the western slopes are relatively smooth and fall to tarn head beck. this runs parallel to the ridge and is the main feeder of seathwaite tarn, a reservoir in a side valley of the duddon system. this was originally a much smaller waterbody, but was dammed 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. on the slopes of brim fell, above the head of the reservoir, are the remains of seathwaite tarn mine. this was worked for copper in the mid-19th century, and also appears as a location in the novel "the plague dogs" by richard adams.