The Geal Charn-Ossian Steep Belt is a major composite synclinal structure cored by an upward-facing Appin Group succession. It is located at the margin of a west-facing sedimentary basin where more than 8 km of Grampian Group sediment was deposited adjacent to an intrabasin structural ‘high’. The ‘high’ comprises mainly gneissose metasedimentary rocks, the Glen Banchor succession, which acted as the basement to the Grampian and Appin group rocks. Major unconformities are recognized at more than one level as the sedimentary basin successions onlapped onto the 'high'. The primary major upright folds and associated slides of the Steep Belt developed when considerable shortening was focused along the basin margin during the Caledonian Orogeny.