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 open cast, 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. There are several small pipe caverns contained within the mine; the most prominent of these is Odin Cave (called Gank Hole by the miners) which was formed by groundwater opening out a void between limestone boulders. There is no evidence that Blue John was ever mined at Odin although the adjacent Treak Cliff Cavern regularly mines the rock in areas not open to visitors.