Reef knoll
（重定向自Bioherm）
A reef knoll is an immense pile of calcareous material on land that accumulated on the ancient sea floor. At the time of its accumulation it may have had enough structure from organisms such as sponges to have been free-standing and to withstand the sea currents as material accumulated, and was likely an atoll. Another possibility is the remains of deep water coral. Such structures are thus often fossil-rich.