Synopsis The Middle Old Red Sandstone at Foyers rests with marked unconformity on an irregular surface of schists, gneisses and rocks of the Caledonian Foyers granitic complex. Basal breccias, conglomerates and grits are widespread, giving way to more persistent grits and sandstones high in the sequence. Deep infilled channels occur, cutting earlier rocks, and in situ scree deposits are common. Field evidence suggests that the sediments may have formed as alluvial fan deposits in an arid intermontane basin, bordered by highlands to the south-east. The highlands probably terminated at a north-east-south-west trending fault scarp, possibly a manifestation of early movements along the Great Glen fault.