Phoenix Caisson off Littlestone-on-Sea


Heritage Category:
Scheduled Monument
List Entry Number:
Date first listed:
Location Description:
Situated in the Straits of Dover, off Littlestone-on-Sea, Kent, approximately 0.66 km from the low water mark, at NGR TR0949324640.


© Crown Copyright and database right 2021. All rights reserved. Ordnance Survey Licence number 100024900.
© British Crown and SeaZone Solutions Limited 2021. All rights reserved. Licence number 102006.006.
Use of this data is subject to Terms and Conditions.

The above map is for quick reference purposes only and may not be to scale. For a copy of the full scale map, please see the attached PDF - 1415588.pdf

The PDF will be generated from our live systems and may take a few minutes to download depending on how busy our servers are. We apologise for this delay.

This copy shows the entry on 19-Jan-2021 at 09:59:37.


The building or site itself may lie within the boundary of more than one authority.

Location Description:
Situated in the Straits of Dover, off Littlestone-on-Sea, Kent, approximately 0.66 km from the low water mark, at NGR TR0949324640.
National Grid Reference:


Phoenix caisson. Built in 1943-4 as part of the 'Mulberry' floating harbour.

Reasons for Designation

The Phoenix caisson off Littlestone-on-Sea, built in 1943-4 as a breakwater component of the pre-fabricated Mulberry Harbours which were a key element of the Normandy landings, is scheduled for the following principal reasons: * Historic interest: as a tangible reminder of Operation Overlord and its significance to national and world history. Its location off Littlestone-on-Sea is illustrative of the logistical preparations and problems involved in the invasion as it remains where it was ‘parked’ prior to D-Day after it proved impossible to refloat and tow it across the Channel; * Innovation: as a component of the innovative feat of engineering which enabled the transport and construction of a vital pre-fabricated port facility, the Mulberry Harbour, off the coast of Normandy; * Rarity: it is one of only six known examples of Phoenix caissons in British Waters; * Survival: it survives remarkably intact, having lost only its anti-aircraft gun mounting and other metalwork, and retains its structural integrity.


D-Day, June 6th 1944, was one of the most significant operations of World War II, defining the start of the final phase of the war in Europe. After two to three years of preparations, the assault phase of `Operation Overlord' - code-named Neptune - lasted for little over three weeks and by 30 June had landed over 850,000 men on the invasion beachheads in Normandy, together with nearly 150,000 vehicles and 570,000 tons of supplies.

Much of the success of Operation Overlord was due to the creation of a pre-fabricated harbour, code named ‘Mulberry', built in sections and towed across the Channel. Two Mulberry harbours were proposed; one in the British sector on 'Gold' beach at Arromanches (Mulberry B); the other further west in the American sector on 'Omaha' beach (Mulberry A). Assembly started on June 9th and by the 18th June the two arcs of caissons were in place. The following day, however, a heavy storm wrecked the incomplete harbour on Omaha beach and severely damaged that at Gold beach. What could be salvaged from Omaha beach was transferred to the British sector. The loss of Mulberry A meant that the capture of the port of Cherbourg now became a strategic imperative for the Americans, although the caissons still provided a sheltered anchorage for transhipment of supplies directly onto the beach. The Mulberry harbours, however, had fulfilled their strategic function in allowing a landing without the necessity of having to immediately capture a heavily fortified port in order to re-supply the invasion. Winston Churchill stated that the harbours were 'a principal part of the great plan', and were decisive in the first days of the invasion. Originally intended to function for a period of 90 days, the surviving Mulberry A harbour eventually continued in use into the winter of 1944 when the facilities at Antwerp were captured, allowing for a new line of supply to the allied armies in Belgium and northern France.

Churchill had conceived of creating rudimentary harbours by sinking sand-filled barges during World War I. Although the concept was not used at that time, Churchill remained a strong advocate of the potential of artificial harbours. In 1941 the War Office established a department to evaluate and refine artificial harbour designs, under Major General DJ McMullen, Brigadier Bruce White and Major Cornick RE. The final decision to proceed with the artificial harbours was, however, not taken until the Quebec Conference in late August 1943 with designs only begun in October and construction in December. This allowed only a very short, six month, construction period. The artificial harbours were made up of three structures: outer breakwaters, pier heads and floating roadways that connected to the shoreline. Each component was given a codeword to maintain secrecy for the operation. The breakwater comprised three elements: a floating outer line of connected hollow steel breakwaters (Bombardons); the Phoenix caissons, which were very large rectangular concrete structures that were airtight enabling them to be sunk and re-floated; and an assembly of 60 obsolete vessels that would be scuttled to form a protective line of block ships (Gooseberries). The pier heads were made up of floating pontoons (Spuds), which were attached to legs that permitted the pier head to move up and down with the tide. A total of 10 miles of floating roadways (Whales) connected the pier heads to the shore.

The reinforced-concrete Phoenix caissons were built with a boat hull, to enable towing, above which was a walkway and upper walls which acted as the actual breakwater; the original units were open at the top. The interior of the caisson was divided into compartments, one of which was used as quarters for transporting the crews which would sink the caissons once they had been towed across the Channel. The naval planning staff who had to arrange for the sinking of the caissons in the correct positions had specified that these be fitted with flooding apertures which would enable them to be sunk in fifteen minutes – the time for which it was estimated that they could be held accurately in position against varying tidal and weather conditions. Mountings for anti-aircraft guns were fitted on top of the larger caissons for protection crossing the Channel and when in situ.

To allow for different depths of water, the caissons were made in six different sizes. The largest (Type A1) were 200 ft long, 56 ft wide, 60 ft tall and weighed 6,044 tons. The smallest (Type D) were 174 ft long, 28 ft wide, 25 ft tall and weighed 1,672 tons. The open-top design, although quicker to assemble, which was essential given the tight time scales involved, proved problematic. Many caissons collapsed when large waves washed over the caisson, filling the open internal compartments with water and causing them to burst outwards from the weight of water inside. After several had failed, the surviving caissons were 'winterised' by filling the internal spaces with sand and covering them with metal shuttering. A later improved design, known as type Ax, included a concrete roof.

The caissons were constructed by civil engineering contractors at various sites around the coast of Britain. 146 caissons were originally constructed (eventually a total of 212 were produced) and it was estimated that they required 330,000 cubic yards of concrete, weighing nearly 600,000 tons and 31,000 tons of steel. On completion the caissons were towed to sites at Selsey or Dungeness where they were sunk, or ‘parked’ on account of a lack of sheltered mooring sites because of other priorities in the naval build-up. The parking of the caissons had been opposed by Brigadier White who was worried about damage resulting from their coming to rest on an uneven seabed. The caissons were later raised and towed by tug across the Channel. The example at Littlestone-on-Sea remains where originally parked, the navy having been unable to refloat it, probably because of damage to the hull when it was sunk, as Brigadier White had feared.


The monument comprises a single Phoenix caisson, resting on the sea bed and exposed at low tide. It is approximately 200 ft (61m) in length, 32 ft (10m) wide. The height is unclear from current information but, depending on the type of caisson, will be between 24 and 60ft high (7-18m). It is constructed of a steel framework with concrete base and walls around 0.3m thick. The interior is divided into nine sections, open to the sky. These are further divided by a spine wall creating 18 square cells. Some of these retain their diagonal steel tension bars. Later navigation lights are mounted at the four corners on steel posts.

EXTENT OF SCHEDULING The scheduled area is restricted to the structure of the caisson itself. A protective margin is not considered necessary as it is thought unlikely that any archaeology relating to the positioning and attempted re-floating of the caisson in June 1944 exists on the sea bed.

EXCLUSIONS The four navigation lights and their mountings are excluded from the scheduling.


Books and journals
Hartcup, G, Codename Mulberry: The Planning Building and Operation of the Normandy Ha, (2006)
White, B, The Artificial invasion harbours called Mulberry, (1980)
“The difficulties will argue for themselves”: Mulberry Harbours and the D-Day Landings, accessed from
The Mulberry Harbours, accessed from


This monument is scheduled under the Ancient Monuments and Archaeological Areas Act 1979 as amended as it appears to the Secretary of State to be of national importance. This entry is a copy, the original is held by the Department for Digital, Culture, Media and Sport.

End of official listing

Your Contributions

Do you know more about this entry?

The following information has been contributed by users volunteering for our Enriching The List project. For small corrections to the List Entry please see our Minor Amendments procedure.

The information and images below are the opinion of the contributor, are not part of the official entry and do not represent the official position of Historic England. We have not checked that the contributions below are factually accurate. Please see our terms and conditions. If you wish to report an issue with a contribution or have a question please email [email protected].