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Winnats Pass is a narrow limestone gorge which is the only direct route from the west of Castleton. It climbs 1300 feet up through a dale, and has a road running the entire length of the ravine. At the foot of the pass is the entrance to Speedwell Cavern. It was once thought that the pass was a collapsed cave, but modern geologists have now discovered a more complex and ancient origin. About 330 million years ago there were two areas of coral reef underwater with a shallow channel between them. After some time the reef became covered in mud and sand and turned to limestone, which then became buried under 3Km of rock. Movements in the Earth's plates lifted the rocks out of the sea and over millions of years the wind and water slowly wore away the layers of sandstone and exposed the buried reef. During the Ice Ages (170000-145000 years ago) the glaciers on the high ground to the north of the area thawed, and torrents of water poured into the Hope valley. The water wore away the last of the shales revealing the limestone and the dramatic pass of Winnats. In more recent times, the limestone rock has been permeated by water and the whole area is now riddled with caves. Some of the larger caves are well documented and explored, but there are still many unexplored caves in and around the area of Winnats Pass.