NY8356 : Entrance to the Blackett Level NY8356 : Blackett Level portalIt is interesting to note the shape of the tunnel cross-section as revealed by the stonework around the portal. This 'egg shape' is typical of that used in 19th century brick sewers, rather than the 'horseshoe' shape (flat bottom, arched roof) more common in mines. It suggests that it was intended principally for drainage rather than man access, and with at least the possibility that it would flow full (or nearly full) at times.