Godard (1965) recognised that there were extensive areas of gentle slopes at 180-300 m asl across the Northern Highlands. Together these represent the Surface écossaise, an erosion surface cut across varied geology related to a period in the mid-Tertiary when reduction of the relief left only small areas of higher ground. Typically, the Surface écossaise has a sharp break of slope against the flanks of residual hills like Scaraben. The erosion surface formed close to sea level and was subsequently uplifted by up to 200 m. Today it has been dissected by rivers and modified by glacial erosion to give mid-level plateaux. This erosion surface is well developed on the southern slopes of Scaraben in the Langwell Forest.