Gray's Reef comprises one of the largest nearshore sandstone reefs in the southeastern United States.It is located 32 kilometers (17.5 nautical miles) off Sapelo Island, Georgia. Designated in 1981, the Gray's Reef National Marine Sanctuary boundaries protect 17 square miles of open ocean. Sandstone outcroppings and ledges up to ten feet in height separate the sandy, flat-bottomed troughs in a reef that combines temperate and tropical flora and fauna. The rocky platform, some 60 to 70 feet below the surface, is covered by a carpet of attached organisms and is known locally as a "live bottom habitat." Gray's reef is not a coral reef. It is a consolidation of marine and terrestrial sediments (sand, shell, and mud) which was laid down as loose aggregate between 6 and 2 million years ago.