A reef knoll is a land-based landform that comprises an immense pile of

calcareous Calcareous () is an adjective meaning "mostly or partly composed of calcium carbonate", in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines. In zoology ''Calcareous'' is used as an ad ...

material that accumulated on a previously existing ancient sea floor. At the time of its accumulation it may have had enough structure from

organism In biology, an organism () is any living system that functions as an individual entity. All organisms are composed of cells ( cell theory). Organisms are classified by taxonomy into groups such as multicellular animals, plants, and fu ...

s such as

sponge Sponges, the members of the phylum Porifera (; meaning 'pore bearer'), are a basal animal clade as a sister of the diploblasts. They are multicellular organisms that have bodies full of pores and channels allowing water to circulate throu ...

s to have been free-standing and to withstand the sea currents as material accumulated, and was likely an

atoll An atoll () is a ring-shaped island, including a coral rim that encircles a lagoon partially or completely. There may be coral islands or cays on the rim. Atolls are located in warm tropical or subtropical oceans and seas where corals can gro ...

. Another possibility is the remains of

deep water coral The habitat of deep-water corals, also known as cold-water corals, extends to deeper, darker parts of the oceans than tropical corals, ranging from near the surface to the abyss, beyond where water temperatures may be as cold as . Deep-water co ...

. Such structures are thus often

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

-rich. A bioherm is a sedimentary rocky landform enclosed or surrounded by rock of different origin. A biostrome is a distinctly bedded or broadly lenticular sedimentary rocky landform. Krumbein defines these terms as types of

stromatolite Stromatolites () or stromatoliths () are layered sedimentary formations ( microbialite) that are created mainly by photosynthetic microorganisms such as cyanobacteria, sulfate-reducing bacteria, and Pseudomonadota (formerly proteobacteria). T ...

s: "Distinctly bedded, widely extensive, blanketlike build-ups are biostromes. Nodular, biscuit-like, dome-shaped or columnar stromatolites are also referred to as bioherms".

England

Examples on the

Derbyshire Derbyshire ( ) is a ceremonial county in the East Midlands, England. It includes much of the Peak District National Park, the southern end of the Pennine range of hills and part of the National Forest. It borders Greater Manchester to the nor ...

Staffordshire Staffordshire (; postal abbreviation Staffs.) is a landlocked county in the West Midlands region of England. It borders Cheshire to the northwest, Derbyshire and Leicestershire to the east, Warwickshire to the southeast, the West Midlands C ...

border include

Thorpe Cloud Thorpe Cloud is an isolated limestone hill (a reef knoll) lying between the villages of Thorpe and Ilam on the Derbyshire/Staffordshire border at the southern end of Dovedale. It is a popular hill amongst the many day-trippers who visit the ar ...

and

Bunster Hill Dovedale is a valley in the Peak District of England. The land is owned by the National Trust and attracts a million visitors annually. The valley was cut by the River Dove and runs for just over between Milldale in the north and a wooded r ...

in southern

Dovedale Dovedale is a valley in the Peak District of England. The land is owned by the National Trust and attracts a million visitors annually. The valley was cut by the River Dove and runs for just over between Milldale in the north and a wooded ...

, and also

Chrome Hill Chrome Hill is a limestone reef knoll in Derbyshire, England, in the upper Dove valley beside the border with Staffordshire, within the civil parish of Hartington Middle Quarter. It is adjacent to Parkhouse Hill, another reef knoll. The ...

and Parkhouse Hill at the northern end. These structures are often most clearly seen where the surrounding rocks are much softer and so can be preferentially eroded. All the Derbyshire examples quoted lie at the edge of the

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

areas; Chrome and Parkhouse lie at the divide between limestone and the much softer

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

. Examples in the

Yorkshire Dales The Yorkshire Dales is an upland area of the Pennines in the historic county of Yorkshire, England, most of it in the Yorkshire Dales National Park created in 1954. The Dales comprise river valleys and the hills rising from the Vale of York w ...

lie on the downthrow side (north) of the Mid Craven Fault. There is one set located around Thorpe (Skelterton, Butter Haw, Stebden, Elbolton, Thorpe Kail, Myra Bank and Hartlington Kail); one set located around Malham (Burns Hill, Cawden, and Wedber); and a set around

Settle Settle or SETTLE may refer to: Places * Settle, Kentucky, United States * Settle, North Yorkshire, a town in England ** Settle Rural District, a historical administrative district Music * Settle (band), an indie rock band from Pennsylvania * ''S ...

(High Hill and Scaleber). It was once proposed that in Lancashire, reef knolls could be seen between the villages of

Worston Worston is a small linear village and civil parish in Lancashire, England. The village is north-west of Pendle Hill, east of Clitheroe, and is in the Ribble Valley district. As it is only a small village, with a population of 76 at the 200 ...

and Downham near

Clitheroe Clitheroe () is a town and civil parish in the Borough of Ribble Valley, Lancashire, England; it is located north-west of Manchester. It is near the Forest of Bowland and is often used as a base for tourists visiting the area. In 2018, the Cl ...

References

{{reflist

External links

British Geological Survey website, on Chrome, Parkhouse and Derbyshire atolls
Landforms

England

See also

References

External links