A reef knoll is an immense pile of calcareous material on land that accumulated on the ancient sea floor. At the time of its accumulation it may have had enough structure from organisms such as sponges to have been free-standing and to withstand the sea currents as material accumulated, and was likely an atoll. Another possibility is the remains of deep water coral. Such structures are thus often fossil-rich.
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 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).
- Cope, F. Wolverson (1976) Geology Explained in the Peak District, David & Charles
- 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: 607–638.
- Kabrna, Paul. "Clitheroe Reef Belt". Craven Basin:Waulsortian Mudmounds. Craven & Pendle Geological Society. Retrieved 8 October 2015.
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