Mulberry Harbour Phoenix Caissons at Portland Harbour


Heritage Category:
Listed Building
List Entry Number:
Date first listed:
Date of most recent amendment:
Location Description:
Statutory Address:
Opposite Castletown Pier, Portland Harbour, Isle of Portland, Dorset, DT5 1BD


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Statutory Address:
Opposite Castletown Pier, Portland Harbour, Isle of Portland, Dorset, DT5 1BD

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

Location Description:
National Grid Reference:


Two Phoenix Caissons, sections of the structure known as a Mulberry Harbour designed for, and used in, the invasion of Normandy in June 1944. The harbour was a part of the vital support structure behind the successful operation. The caissons are moored in-line to the north of Castletown Pier in Portland Harbour.

Reasons for Designation

The Mulberry Harbour (two Phoenix Caissons) at Portland Harbour is listed at Grade II for the following principal reasons:

Architectural interest: * For the Mulberry Harbour design as an innovative construction created specifically for its critical role in the invasion of Normandy in 1944; * The caissons survive largely unaltered.

Historic interest:  * As part of the vital support and supply structure that helped secure an Allied victory in Operation Overlord of June 1944; * The fabrication, deployment and installation of the Mulberry Harbour was a formidable task and testament to the ingenuity and heroism involved in the invasion of Normandy.

Group value: * As part of a complete naval base of considerable importance, specifically designed as the first safe anchorage for the replenishment of the navy’s fleet of steam-driven warships; * Portland Harbour and the nearby coast of the Isle of Portland has a significant collection of designated assets associated with the military history of the area, including Portland Castle (Grade I) and the East Weare Defences.


Due to the lack of a suitable port an absolutely essential part of the Allies’ planning for the invasion of Normandy in 1944 was the provision of ‘Gooseberry’ and ‘Mulberry’ harbours. The ‘Gooseberries’ were anchorages of calm water formed by sinking a number of ships to form a sea wall off Port-en-Bessin, Varreville, Courseulles and Ouistreham. The ‘Mulberries’ were altogether more sophisticated pre-fabricated concrete harbours and their design was based on a concept originally proposed by Winston Churchill in 1917 for an operation in the Friesian Islands. ‘Mulberry ‘A’ serving the American forces at Saint-Laurent-sur-Mer (Omaha Beach) and 'Mulberry ‘B’ serving the British forces at Arromanches (Gold Beach). This was a considerable undertaking: 4,500 men were involved in their construction, and each ‘Mulberry’ was intended to be roughly equivalent in area to Dover Harbour and be capable of handling 12,000 tons of supplies daily. They consisted of a number of exotically code-named components: ‘Phoenix’ (a hollow concrete caisson); ‘Corncob’ (a sunken blockship); ‘Whales’ (floating pierheads); ‘Spuds’ (extendable steel legs); ‘Beetles’ (concrete pontoon barges); and ‘Bombardons’ (steel mooring buoys).

The two operational harbours were built within two weeks. Although "Mulberry "A" had to be abandoned after a storm in late June 1944, Mulberry "B" remained in use for ten months for the landing of over 2.5 million men, 500,000 vehicles, and 4 million tons of supplies. The Mulberry Harbour initiative undoubtedly contributed significantly to the successful invasion of Normandy and the subsequent liberation of Europe.

Ten of the ‘Phoenix’ caissons were towed to Portland in 1946 and were positioned to the west of the harbour to protect berthed vessels from prevailing westerly winds. In the early 1950s they provided sheltered protection during the construction of a new pier within Portland's dockyard, known as Queen's Pier, (or 'Q Pier'). Eight of the caissons were then sent by the Admiralty to the Netherlands to repair and block breaches in the dykes, following a great storm in January 1953. Two caissons remain moored about 115m north of Castleton Pier as a reminder of the remarkable technical achievement of the harbours and the Normandy invasion as a whole.


Two ‘Phoenix’ caissons of 1944 moored end-to-end.

MATERIALS: built of reinforced concrete each weighs 7,000 tons (7,113.8 tonnes).

DESCRIPTION: each caisson is rectangular on plan and 12.19m long, 9.14m wide and 12.19m high. Of monolithic appearance, the concrete walls rise above a wider concrete base that provides a walkway around the caisson. At the centre of each end elevation is a vertical concrete pier with an opening to allow passage along the walkway. The pier can be used for ladder access to the roof and there are fixed ladders at the adjacent ends, and a gangplank between the two caissons. Each caisson has steel railings at parapet level and on the roofs are fixings for former 40-mm Bofors light anti-aircraft gun mounted on a squat tower. Internally, they are subdivided into a number of open transverse chambers that could be flooded to sink the caissons to the sea floor to form a breakwater.


The contents of this record have been generated from a legacy data system.

Legacy System number:
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Books and journals
Mallory, K, Walls of War: A history of military architecture in North West Europe 1900-1945, (1979), 200-213
English Heritage, ‘Thematic Survey of English Naval Dockyards. Summary Report. Thematic Listing Programme’ (1998), accessed 20/09/2017 from
The Encyclopedia of Portland History - Mulberry Harbour Phoenix Caissons, accessed 09/06/2017 from
Keystone (Historic Buildings Consultants), 'The Portland Naval Base, Dorset K/428', Volume 1 and Volume 2. 1993.


This building is listed under the Planning (Listed Buildings and Conservation Areas) Act 1990 as amended for its special architectural or historic interest.

End of official listing

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Date: 02 Jul 2001
Reference: IOE01/04605/20
Rights: Copyright IoE Mr Reg Perry. Source Historic England Archive
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