On Watson's Dodd this formation is represented by two members. The Tarn Crags Member makes up Castle Rock, as well as a thin layer higher up the fellside at about the 650 m contour. It is a coarse-grained dacitic lapilli-tuff. Above this thin layer are rocks of the Thirlmere Tuff Member, which covers the top of the fell. First is an area of volcaniclastic breccia, which forms a series of small rocky outcrops on the fellside, and above this is a thick sheet of welded rhyo-dacitic lapilli-tuff in which the individual pieces of semi-molten lava (the lapilli) were flattened under the weight of deposits above them. This rock weathers to a white or pink colour, but it is covered by the smooth grassy turf characteristic of Great Dodd, Watson's Dodd and Stybarrow Dodd, all of which are covered by the same sheet.