The lowest and oldest rocks on Helvellyn are those of this Lincomb Tarns Tuff Formation, which outcrop along the western side, up to roughly the contour on Whelp Side. The lowest part of the formation here is the densely welded lapilli-tuff of the Thirlmere Member, in which the individual pieces of semi-molten lava were flattened under the weight of deposits above them. Contemporaneous movement on the caldera's boundary fault has produced a thick deposit of breccia above the Helvellyn Screes and on Browncove Crags. The Thirlmere Member is overlain by a deposit of sandstone, the Raise Beck Member, deposited in water during a break in the volcanism, but succeeded by further thick ignimbrite deposits.