Tertiary strata with a low northeasterly dip, truncated by an erosion surface with low westerly dip, form the basement in the Sandettie-Fairy Bank gap except in the northwest part of the area where a basin is cut into the Tertiary strata. This basin is filled with a sequence of late Pleistocene sands overlain by Holocene sands which form the major topographic features of the Sandettie and Fairy Banks and the low sea-floor col linking them. The surface of the banks and col are moulded into transverse ridges up to 10 m in amplitude and 200 m in wavelength. Three types of ridges are present: at the north end of the col, linear transverse ridges of irregular amplitude and wavelength occur with a flint gravel base grading up into medium sand; these ridges are fossil features of Preboreal-Subboreal age. At the south end of the col there are symmetrical sandwaves of regular amplitude and wavelength which, from geological evidence, appear to be stable. To the west of Sandettie Bank and the col there are south-facing asymmetrical sandwaves which on geological evidence, are potentially mobile and which later sedimentological investigations revealed to be moving southwards in one area.