SO9389 : Dudley Tunnel south portal SO9389 : Dudley No.1 Canal TunnelThe gauge at the tunnel mouth is explained by the following text from the Dudley Canal Trust web site
http://www.dudleytunnel.co.uk/ninety_six.htm1884- The southern section of Dudley Tunnel collapsed due to mining subsidence. This was rebuilt during that year to larger dimensions. The theory was that over time the tunnel would sink and the roof would line up with the 1792 section. This has failed to happen and the tunnel entrance give a false indication of the headroom in the tunnel. Because of this countless boats have got stuck over the years, and so a gauge has been fitted to prevent this happening.
For an interactive map of the tunnels see
http://www.beneath-dudley.co.uk/flash-map.htm