The shallow marine area surrounding Sable Island is floored by sand. The grain size of the seafloor sediment becomes finer eastward, with gravel occurring on the west end of Sable Island Bank, passing eastward to medium sand around most of the Island, with the proportion of fine sand increasing toward the east end of the Island. This sediment is subjected to an intense wave and current regime during storms. Storm-generated currents, supplemented by intense wave action, cause sediment movement from west to east, and has created a complex seafloor morphology. In water depths greater than 15-20 m, a series of elongate sand ridges occurs along the entire southern side of the Island and its submerged extensions. Additional ridges occur north of the Island. The southern ridges reach 20 km in length, have spacings of 1.5-8 km, and heights of 3-10 m. Their internal structure shows they are migrating eastward. More than one metre of sediment can be deposited on their eastern flank in a single storm, causing migration rates up to 50 m/a. These ridges are unusual globally because they are more nearly orthogonal to the shoreline than such ridges elsewhere. This is attributed to the existence of obliquely shoreward-directed bottom currents during storms that move sand onshore; this presumably nourishes the southern side of the Island. Thus, the stability of the Island is linked with the dynamics of the shallow-water areas around it. Almost nothing is known, however, about the shoreface that links the shallow seafloor with the beach.