Castleton geology is both complex and fascinating. It is also an important geological area of the Peak District as it has such a variety of rocks. It lies on the border of an unconformity between the Dinantian limestones and Namurian Millstone Grit. This boundary can be observed in local streams.
Overall, the area represents an apron reef facies, consistent with the reef structures found in shallow Pacific seas. There has also been some volcanic activity in the area and a great deal of mineralisation.
Winnats Pass is a fossilised inter-reef channel, formed during the carboniferous and filled in with younger rocks. The infill was eroded away during more recent times leaving the narrow, curved and steep sided 'nick' leading to the limestone plateau between Castleton and Chapel-en-le-Frith.
Controversial Castleton Geology
In the Castleton region, the underlying limestone is gently dipping, poorly fossiliferous with some coral bands. Highly fossiliferous strata does however occur in the Treak Cliff cavern. The limestone forms a steep north-facing slope as you travel west from Castleton via Winnats Pass..
The Winnats Pass limestone is very different to nearby limestones, for example, those of Cave Dale. They are very fine grained, show little or no bedding with occasional pockets of well preserved fossils. These are therefore reef limestones but controversy surrounds their origin.
Winnats Pass is a narrow gorge that leads steeply up from west of Castleton onto the higher ground. Geologists debated how this gorge was formed for many decades but there is now a general consensus. The gorge formed during the reef phase of Castleton geology and has been re-excavated by erosion.
Contrasting Limestone Geology
The Castleton reefs do not show a mound-like form, causing problems with interpretation. The consensus is that they represent an apron-reef. These are formed in a marginal facies between a basin and a shelf. On a shelf, the angle of the sea floor is reasonably constant. The angle is changed where the shelf meets the basin.
The partial infilling of brachiopods found in the area of the Blue John cavern provide some evidence for this being a marginal facies. The infill indicates a sloping sea floor at the time of deposition. At Castleton, the shelf was to the south with the basin to the north.
There is a little more on reef geology of Derbyshire here >>
The structure of the reef is in 3 parts, the back-reef, the reef core and the fore-reef. The back reef is found at the top of Winnats Pass, exposed in Windy Knoll (disused) quarry. The limestone here shows that the back reef was therefore a lower energy environment - it is finer grained and muddier. This area also comprises ooliths and calcareous algal deposits.
The reef core contains coarse grained shell fragments of crinoids, corals and brachiopods because it was a higher energy environment. The fore reef is made up from a bioclastic limestone with fragments that washed off the main reef. These were deposited on a sea floor that dips at a little less than thirty degrees.
The fore-reef is exposed in Cow Low Nick close to the south-east foot of Winnats Pass. Winnats Pass itself offers good exposures also but is private land. Fossils in the reef limestones are noticeably different to the fossils of the shelf limestones. Pugnax is virtually absent in the latter but abundant in the reef, however, corals are almost absent in the reef but present in places in the shelf limestone (mainly Lithostrotion and Arplexus).
The beach beds are a cause of controversy because of the uncertainty about how they formed. They are found as a fringe to the lower parts of the fore reef but the fossil fragments are associated with a back-reef lagoonal facies. There lies the puzzle. Did they form in situ or were they from the back-reef and were transported there?
That offers up three possibilities. Firstly, the Beach could have been established at the base of the reef due to uplift or a lowering of sea level. Secondly, they are contemporaneous with the reef and were formed by a strong submarine current carrying sediments from the lagoon through the reef. Thirdly, Winnats Pass and the Beach Beds were formed by post-Carboniferous erosion. Professional geologists favour the second explanation.
Later Castleton Geology
Mam Tor is known as the 'Shivering Mountain'. The eastern face is of Namurian Edale Shale which is loose and friable, hence the ground has slid down into the Hope Valley. The landslide is currently still moving slowly as groundwater circulates.
This landslip has destroyed the main road between Castleton and Chapel-en-le-Frith. The authorities gave up mending the road in the 1979 as they realised it was a waste of time and money. Prior to that, works were needed once every 10 years or so to keep it passable. The road through Winnats Pass has now become the main road as the limestone is a tad more stable than the shale!
The landslide started over 4000 years ago and has advanced at an average rate of about 10 cm per year. Recent research indicates that the rate of slippage is not uniform, the top is moving faster than the toe. Current slippage is greater than average (possibly up to 1 metre per year), probably due to a generally wetter climate.
The Peak District mineral deposits form the southern part of the South Pennine orefield. Mineralisation is generally of the type known as fracture hosted vein mineralisation, in other words, the veins follow joints and faults. Groundwater circulation during Permian to Jurassic times lead to extensive mineralisation.
Minerals leached from the shales deposited north of the Castleton reefs. Additionally, the circulating hot groundwater hosted secondary alteration products too. The mineral rich fluid passed through fissures in the limestone and where it met cooler water, crystallisation occurred. That gave rise to the lead veins worked at the Odin mine and the coloured fluorite deposits known as Blue John. Castleton is the only known location for Blue John in the UK.
The remains of the Odin mine can be visited, but there is only access to the interior of the mine to experienced cavers. The bedrock of the mine is a dark crinoidal limestone deposited immediately before the start of Namurian sedimentation. The Blue John horizon underlies this pre-Namurian boulder bed.
The mine is not the obvious cave, it is the fissure to the right of the cave. If you look carefully, you can see slickenslides on the walls of the fissure indicating wrench faulting. The fractures hosting the mineralisation around Castleton show generally an east or east-northeast trends or a northwesterly trend.
Hydrocarbon Extraction at Windy Knoll
On the plateau above Winnats pass and the Blue John Mine lies Windy Knoll. This is a small cave (excavated in the 1870s and found to contain thousands of animal bones) and a small quarry. The quarry provided hydrocarbons for fuel rather than the more usual rock. Some of the colouration of the Blue John may have been caused by these hydrocarbons in the groundwater.
The Speedwell Vent is a volcanic plug near to the lower opening of Winnats Pass. It is not well understood and may be associated with the Cave Dale lava flow rather than an actual volcanic vent. The (amygdoloidal) basalt is exposed in Cave Dale and has been correlated with the Lower Lava of Miller's Dale.
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