Water depths increase rapidly offshore but remain modest within the archipelago. A drop in sea level of just 35 m would reunite the islands. The contours suggest that two submerged cliff lines and platforms exist - one at around -10 m and another at -50 m. The lower features may relate to a submerged rock shoreline around St Kilda at -40 m. The NNE alignment of the break of slope east of Orkney runs parallel to but 20 km west of the Walls Boundary Fault and follows no known structural weakness.