Shute Shelve Tunnel on the Strawberry Line Trail in Somerset and North Somerset, in the South West of England.
The old railway tunnel passes through a ridge between Shute Shelve Hill and Winscombe Hill in the Mendip Hills Area of Outstanding Natural Beauty. Although the tunnel is only about 180m long, it passes through a very marked transition in rock strata about two thirds of the way through, when entering through the North portal, (coming from Winscome and heading South towards Axbridge) - changing abruptly from soft marl to hard limestone.
Where the tunnel was cut through the soft marl, it has a lining of brick and stone, whereas the section through the hard limestone has bare, unlined walls.
The photo shows the transition between the lined and unlined sections of the tunnel.