Arkosic is the adjective. Ash Sand sized or finer-grained volcanic ejecta. Bench A horizontal shelf along the wall of a lava tunnel, generally close to floor level. It is formed at the edge of a lava flow level, by cooling and solidification of the edge followed by a lowering of the level. Some 'benches' form differently by the rolling down of a long flap of wall lining.
Gradations can occur, eg. where a thin bench breaks free and starts to roll downwards.
Bioclastic Containing grains composed of fragmented and transported organic material, eg. shell fragments. A valley that is closed off at its downstream end, all its water sinking at or near that point. Strictly, a cave formed by gas pressure pushing up a surface crust. True blisters are rare, and generally too small to enter. Some large ones are known overseas.