The Hebridean Terrane is the westernmost strip of mainland Scotland, most of the Inner Hebrides and all of the Outer Hebrides. Similar rocks are also thought to be present on Shetland. They have been proved west and north of the Outer Hebrides by BGS shallow boreholes and hydrocarbon exploration wells. The full extent of this terrane to the west is obscured by the effects of Mesozoic rifting.