A reef knoll is a
landform
A landform is a land feature on the solid surface of the Earth or other planetary body. They may be natural or may be anthropogenic (caused or influenced by human activity). Landforms together make up a given terrain, and their arrangement ...
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. Reef knolls are often fossil-rich, with prehistoric
corals,
sponges
Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a basal clade and a sister taxon of the diploblasts. They are sessile filter feeders that are bound to the seabed, and ar ...
,
calcareous algae
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
, and other reef-builders contributing to a large portion of the structure's volume. This density of skeletal material allows the structure to 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. A biostrome is a distinctly bedded or broadly lenticular sedimentary rock landform.
Krumbein additionally used these terms to distinguish different shapes of
stromatolite
Stromatolites ( ) or stromatoliths () are layered Sedimentary rock, sedimentary formation of rocks, formations (microbialite) that are created mainly by Photosynthesis, photosynthetic microorganisms such as cyanobacteria, sulfate-reducing micr ...
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/
Staffordshire border include
Thorpe Cloud and
Bunster Hill in southern
Dovedale, and also
Chrome Hill 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 is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...
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 mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...
.
Examples in the
Yorkshire Dales
The Yorkshire Dales are a series of valleys, or Dale (landform), dales, in the Pennines, an Highland, upland range in England. They are mostly located in the Ceremonial counties of England, ceremonial county of North Yorkshire, but extend into C ...
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 (High Hill and Scaleber).
It was once proposed that in Lancashire, reef knolls could be seen between the villages of
Worston and
Downham near
Clitheroe.
See also
*
Waulsortian mudmound
References
{{reflist
External links
British Geological Survey website, on Chrome, Parkhouse and Derbyshire atolls
Landforms