Twin lighthouses were built, on the banks of the mouth of the River Nene in 1831 to commemorate the opening of the Nene outfall cut. Contrary to belief these were lit and acted as lighthouses although they were not lit throughout the night, there being no rocks to protect ships from. The River Nene has however always been an important navigation for shipping and if a high tide occurred after dark, they were lit for approximately one and a half hours before and after high tide to guide ships through the sand banks and into the river. The towers are circular but the top lantern sections are hexagonal. Both have a circular window facing the channel entry. Each has in addition a half moon window to the north on the west bank lighthouse and to the south on the east bank lighthouse. A ship picking up either side light is not in the channel. The side lights would however be used by skilled pilots to triangulate their way through the twisting sand banks by picking them up and losing them.