Reef knoll

A reef knoll is a landform that comprises an immense pile of calcareous material that had previously accumulated on an ancient sea floor. Reef knolls are geological remnants of reefs and other organic concentrations of calcareous organisms. They are often rich in fossils, with prehistoric corals, sponges, calcareous algae and other reef-builders making up a large part of the structure's volume, and this density of skeletal material lets the structure withstand sea currents and stand freely. Reef knolls can be divided into bioherms and biostromes. A bioherm is a landform of organic sedimentary rock enclosed or surrounded by rock of different origin, while a biostrome is a distinctly bedded or broadly lenticular sedimentary rock landform.