Who exactly is this Blue John ? EarthCache

The Carboniferous period lasted for 65 million years and it was followed by an era when this region was subjected to massive earth movements which raised and folded the rocks of the area. This folding was not even - the rocks to the West were folded more than those to the East, and even as the folding took place the rocks at the top of the arch were being eroded by the elements. The region was raised in a North-South line which resulted in the dome-like shape of the modern Peak District and the rocks were worn away until even the Limestone beneath was exposed. At the end of this period the Earth's crust sank in this area and the whole region was covered by sea which deposited a whole range of new rocks over it. This explains why you find different rocks in the Peak District – Limestone is prominent in the White Peak but in this area you are in the Dark Peak, where Gritstone is more prominent. However, around the locality, you will find notable areas of limestone such as Winnat’s Pass and the caves around Castleton.

During this period the rock, especially the limestone, often cracked under the pressures and molten rock (magma) was forced into the fissures. This was often rich in minerals, such as galena (lead sulphide - grey metallic coloured rectangular crystals), fluorspar (calcium fluoride - translucent white crystals, sometimes coloured blue or yellow), barytes (barium sulphate), calcite (a form of calcium carbonate - white crystals) and sometimes copper, all of which crystallised as the magma slowly cooled. The result was the many mineral veins or rakes which are to be found in the limestone areas and which have been mined for lead and other minerals since at least Roman times. Odin Mine at the foot of the old Mam Tor road is an excellent example of an ancient lead mine.

Dirtlow Rake

At the location of this earthcache you find yourself at Dirtlow Rake, one of the most principal epigenetic mineral veins in Derbyshire. It is a north-west trending replacement deposit with a downthrow to the south of about 70m and considerable lateral movements in places (observed as mineralised slickensides of the footwall and hanging wall). In geology, a slickenside is a smoothly polished surface caused by frictional movement between rocks along the two sides of a fault. This surface is normally striated in the direction of movement. The plane may be coated by mineral fibres that grew during the fault movement, known as slickenfibres, which also show the direction of displacement. Due to irregularities in the fault plane exposed slickenfibres typically have a stepped appearance that can be used to determine the sense of movement across the fault. The surface feels smoother when the hand is moved in the same direction that the eroded side of the fault moved, as the surface steps down in that direction, like the scales on a fish when stroked from the head.

The entire length of the rake contains three complex, parallel, distinct veins. Where the vein crosses reef-complexes and natural caves, fluorite lenses are well developed, sometimes associated with chert. The rake has been mined by both surface and underground extractions (for example at Ashton’s, Hollandtwine and the Hazard mines). Towards Pin Dale (the gorge to your north-east) evidence of historical workings exist and the surface outcrop of the vein has been completely extracted to leave a deep, steeply inclined gorge. A small open-pit mining operation presents an ideal opportunity to inspect the mineral vein in cross section which is seen to dip at about 70 degrees to the south. The open pit is situated where Dirtlow Rake crosses pre-Namurian solution collapse structures.

The main economic and productive part of the vein is along the wall rocks which contain large masses of radiating columnar jointed calcite and veins of fluorite, baryte and galena. The limestone host rock has been altered and tectonised and in places silicification is common extending over 400m parallel to the vein on the south side. To the east Dirtlow Rake and associated scrins are marked by numerous shafts, lines of spoil heaps and narrow ‘gorges’ representing worked-out sections of the vein. Several hundred thousand tonnes of fluorite have been mined from Dirtlow Rake. Lead is no longer exploited and the fluorite, baryte and calcite historically discarded as ‘gangue’ or waste, now form important commodities. Fluorite is needed as a raw material in the chemical industry as well as being required for the fluoridation of water and in toothpaste. Baryte in used as a filler in paper and as an additive to drilling mud in the North Sea hydrocarbon industry to help control high gas pressures.

To my knowledge, Blue John was not extracted from Dirtlow Rake but from closeby in Castleton as it tends to have only been found below ground at locations not accessible for an earthcache. However, Blue John is simply another variety of fluorite (also known as fluorspar or calcium fluoride, CaF2) which has been mined for ornamental and industrial purposes since at least the Middle Ages. Apparently the Romans discovered the Blue John mine, some 2000 years ago. Two vases of ‘Blue John’ are reported to have been discovered in the ruins of Pompeii. Although now a tourist cave this was a working mine and is one of 4 ‘show caves’ in the area. It was discovered by miners’ in 1770 and worked in the 17th and 18th Century for galena and fluorite. A steep descent along a narrow fissure leads into a series of vadose stream caverns, most of which are now dry. The cavern terminates at a sump adjacent to Odin Mine. The origin of this cave system so close to the northern edge of the limestone is inexplicable since there does not appear to be sufficient catchment for a stream large enough to erode these large caverns

Treak Cliff Cavern contains the only known workable deposits of ‘Blue John’ fluorite in a ‘pipe’ vein, following the inclination of the bedding in the limestone. The vein may be followed up the steeply dipping rocks forming the Treak Cliff reef escarpment. Records for mining date back to at least 1757 and the mine has worked since then on a discontinuous basis. The Old Tor Mine, which is situated on Treak Cliff, was once a major producer of ‘Blue John’. Mining resumed after the World War I in 1926, to provide fluorite as a fluxing material in electrical furnaces. At this time miners discovered old shafts leading to a spectacular cave networks containing stalactites and stalagmites lined with ‘Blue John’ fluorspar. The cave was opened to the public in 1935 when electricity had been installed. Outward dips of between 25 to 30 degrees can be seen on the fore-reef limestones at Treak Cliff. The limestone is highly fossiliferous and contains abundant well preserved fossils including; brachiopods, polyzoa, rare corals, sponges, nautilus, lamellibranchs, gastropods, ostracods, bryozoans, goniatites, crinoids, stromatolitic algae and some trilobites. Near the foot of Treak Cliff is a large, distinct, circular depression. Often mistaken for a karst doline or old mine workings, but it is actually a Nazi bomb crater formed in 1941.

You need to walk through Dirtlow Rake, the earthcache co-ordinates are roughly in the middle of the rake but are not at any specific feature. Note, this area is open access, you are free to roam, but please enter the site only at the gate. If it is locked, you may need to climb over it; you do have a legal right to access the land but please do so in a way that won't damage it. You will not be able to answer the questions unless you spend a little time exploring the area and walk the full length of the rake. You should send in your answers before you log a find. You do not need to wait for my response before logging but your log will be more than likely to be deleted if your answers are not received shortly following your log. Warning would be given before the deletion of any log.

1. IN YOUR LOG – please explore all of Dirtlow Rake (the full area you see in the aerial image above). You will see the remains of the mining and mineral extraction that took place here so many years ago. Please look for evidence of the mineral veins and explain what you spotted. There may be marks in the rock. Are they natural, or man made. For example pick axe marks, natural layers or water damage? You can include photographs and descriptions in your logs. Can you find evidence of the slickenside? Again, please include photographs and descriptions. A photograph or yourself or GPSr in the rake would help to prove you were there and would be appreciated - it is not a requirement.

   
   

2. BY EMAIL – How deep is Dirtlow Rake at its deepest point? Estimate the width at the narrowest and widest points. Can you confirm the rake is entirely limestone? Can you describe it’s texture visually and it's colour? Is this the same throughout the rake or does it change in any way? Also, what does epigenetic mean, and why do you think Dirtlow Rake is epigenetic?