A N.–S. orientated region of low gravity adjacent to the W. coast of the S. Shetland mainland is partly caused by a basin of probable Mesozoic age, as indicated by seismic refraction, sparker and bathymetric observations. The floor of the basin is estimated to be up to 2.3 km deep. The sediments of the western part of the basin are thin and of variable dip, but the eastern part takes the form of an eastward dipping sediment wedge or synclinal structure truncated on the east side by a normal boundary fault of contemporaneous or later age. The basin divides at its southern end into two tongues. The gravity low is also partly attributed to low density basement rocks such as granite, both at its north end and in the vicinity of Fair Isle, and to westward thickening of upper Palaeozoic (O.R.S.?) strata beneath the basin.
The postulated Mesozoic basin appears to be a subsidence structure along a major underlying dislocation approximately continuing the line of the Walls boundary fault. The new evidence is consistent with this being the Great Glen fault or a splay of it and interpretation of the linear magnetic gradient coinciding with the eastern tongue supports this hypothesis. The eastern boundary fault of the basin, however, appears to be normal. Tertiary transcurrent movement along the line appears to be improbable.