sentences:  The tarn was originally a much smaller waterbody, but was raised early in the 20th century to provide drinking water for the Barrow in Furness area.The dam is almost 400 yards long and is concrete cored with slate buttresses, the resulting depth of the tarn being around 80 ft. Water is not abstracted directly from the tarn, but flows some distance downriver to an off-take weir.Since Grey Friar curves southward, the main ridge stands across the tarn, Brim Fell and the back of Dow Crag forming the opposing slope.