A reef knoll is an immense pile of calcareous material which has accumulated on the sea floor . At the time of its accumulation it must have had enough structure from organisms such as sponges to have been free-standing and to withstand the sea currents as material accumulated. Such structures are thus often fossil-rich. At the time of their creation they were probably atolls.

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 area; and Chrome and Parkhouse lie at the divide between limestone and the much softer shale (see for example the British Geological Survey website, on Chrome, Parkhouse and Derbyshire atolls: ^*).

Reference

1. Cope, F. Wolverson (1976) Geology Explained in the Peak District, David & Charles

Landforms