Humberside Geologist No. 13
‘In search of Blue John’
The Castleton area, Derbyshire is one of the classic areas of British Geology. There is much to interest the sedimentologist, geomorphologist, karstologist or mineralogist amongst others for far more than the day Hull Geological Society spent in the area. So much is packed into this small area that it gets a whole
Geologist’s Association Guide to itself (Ford 1996). Witham (in litt.) has commented on some of the geological and palaeontological aspects of the field trip and these notes concentrate on some of the karstic features seen.
The party drove down Rushup Vale via Sparrowpit, returning via the A625 along the northern slopes. Passing the large (and somewhat disfiguring) limestone quarry on Eldon Hill the streams running off the Edale Shales could be seen incising deep into the overlying solifluction deposits before disappearing into a series of swallets. There is no lateral surface drainage to this valley, all the water sinking at the swallets, even in flood, and resurging in the Hope Valley at Russet Well in Castleton.
Blue John Cavern
One of several natural caves (much modified) that have been worked for the banded form of Fluorospar known as ‘Blue John’ which is used for ornaments and jewellery. The cave shows a variety of speleogenic features with the Blue John deposits in paleokarst caverns of mid-Carboniferous age (Ford 1985:86, Ford 1996: 48) and later periglacial vadose modification (Ibid.)
It was amusing to hear the guide, well informed on the historical aspects of the workings repeat some classic errors concerning speleogenesis. Speleothems such as stalactites are not formed by ‘evaporation’ as is commonly thought but by degassing resulting in deposition of calcite in the reversible reaction:
CaCO3 + H2CO3 <==> Ca (2+) + 2HCO(3-)
Removal of CO2 as saturated solutions reach the atmosphere of the cave result in calcite being precipitated. The simple equation above hides a complex series of equilibria of the differing ions (see Sweeting 1972: 25 ff) this means that when two karst waters mix, even if both are saturated with calcite and incapable of corrosion on their own the mixing usually results in water capable of further
dissolution (Ford & Williams 1989: 69). It is this increased aggressiveness of mixing karst
waters that accounts for the sculpted pockets in the roof of caverns. The guide’s explanation of such hollows resulting from "Water swirling round" (unless that swirling caused mixing of waters with differing saturation, though it is doubtful that this was what the guide had in mind) is repeated in a guide book for the caverns, but is incorrect.(Ollerenshaw,N.D: 16.)
Even in low gear this steep, narrow, deeply incised but dry gorge running down into the Hope Valley represents a challenge for a loaded minibus. What has puzzled geomorphologists for a long time is the very limited catchment area at the head of this valley, far too small to provide enough water to cut the gorge. (Ford, 1996:72) This has lead to a number of attempts to explain this feature, among others that it is a glacial spillway or a collapsed cave system.(Ford, 1985: 84) The current view is that the gorge represents an exhumed submarine inter-reef channel of Brigantian age in the flanks of the reef facies that forms the main escarpment along the Hope Valley. (Collinson. Et al.1991: 18. Ford 1996:72) Much of the removal of the late Namurian shales must have been carried out by glacial meltwater and Ford postulates a tongue of ice occupying the Rushup Valley in late Wolstonian times to provide the water (Ford 1996: 33)
Collinson, John D. Holdsworth, Brian K, Martinson, Ole J. & Bristow, Charlie S.(1991) Namurian Delta Systems of the Pennine Area: Field Guide No. 14. The British Sedimentological Research Group. Cambridge.
Ford, Derek C. & Williams, Paul W. (1989) Karst Geomorphology and Hydrology. Unwin Hyman. London
Trevor D. (1996) The Castleton Area , Derbyshire: The Geologist’s Association
Guide No. 56 The Geologist’s Association
Ford, Trevor D. (1985) Castleton Caves: results of speleothem dating. In Briggs, D.J. Gilbertson, D. & Jenkinson, R.D.S. (Eds.) Peak District and Northern Dukeries- Field Guide Quaternary Research Association. Cambridge
Sweeting, Marjorie M. (1972) Karst Landforms Macmillian
Ollerenshaw, Arthur E. (Not Dated, ?1970’s) Blue John Caverns and Blue John Mine: Castelton via Sheffield Privately Published. Castleton
(c) Hull Geological Society 1999 + 2001