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The Odin Fissure, a narrow man-made limestone gorge created by miners in pursuit of lead ore. This represents the earliest workings at Odin Mine before excavations moved underground.
Odin is the oldest documented mine in Derbyshire and is mentioned in an official record dating from 1280 when one John of Bellhag, a poacher, was put on trial for hunting near the entrance. However, its origins may well be much older, possibly going back into the Roman period, but there is no firm evidence to prove this. It continued to be worked for lead until 1869 but was then abandoned apart from a brief revival for fluorspar and barite in 1908-09. It was later designated a Scheduled Ancient Monument, passing into the custodianship of The National Trust.
The Wikipedia entry for the mine (accessed March 2015) includes the following:
'The veins of lead in the Castleton area formed 280 million years ago when a fault in the local Carboniferous limestone allowed mineralising fluids to flow into fissures in the rock, pushed up by great pressure from beneath the Earth's surface. Lead and sulphate combined to form the lead ore galena. In its heyday, the mine was a complex system of levels and shafts that extended for approximately 1500 metres into the Edale shales beneath the nearby Mam Tor. In the early days the mining was opencast, forming a gorge in the hillside with the water diverted by a leat to the north to keep the workings dry. Later the miners followed the vein of lead underground. The vein is exposed on the surface in a small limestone outcrop at the entrance to the mine and then continues underground just south of due west in the limestone under the Edale shales of Mam Tor. The 1769 plans of the mine show that there were several branch veins leading from the main lead workings.'