Marine geophysical surveys in the Sea of the Hebrides show that there are several basins containing sediments up to 1 km in thickness resting on the offshore extensions of the Palaeocene–Eocene lava piles of Skye and Mull. Close geological and geophysical similarities with the Lough Neagh Clays of Northern Ireland suggest that the sediments are argillaceous deposits of probable Oligocene age; rocks of this age do not occur onshore in W Scotland. The early Tertiary history of the inner continental shelf now closely resembles that of the continental margin and Rockall trough, where an important Eocene–Oligocene unconformity preceded rapid Oligocene subsidence. South-west of Mull the main branch of the Great Glen fault downthrows the Tertiary to the SE. by about 0 5 km.