an Entity references as follows:
A reef knoll is a land-based landform that comprises an immense pile of calcareous material that accumulated on a previously existing 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.