A reef knoll is a landform that comprises an immense pile of calcareous material that had previously accumulated on an ancient sea floor.[1] Reef knolls are geological remnants of reefs and other organic concentrations of calcareous organisms. Reef knolls are often fossil-rich, with prehistoric corals, sponges, calcareous algae, 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.[2] A biostrome is a distinctly bedded or broadly lenticular sedimentary rock landform.[3] Krumbein additionally used these terms to distinguish different shapes of stromatolites: "Distinctly bedded, widely extensive, blanketlike build-ups are biostromes. Nodular, biscuit-like, dome-shaped or columnar stromatolites are also referred to as bioherms".[4]
England
editExamples 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 areas; Chrome and Parkhouse lie at the divide between limestone and the much softer shale.
Examples in the Yorkshire Dales[5] 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.[6][7][8]
See also
editReferences
edit- ^ Cope, F. Wolverson (1976) Geology Explained in the Peak District, David & Charles
- ^ "Definition of BIOHERM".
- ^ "Definition of BIOSTROME".
- ^ Krumbein, W.E., Brehm, U., Gerdes, G., Gorbushina, A.A., Levit, G. and Palinska, K.A. (2003). "Biofilm, Biodictyon, Biomat Microbialites, Oolites, Stromatolites, Geophysiology, Global Mechanism, Parahistology". In Krumbein, W.E.; Paterson, D.M.; Zavarzin, G.A. (eds.). Fossil and Recent Biofilms: A Natural History of Life on Earth (PDF). Kluwer Academic. pp. 1–28. ISBN 978-1-4020-1597-7. Archived from the original (PDF) on January 6, 2007. Retrieved 2008-07-09.
{{cite book}}: CS1 maint: multiple names: authors list (link) - ^ Ramsbottom, W.H.C.; R.F.Goosens; E.G. Smith; M.A. Calver (1974). D.H. Rayner and J.E. Hemingway (ed.). The Geology and Mineral Resources of Yorkshire. Yorkshire Geological Society. pp. 61–64.
- ^ "English Nature: Lancashire Geology". Archived from the original on 2 July 2007. Retrieved 13 September 2015.
- ^ Miller, J.; Grayson, R.F. (1972). "Origin and structure of Lower Viséan "reef"limestones near Clitheroe, Lancashire". Proceedings of the Yorkshire Geological Society. 38 (4): 607–638. doi:10.1144/pygs.38.4.607.
- ^ Kabrna, Paul. "Clitheroe Reef Belt". Craven Basin:Waulsortian Mudmounds. Craven & Pendle Geological Society. Retrieved 8 October 2015.