At Seven Arches Farm the River Colne flows beneath a railway viaduct - with 7 arches. Underneath the westernmost arch, with its entrance hard against the western buttress, is a hexagonal concrete pillbox. It measures 19ft 6in across with each wall 11ft in length. It is 3ft 6in thick and there are five small loopholes with steel side framing - no loophole in the western side facing the buttress.

From this position the pillbox could fire each side of the viaduct - north across the river to the meadows beyond or south in support of the anti-tank obstacles on the river bank, while being itself protected from air attack by the viaduct.

Five photos of site.<1>

See also <2>.