The Scottish Highlands are both an ancient mountain belt and, more recently, part of a passive continental margin, both features where the long-term controls on topography are uncertain. During the Mesozoic the area may have been either a net sediment source or, at least in parts, a net sediment sink. Balancing preserved sediment volumes in surrounding basins with the palaeo-surface area of the Highlands exposed to erosion suggests that a c. 2000 – 2400 m average thickness of (zero-porosity) rock has been eroded from the Highlands in the Cenozoic. Assuming that a change from sand-dominated sediments in the Paleocene and Eocene to later mud-dominated sediment corresponds to the change from the erosion of sediment to erosion of metamorphic basement, then 1900 – 2400 m average thickness of clastic sediment was present at the start of the Cenozoic, plus any overlying Chalk. This has been subsequently eroded away except along the east and west coasts, where preserved Mesozoic sequences suggest a more extensive original cover. Prior to uplift associated with rifting of the North Atlantic, the Highlands may have been an area of sediment-filled half-graben, much like the Inner Hebrides at the present day.