Aspects of speleogenesis in the Carboniferous limestone of North Derbyshire
Karstification of the Carboniferous Limestone of the northern part of the Derbyshire limestone outcrop began to a limited extent prior to deposition of the Namurian, and continued in some areas with the development of a hydrothermal karst system during the mineralisation phases of the Permo-Triassic. Extensive cavernisation by allogenic streams and by percolating meteoric water began after the final stripping of the cover of younger rocks during the late Tertiary, and the cave systems were extended and modified throughout the Pleistocene, A complex series of erosional events, apparently related to successive glacial phases, gave rise to a series of abandoned cave levels in some areas. The cavities produced during the Permo- Triassic were of importance in determining the nature and orientation of the later karst drainage systems. Where such pre-existing cavity systems failed to correspond with the hydraulic gradients of the Pleistocene, bedding controlled tube networks developed at preferred horizons in the limestone, often where a fossil horizon gave a relatively higher primary permeability and an underlying clay 'wayboard' arrested downward percolation. Interbedded impermeable horizons are important in the development of perched groundwater areas. Where such horizons have been breached they often form the upper limits of large caverns, since oxidation of sulphide minerals which they contain has locally increased the aggressiveness of circulating groundwater. The concentration of large cave systems near the margins of the present outcrop suggests that stripping of the cover during the late Tertiary occurred fast, and was completed prior to the establishment of significant hydraulic gradients within the limestone.