The Island is actually the summit of an ancient, now submerged mountain, linked by an undersea saddle to the Blouberg. Its lower strata consists of Malmesbury shale forming a rocky and somewhat inhospitable coastline. Above this lies a thick limestone and calcrete deposit covered by windblown sands and shell fragments.
The Island is low-lying with the highest point being 24 metres, also known as Minto's Hill, (named after a nineteenth-century Surgeon-Superintendent of the General Infirmary) above sea-level. The climate is Mediterranean, as in nearby Cape Town, but the Island experiences stronger winds and comparative extremes in temperature.