The sill is the curved stone. Normally the gate would be moved across the entrance of the drydock on the rollers shown just below the sill. In this instance the drydock is being repaired thus the gate in the outside position. This remarkable piece of industrial archaeology is intact as shown in this 1950s photograph . Detail of gate shown above.
Rollers support the gate as it is moved across the entrance to the drydock.
Marine Museum of the Great Lakes at Kingston 55 Ontario Street, Kingston, Ontario.