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Concrete barges (Shaded Yellow) - the bedrock of the Maunsell forts.
"in 1940 Mr. Guy Maunsell a Civil Engineer was given the task of producing & constructing the Forts for both the Thames Estuary & the Humber Estuary. 2 designs were finally produced to be used in the Thames Estuary both designs were pre-built fully assembled & towed to their permeate location site. The method chosen for sighting these forts was to have them built on concrete barges, which were used to tow the forts to there chosen location which once in position they were sunk in a controlled sinking. The fact that all of the Forts were located on sand banks helped to ensure that they did not move even in the roughest conditions."

Excerpt taken from:- www.gnometech.freeserve.co.uk