reef (Geography Dictionary)

reef : A submerged ridge-like or mound-like structure built by sedentary calcareous organisms, especially corals, in shallow marine waters, and consisting primarily of their skeletal remains, though often still supporting living colonies as well. Reefs may also be partially composed of rocks, sand, gravel, or seashells. They are locally prominent above surrounding sediments deposited on the sea floor, rising to or nearly to the water’s surface