From Swirl How the east ridge drops steeply down Prison Band to Swirl Hawse, before rising again to the summit of Black Sails. Black Sails has a descending southern spur which steps down over High and Low Wether Crags. Between this and the main Coniston range is the valley of Swirl Hawse Beck and Levers Water. This tarn has been raised by damming to a depth of 125 ft, originally to supply water to the Coniston Copper Mines. Following the decline of mining a water treatment plant was built, and since the 1970s the tarn has supplied drinking water for Coniston and other local villages as far east as Sawrey.