Cogra Moss (or Arlecdon Reservoir) was created in 1880 by damming the Rakegill Beck valley to create an attractive reservoir, about 40 acres in area, to supply water for Arlecdon. It is no longer used for drinking water, but is now used as a fishing tarn. Surrounded by a horseshoe-shaped valley on the edge of Lamplugh Fell, the shores are backed by conifer plantations.

It may be accessed via a track leading from a small car park in Felldyke, to the west.