Concretionary tuff on Pavey Ark

The rocks in this photo belong to the uppermost part of the Dungeon Ghyll Member in the Borrowdale Volcanic Group (Ordovician). They are coarse, crudely bedded volcaniclastic pebbly sandstones, probably resulting from ash-fall into water, with accompanying andesite lava spatter; the whole lot subsequently disrupted by slumping and mass movement (Millward et al, 2000).

The 'strangeness' is due to the presence of curious, concentric ring-like structures which appear to be secondary (i.e. later) concretions - expanding growths of secondary minerals (epidote?) to form spheroidal masses. The origin of these structures is unclear, and they do not seem to be documented from anywhere else in the district, although I have seen similar but less well developed structures on Side Pike in Great Langdale and near Levers Water in the Coniston Fells.

Reference:
Millward, D. et al. 2000. Geology of the Ambleside district, British Geological Survey Memoir for 1:50000 Geological Sheet 38, London HMSO.