The monument consists of the surviving elements of a Mulberry harbour, a prototype floating artificial harbour, which underwent sea trials near Garlieston in 1943 during the Second World War. The remains comprise seven floating pontoons and a stone and concrete plinth. All seven pontoons lie beached on rocks, with five situated in a small cove known as Port Whapple approximately 2070m east-north-east of Garlieston, and two situated about 1035m east of Garlieston. The stone base is situated on the shore at Rigg Bay, approximately 1860m south of Garlieston and 455m south of Garlieston House.

The seven pontoons, codenamed 'Beetles' during the Second World War, were fabricated from reinforced concrete and originally supported a roadway on a steel superstructure capable of bearing substantial loads. The surviving pontoons are each approximately 15m by 7m and stand approximately 2m in height. The stone plinth relates to a rejected prototype for an alternative harbour design, being used as a tethering point to secure a roadway. It measures about 2.5m by 7.5m and stands about 4m in height.