Reef Knoll

A reef knoll is a landform built from a large accumulated mass of calcareous material laid down on an ancient sea floor, composed of the fossil rich remains of corals, sponges, calcareous algae and other reef building organisms whose dense skeletal material let the structure resist ocean currents and stand as a freestanding mound. Reef knolls are classified as either bioherms, organic sedimentary rock bodies enclosed within rock of a different origin, or biostromes, distinctly bedded or broadly lens shaped sedimentary landforms. Well known examples in England include Thorpe Cloud, Bunster Hill, Chrome Hill and Parkhouse Hill along the Derbyshire and Staffordshire border, where the surrounding softer rock has eroded away to expose the resistant knolls, along with further examples in the Yorkshire Dales near Thorpe, Malham and Settle.