fault system of Devon and Cornwall can be shown to extend into the area.