GEOLOGY

Leader: Derek Brumhead    

Visit to Mam Tor, Treak Cliff and the Winnats.

Monday, 25 July 2011

[Click on images to enlarge]

The geology group, led by Derek Brumhead  travelled to the car park opposite Blue John Cavern. Standing here in front of Mam Tor we looked eastwards over the Mam Tor - Lose Hill - Win Hill ridge and the beautiful Hope valley. Derek outlined the geological history of the immediate area.

Treak Cliff. The wide vale of Hope, underlain by the Edale Shales (a mudstone) emphasised the contrast with the steep limestone cliff, which is a former coral reef (apron reef), formed about 350m years ago, which sloped down into the deep water in which the Edale Shales were being deposited. This submarine cliff was buried by the onset of deposits of the Millstone Grit delta from the north and is only seen today because it has been re-exposed by tens of millions of years of erosion.

Close-up of the Mam Tor Sandstones. 318-332 million years ago, when what is now western Europe was situated in the latitude of the equator, a huge mountain chain spread over what is now Scandinavia. The continents were still joined and there was no North Sea or Atlantic Ocean. Rivers spread huge amounts of sediments, particularly sand and mud southwards and a huge delta spread (Millstone Grit sediments) into the sea which was covering what is now the Pennine region. The Mam Tor Sandstones were the first to be deposited on the delta front. Repeated currents swept down the delta front laying sheets of sand on the sea floor which alternated with thicknesses of mud, deposited in quieter periods. Each new turbidity current of sediment-laden water scoured the underlying mud and left various marks (known as sole marks) which are now preserved on the underside of the sandstone units. The resultant formation is a series of alternating sandstone units separated by shale, dramatically displayed on the face of Mam Tor. Beneath the Mam Tor Sandstones are the Edale Shales which formed the muds of the basin depths into which the delta was prograding.

Sole marks on the base of a sandstone block.

The party walked to inspect the fallen rocks below the face of Mam Tor and  Derek pointed out several blocks of sandstone, explaining how these illustrated the ancient turbidity currents. Several bedding planes showed sparkling flakes of mica, proof that they were derived from the igneous and metamorphic rocks making up the mountains to the north.

From this location there is also  is a dramatic view’ of the Mam Tor landslip, the largest active landslip in the country, at least 3,600 years old. The road was built as a turnpike in 1810 to replace the steep Winnats Pass road. This turned out to be an unwise choice. Due to movements of the slip, repairs were made in the 1940s, 1960s and 1970s, with repeated filling and levelling. But massive movement finally destroyed the road and it was closed in 1979, never to be reopened. Walking over it is an extraordinary experience for the successive layers of tarmac, oldest at the bottom, are a perfect example of the principle of geological succession, while the various separated blocks illustrate mini fault scarps.

Outcrop of volcanic rock. The group then took the cars down the Winnats gorge to the car park opposite Speedwell Cavern. From here, a path lead across fields in the direction of Castleton to the next locality where the leader pointed out a discrete triangular-shaped piece of land which appeared to have resisted erosion. Some small outcrops exhibited a crumbly brownish rock, obviously not limestone. Derek explained that the rock was volcanic and had been deposited and fragmented in water, probably at the same time as the basalt which can be seen in nearby Cave Dale.

The party returned to Speedwell Cavern where it inspected an outcrop of the 'Beach Beds' in a low quarry face. A wire fence prevented close inspection but binoculars showed numerous fragments of rolled shells. This fan-shaped accumulation at the foot of Winnats Pass is thought to be material washed down from a lagoon above the Winnats and is considered to be evidence that the gorge is contemporaneous with the formation of the reef, 300 m years ago.

View up the Winnats gorge. The crag on the right is that in the next photograph.

Walking up the gorge for a short distance Derek pointed out that the tilted limestone represented a depositional dip not a structural one (ie not due to folding). In other words, it was evidence that the limestone had been deposited at an angle to form the reef slope represented by the present-day Treak Cliff.

One fascinating piece of evidence for this is that fossil shells which have been half filled with liquid lime mud show a fossil ‘spirit level’ which is horizontal within the rock which is tilted, thus proving the depositional dip. These are known as geopetals. They are not easy to find, but we were lucky that Gerald had an eye for them and found two excellent examples.

Walking up Winnats, a short way it was noted that the limestones high up on the opposite side of the gorge had become horizontal and represented the limestone deposited in shallow water at the back of the reef.

Back at the car park, a view towards Treak Cliff Cavern showed a limestone crag jutting out from the cliff face. It was explained that studies by Fred Broadhurst and a colleague showed that the geopetals in this crag were upside down. The crag therefore was not part of the cliff face, but were huge boulders that had slipped down and turned over during the formation of the reef.

The party then drove to the site of Odin mine, an open fissure below Mam Tor, where lead ore (galena) had been extracted from at least the thirteenth century, perhaps even earlier. The fissure coincided with a geological fault in which the ore had been deposited by hydrothermal solutions rising from below. Unfortunately it is now no longer possible to safely get into the fissure, where horizontal slikensides on the rock wall demonstrate the movement of the fault; also the marks of the miners' picks where the last vestiges of ore were scraped off the limestone.

The galena needed to be separated from the rock and to do this it was first crushed into small fragments or powder. This was done by the Odin crusher wheel and track on the opposite side of the road. A large gritstone wheel with an iron tyre was pulled round a circular iron track by a horse to crush the ore. It was then buddled or riddled in settling tanks to separate the galena (specific gravity 7) from the rock which was then taken away to a lead smelter - there was one at Bradwell. This crusher is the best preserved in the Peak District and dates from 1823.

Words by Derek Brumhead. Photographs taken by Pat Stanway.

References:

• Allen J P L, ‘The Mam Tor Sandstones - a turbidite facies of the Namurian deltas of Derbyshire’. Journal of Sedimentary Petrology,30 (1960), pp. 193-208.
• Trevor D Ford, The Castleton Area, Geologists’ Association Guide No. 56, London, 1996.
• Simpson I M and Broadhurst F M, ‘A Boulder Bed at Treak Cliff, north Derbyshire', Proceedings Yorkshire Geological Society, 37 (1969), pp. 141-152.
• Skempton A W, Leadbetter A D and Chandler R J, ‘The MamTor Landslide, north Derbyshire', Philosophical Transactions of the Royal Society, A329 (1989), pp. 503-547.
• I P Stevenson and G D Gaunt, Geology of the Country around Chapel en le Frith, HMSO, London 1971.